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Ikapati Crater

A deep-space robotic emissary from Earth is continuing to carry out its extraordinary mission at a distant dwarf planet.

Orbiting high above Ceres, the sophisticated Dawn spacecraft is hard at work unveiling the secrets of the exotic alien world that has been its home for almost two years.

Dawn’s primary objective in this sixth orbital phase at Ceres (known as extended mission orbit 3, XMO3 or "this sixth orbital phase at Ceres") is to record cosmic rays. Doing so will allow scientists to remove that "noise" from the nuclear radiation measurements performed during the eight months Dawn operated in a low, tight orbit around Ceres. The result will be a cleaner signal, revealing even more about the atomic constituents down to about a yard (meter) underground. As we will see below, in addition to this ongoing investigation, soon the adventurer will begin pursuing a new objective in its exploration of Ceres.

With its uniquely capable ion propulsion system, Dawn has flown to orbits with widely varying characteristics. In contrast to the previous five observation orbits (and all the observation orbits at Vesta), XMO3 is elliptical. Over the course of almost eight days, the spacecraft sails from a height of about 4,670 miles (7,520 kilometers) up to almost 5,810 miles (9,350 kilometers) and back down. Dutifully following principles discovered by Johannes Kepler at the beginning of the 17th century and explained by Isaac Newton at the end of that century, Dawn’s speed over this range of altitudes varies from 210 mph (330 kilometers per hour) when it is closest to Ceres to 170 mph (270 kilometers per hour) when it is farthest. Yesterday afternoon, the craft was at its highest for the current orbit. During the day today, the ship will descend from 5,790 miles (9,310 kilometers) to 5,550 miles (8,930 kilometers). As it does so, Ceres’ gravity will gradually accelerate it from 170 mph (273 kilometers per hour) to 177 mph (285 kilometers per hour). (Usually we round the orbital velocity to the nearest multiple of 10. In this case, however, to show the change during one day, the values presented are more precise.)

As we saw last month, the angle of XMO3 to the sun presents an opportunity to gain a new perspective on Ceres, with sunlight coming from a different angle. (We include the same figure here, because we will refer to it more below.) Last week, Dawn took advantage of that opportunity, seeing the alien landscapes in a new light as it took pictures for the first time since October.

Figure illustrates orbits at Ceres
This illustrates (and simplifies) the relative size and alignment of Dawn’s six science orbits at Ceres. We are looking down on Ceres’ north pole. The spacecraft follows polar orbits, and seen edge-on here, each orbit looks like a line. (Orbits 1, 2 and 6 extend off the figure to the lower right, on the night side. Like 3, 4 and 5, they are centered on Ceres.) The orbits are numbered chronologically. The first five orbits were circular. Orbit 6, which is XMO3, is elliptical, and the dotted section represents the range from the minimum to the maximum altitude. With the sun far to the left, the left side of Ceres is in daylight. Each time the spacecraft travels over the illuminated hemisphere in the different orbital planes, the landscape beneath it is lit from a different angle. Ceres rotates counterclockwise from this perspective (just as Earth does when viewed from the north). So higher numbers correspond to orbits that pass over ground closer to sunrise, earlier in the Cerean day. (Compare this diagram with this figure, which shows only the relative sizes of the first four orbits, with each one viewed face-on rather than edge-on.) Click on this image for a larger view. Image credit: NASA/JPL

Dawn takes more than a week to revolve around Ceres, but Ceres turns on its axis in just nine hours. Because Dawn moves through only a small segment of its orbit in one Cerean day, it is almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath it. During one such period on Jan. 27, Dawn’s high perch moved only from 11°N to 12°S latitude as Ceres presented her full range of longitudes to the explorer’s watchful eye. This made it very convenient to take pictures and visible spectra as the scenery helpfully paraded by. (The spacecraft was high enough to see much farther north and south than the latitudes immediately beneath it.) Dawn will make similar observations again twice in February.

As Dawn was expertly executing the elegant, complex spiral ascent from XMO2 to XMO3 in November, the flight team considered it to be the final choreography in the venerable probe’s multi-act grand interplanetary performance. By then, Dawn had already far exceeded all of its original objectives at Vesta and Ceres, and the last of the new scientific goals could be met in XMO3, the end of the encore. The primary consideration was to keep Dawn high enough to measure cosmic rays, meaning it needed to stay above about 4,500 miles (7,200 kilometers). There was no justification or motivation to go anywhere else. Well, that’s the way it was in November anyway. This is January. And now it’s (almost) time for a previously unanticipated new act, XMO4.

Always looking for ways to squeeze as much out of the mission as possible, the team has now devised a new and challenging investigation. It will consume the next five months (and much of the next five Dawn Journals). We begin this month with an overview, but follow along each month as we present the full story, including a detailed explanation of the underlying science, the observations themselves and the remarkable orbital maneuvering entirely unlike anything Dawn has done before. (You can also follow along with your correspondent’s uncharacteristically brief and more frequent mission status updates.)

Map of Ceres
This map of Ceres has all 114 feature names approved so far by the International Astronomical Union (IAU). (We described the naming convention here.) As more features are named, this official list and map are kept up to date. We saw an earlier version of this map before Dawn had flown to its lowest orbit and obtained its sharpest pictures. The dwarf planet is 1.1 million square miles (2.8 million square kilometers). That’s about 36 percent of the land area of the contiguous United States, or the combined land areas of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. The scales for horizontal distance in this figure apply at the equator. Rectangular maps like this distort distances at other latitudes. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

From the XMO3 vantage point, with sunlight coming from the side, Ceres is gibbous and looks closer to a half moon than full. The new objective is to peer at Ceres when the sun is directly behind Dawn. This would be the same as looking at a full moon. (In the figure above, it would be like photographing Ceres from somewhere on the dashed line that points to the distant sun.)

While Dawn obtained pictures from near the line to the sun in its first Ceres orbit, there is a special importance to being even closer to that line. Let’s see why that alignment is valuable.

Most materials reflect light differently at different angles. You can investigate this yourself (and it’s probably easier to do at home than it is in orbit around a remote dwarf planet). To make it simpler, take some object that is relatively uniform (but with a matte finish, not a mirror-like finish) and vary the angles at which light hits it and from which you look at it. You may see that it appears dimmer or brighter as the angles change. It turns out that this effect may be used to help infer the nature of the reflecting material. (For the purposes of this exercise, if you can hold the angle of the object relative to your gaze fixed, and vary only the angle of the illumination, that’s best. But don’t worry about the details. Conducting this experiment represents only a small part of your final grade.)

Now when scientists carefully measure the reflected light under controlled conditions, they find that the intensity changes quite gradually over a wide range of angles. In other words, the apparent brightness of an object does not vary dramatically as the geometry changes. However, when the source of the illumination gets very close to being directly behind the observer, the reflection may become quite a bit stronger. (If you test this, of course, you have to ensure your shadow doesn’t interfere with the observation. Vampires don’t worry about this, and we’ll explain below why Dawn needn’t either.)

If you (or a helpful scientist friend of yours) measure how bright a partial moon is and then use that information to calculate how bright the full moon will be, you will wind up with an answer that’s too small. The full moon is significantly brighter than would be expected based on how lunar soil reflects light at other angles. (Of course, you will have to account for the fact that there is more illuminated area on a full moon, but this curious optical behavior is different. Here we are describing how the brightness of any given patch of ground changes.)

A full moon occurs when the moon and sun are in opposite directions from Earth’s perspective. That alignment is known as opposition. That is, an astronomical body (like the moon or a planet) is in opposition when the observer (you) is right in between it and the source of illumination (the sun), so all three are on a straight line. And because the brightness takes such a steep and unexpected jump there, this phenomenon is known as the opposition surge.

Yalode Crater
Dawn observed this scene inside Yalode Crater on Oct. 13, 2015, from its third mapping orbit at an altitude of 915 miles (1,470 kilometers). At 162 miles (260 kilometers) in diameter, Yalode is the second largest crater on Ceres. (Scientists expected to see much larger craters than Ceres displays.) The two largest craters within Yalode are visible in this picture. Lono Crater, at top right, is 12 miles (20 kilometers) in diameter. (Lono is a Hawaiian god of agriculture, rain and other roles.) Below Lono is the 11-mile (17-kilometer) Besua Crater. (Besua is one of at least half a dozen Egyptian grain gods.) Note several chains of craters as well as fractures on the left and lower right. We saw a much more fractured area of Yalode, now named Nar Sulcus, here. (Nar is from a modern pomegranate feast in part of Azerbaijan. A sulcus is a set of parallel furrows or ridges.) You can locate this scene in the eastern part of Yalode on the map above near 45°S, 300°E. The photo below shows a more detailed view. You can see all of Yalode starting at 2:32 in the animation introduced here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The observed magnitude of the opposition surge can reveal some of the nature of the illuminated object on much, much finer scales than are visible in photos. Knowing the degree to which the reflection strengthens at very small angles allows scientists to ascertain (or, at least, constrain) the texture of materials on planetary surfaces even at the microscopic level. If they are fortunate enough to have measurements of the reflectivity at different angles for a region on an airless solar system body (atmospheres complicate it too much), they compare them with laboratory measurements on candidate materials to determine which ones give the best match for the properties.

Dawn has already measured the light reflected over a wide range of angles, as is clear from the figure above showing the orbits. But the strongest discrimination among different textures relies on measuring the opposition surge. That is Dawn’s next objective, a bonus in the bonus extended mission.

You can see from the diagram that measuring the opposition surge will require a very large change in the plane of Dawn’s orbit. Shifting the plane of a spacecraft’s orbit can be energetically very, very expensive. (We will discuss this more next month.) Fortunately, the combination of the unique capabilities provided by the ion propulsion system and the ever-creative team makes it affordable.

Ceres
Dawn had this view on June 7, 2016, from its fourth mapping orbit. Taken at an altitude of 240 miles (385 kilometers), this picture shows greater detail in a smaller area than the picture above. Part of Lono Crater is at the bottom. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Powered by an insatiable appetite for new knowledge, Dawn will begin ion thrusting on Feb. 23. After very complex maneuvers, it will be rewarded at the end of April with a view of a full Ceres from an altitude of around 12,400 miles (20,000 kilometers), about the height of GPS satellites above Earth. (That will be about 50 percent higher than the first science orbit, which is labeled as line 1 in the figure.) There are many daunting challenges in reaching XMO4 and measuring the opposition surge. Even though it is a recently added bonus, and the success of the extended mission does not depend on it, mission planners have already designed a backup opportunity in case the first attempt does not yield the desired data. The second window is late in June, allowing the spacecraft time to transmit its findings to Earth before the extended mission concludes at the end of that month.

Occator Crater
Occator Crater is shown in this mosaic of photos Dawn took at its lowest altitude of 240 miles (385 kilometers). The central bright area, Cerealia Facula, is the prime target in the planned opposition surge measurements. Dawn’s infrared spectra show that this reflective material is principally sodium carbonate, a kind of salt. We described more about this mosaic here. For other views of Occator and its mesmerizing reflective regions, follow the links in the paragraph below. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For technical reasons, the measurements need to be made from a high altitude, and throughout the complex maneuvering to get there, Dawn will remain high enough to monitor cosmic rays. Ceres will appear to be around five times the width of the full moon we see from Earth. It will be about 500 pixels in diameter in Dawn’s camera, and more than 180,000 pixels will show light reflected from the ground. Of greatest scientific interest in the photographs will be just a handful of pixels that show the famous bright material in Occator Crater, known as Cerealia Facula and clearly visible in the picture above. Scientists will observe how those pixels surge in brightness over a narrow range of angles as Dawn’s XMO4 orbital motion takes it into opposition, exactly between Occator and the sun. Of course, the pictures also will provide information on how the widespread dark material covering most of the ground everywhere else on Ceres changes in brightness (or, if you prefer, in dimness). But the big reward here would be insight into the details of Cerealia Facula. Comparing the opposition surges with laboratory measurements may reveal characteristics that cannot be discerned any other way save direct sampling, which is far beyond Dawn’s capability (and authority). For example, scientists may be able to estimate the size of the salt crystals that make up the bright material, and that would help establish their geological history, including whether they formed underground or on the surface. We will discuss this more in March.

Most of the data on opposition surges on solar system objects use terrestrial observations, with astronomers waiting until Earth and the target happen to move into the necessary alignment with the sun. In those cases, the surge is averaged over the entire body, because the target is usually too far away to discern any details. Therefore, it is very difficult to learn about specific features when observing from near Earth. Few spacecraft have actively maneuvered to acquire such data because, as we alluded to above and will see next month, it is too difficult, especially at a massive body like Ceres. The recognition that Dawn might be able to complete this challenging measurement for a region of particular interest represents an important possibility for the mission to discover more about this intriguing dwarf planet’s geology.

Meeting the scientific goal will require a careful and quantitative analysis of the pixels, but the images of a fully illuminated Ceres will be visually appealing as well. Nevertheless, you are cautioned to avoid developing a mistaken notion about the view. (For that matter, you are cautioned to avoid developing mistaken notions about anything.) You might think (and some readers wondered about this in a different phase of the mission) that with Dawn being between the sun and Ceres (and not being a vampire), the spacecraft’s shadow might be visible in the pictures. It would look really cool if it were (although it also would interfere with the measurement of the opposition surge by introducing another factor into how the brightness changes). There will be no shadow. The spacecraft will simply be too high. Imagine you’re standing in Occator Crater, either wearing your spacesuit while engaged in a thrilling exploration of a mysterious and captivating extraterrestrial site or perhaps instead while you’re indoors enjoying some of the colony’s specially salted Cerean savory snacks, famous throughout the solar system. In any case, the distant sun you see would be a little more than one-third the size that it looks from Earth, comparable to a soccer ball at 213 feet (65 meters). Dawn would be 12,400 miles (20,000 kilometers) overhead. Although it’s one of the largest interplanetary spacecraft ever to take flight, with a wingspan of 65 feet (20 meters), it would be much too small for you to see at all without a telescope and would block an undetectably small amount of sunlight. It would appear smaller than a soccer ball seen from 135 miles (220 kilometers). Therefore, no shadow will be cast, the measurement will not be compromised by the spacecraft blocking some of the light reaching the ground and the pictures will not display any evidence of the photographer.

Ceres
Dawn took this picture on Oct. 21, 2016, in its fifth observation orbit, at an altitude of 920 miles (1,480 kilometers). The two largest craters here display very different kinds of topography on their floors. The larger, Jarimba, is 43 miles (69 kilometers) across. (Jarimba is a god of fruit and flowers among the Aboriginal Aranda of central Australia.) Above Jarimba is part of Kondos Crater, which is 27 miles (44 kilometers) in diameter. (Kondos is a pre-Christian Finnish god of sowing and young wheat.) This scene is centered near 21°S, 27°E on the map above. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even as the team was formulating plans for this ambitious new campaign, they successfully dealt with a glitch on the spacecraft this month. When a routine communications session with the Deep Space Network began on Jan. 17, controllers discovered that Dawn had previously entered its safe mode, a standard response the craft uses when it encounters conditions its programming and logic cannot accommodate. The main computer issues instructions to reconfigure systems, broadcasts a special radio signal through one of the antennas and then patiently awaits help from humans on a faraway planet (or anyone else who happens to lend assistance). The team soon determined what had occurred. Since it left Earth, Dawn has performed calculations five times per second about its location and speed in the solar system, whether in orbit around the sun, Vesta or Ceres. (Perhaps you do the same on your deep-space voyages.) However, it ran into difficulty in those calculations on Jan. 14 for the first time in more than nine years of interplanetary travel. To ensure the problematic calculations did not cause the ship to take any unsafe actions, it put itself into safe mode. Engineers have confirmed that the problem was in software, not hardware and not even a cosmic ray strike, which has occasionally triggered safe mode, most recently in September 2014.

Mission controllers guided the spacecraft out of safe mode within two days and finished returning all systems to their standard configurations shortly thereafter. Dawn was shipshape the subsequent week and resumed its scientific duties. When it activated safe mode, the computer correctly powered off the gamma ray and neutron detector, which had been measuring the cosmic rays, as we described above. The time that the instrument was off will be inconsequential, however, because there is more than enough time in the extended mission to acquire all the desired measurements.

The extended mission has already proven to be extremely productive, yielding a great deal of new data on this ancient world. But there is still more to look forward to as the veteran explorer prepares for a new and adventurous phase of its extraordinary extraterrestrial expedition.

Dawn is 5,650 miles (9,100 kilometers) from Ceres. It is also 2.87 AU (266 million miles, or 429 million kilometers) from Earth, or 1,135 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.

TAGS: CERES, DAWN, DWARF PLANET

  • Marc Rayman
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Occator Crater

Blue rope lights adorn Dawn mission control at JPL, but not because the flight team is in the holiday spirit (although they are in the holiday spirit).

The felicitous display is more than decorative. The illumination indicates that the interplanetary spacecraft is thrusting with one of its ion engines, which emit a lovely, soft bluish glow in the forbidding depths of space. Dawn is completing another elegant spiral around dwarf planet Ceres, maneuvering to its sixth science orbit.

Dawn’s ion propulsion system has allowed the probe to accomplish a mission unlike any other, orbiting two distant extraterrestrial destinations. Even more than that, Dawn has taken advantage of the exceptional efficiency of its ion engines to fly to orbits at different altitudes and orientations while at Vesta and at Ceres, gaining the best perspectives for its photography and other scientific investigations.

Dawn has thrust for a total of 5.7 years during its deep-space adventure. All that powered flight has imparted a change in the ship’s velocity of 25,000 mph (40,000 kilometers per hour). As we have seen, this is not the spacecraft’s actual speed, but it is a convenient measure of the effect of its propulsive work. Reaching Earth orbit requires only about 17,000 mph (less than 28,000 kilometers per hour). In fact, Dawn’s gentle ion engines have delivered almost 98 percent of the change in speed that its powerful Delta 7925H-9.5 rocket provided. With nine external rocket engines and a core consisting of a first stage, a second stage and a third stage, the Delta boosted Dawn by 25,640 mph (41,260 kilometers per hour) from Cape Canaveral out of Earth orbit and onto its interplanetary trajectory, after which the remarkable ion engines took over. No other spacecraft has accomplished such a large velocity change under its own power. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

Early this year, we were highly confident Dawn would conclude its operational lifetime in its fourth orbit at Ceres (and remain there long after). But unexpectedly healthy and with an extension from NASA, Dawn is continuing its ambitious mission. After completing all of its tasks in its fifth scientific phase at Ceres, Dawn is pursuing new objectives by flying to another orbit for still more discoveries. Although we never anticipated adding a row to the table of Dawn’s orbits, last presented in December 2015, we now have an updated version.

Ceres
orbit
Dawn code
name
Dates
(mo.day.yr)
Altitude
in miles
(km)
Resolution
in ft (m)
per pixel
Orbit
period
Equivalent
distance of a soccer ball
1 RC3   04.23.15 – 05.09.15 8,400
(13,600)
4,200
(1,300)
15
days
10 ft
(3.2 m)
2 Survey   06.06.15 –06.30.15 2,700
(4,400)
1,400
(410)
3.1
days
3.4 ft
(1.0 m)
3 HAMO   08.17.15 – 10.23.15 915
(1,470)
450
(140)
19
hours
14 in
(34 cm)
4 LAMO/
XMO1
  12.16.15 – 09.02.16 240
(385)
120
(35)
5.4
hours
3.5 in
(9.0 cm)
5 XMO2   10.16.16 – 11.04.16 920
(1,480)
450
(140)
19
hours
14 in
(35 cm)

As with the obscure Dawn code names for other orbits, this fifth orbit’s name requires some explanation. The extended mission is devoted to undertaking activities not envisioned in the prime mission. That began with two extra months in the fourth mapping orbit performing many new observations, but because it was then the extended mission, that orbit was designated extended mission orbit 1, or XMO1. (It should have been EMO1, of course, but the team’s spellchecker was offline on July 1, the day the extended mission started.) Therefore, the next orbit was XMO2. Dawn left XMO2 on Nov. 4, and we leave it to readers’ imaginations to devise a name for the orbit the spacecraft is now maneuvering to.

Surprisingly, Dawn is flying higher to enhance part of the scientific investigation that motivated going to the lowest orbit. We have explained before that Dawn’s objective in powering its way down to the fourth mapping orbit was to make the most accurate measurements possible of gravity and of nuclear radiation emitted by the dwarf planet.

For more than eight months, the explorer orbited closer to the alien world than the International Space Station is to Earth, and the gamma ray spectra and neutron spectra it acquired are outstanding, significantly exceeding all expectations. But ever-creative scientists have recognized that even with that tremendous wealth of data, Dawn can do still better. Let’s look at this more carefully and consider an example to resolve the paradox of how going higher can yield an improvement.

Ceres
Dawn had this view of Ceres’ limb on Oct. 16 at an altitude of 920 miles (1,480 kilometers). The probe took this picture about 12 minutes after the picture above of Occator Crater. By this time, Dawn’s orbital motion had taken the center of Occator out of the view, but most of the shadowy eastern part is still visible at upper left. A Cerean day lasts about nine hours, so in the time between these two pictures, Ceres rotated as much as Earth would rotate in about 32 minutes. As a result, the change in the sun angle is quite noticeable. You can compare some craters in the two pictures to see how the lighting has changed. This is particularly evident not only in Occator but also in the crater near the center of the large crater visible here (on the lower right of the first picture) as well as the craters below and to the left of it. At the bottom right of this picture is part of the 45-mile (72-kilometer) Kaikara Crater. (Kaikara is a harvest goddess in the kingdom of Bunyoro in Uganda.) You can locate this scene on this map, with Kaikara at 43°N, 222°E and Occator at 20°N, 239°E. Full image (rotated differently and with different picture adjustments) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The gamma ray and neutron detector (GRaND) reveals some of Ceres’ atomic constituents down to about a yard (meter) underground. The principal limitation in analyzing these spectra is "noise." In fact, noise limits the achievable accuracy of many scientific measurements. It isn’t necessarily the kind of noise that you hear from loud machinery (nor from the mouth of your unhelpful parent, inattentive progeny or boring and verbose coworker), but all natural systems have something similar. Physical processes other than the ones of interest make unwanted contributions to the measurements. The part of a measurement scientists want is called the "signal." The part of a measurement scientists don’t want is called the "noise." The quality of a measurement may be characterized by comparing the strength of the signal to the strength of the noise. (This metric is called the "signal to noise ratio" by people who like to use jargon like "signal to noise ratio.")

We have discussed that cosmic rays, radiation that pervades space, strike atomic nuclei on Ceres, creating the signals that GRaND measures. Remaining at low altitude would have allowed Dawn to enhance its measurement of the Cerean nuclear signal. But scientists determined that an even better way to improve the spectra than to increase the signal is to decrease the noise. GRaND’s noise is a result of cosmic rays impinging directly on the instrument itself and on nearby parts of the spacecraft. With a more thorough measurement of the noise from cosmic rays, scientists will be able to mathematically remove that component of the low altitude measurements, leaving a clearer signal.

For an illustration of all this, suppose you want to hear the words of a song. The words are the signal and the instruments are the noise. (This is a scientific discussion, not a musical one.) It could be that the instruments are so loud and distracting that you can’t make the words out easily.

You might try turning up the volume, because that increases the signal, but it increases the noise as well. If the performance is live, you might even try to position yourself closer to the singer, perhaps making the signal stronger without increasing the noise too much. (Other alternatives are simply to Google the song or ask the singer for a copy of the lyrics, but those methods would ruin this example.)

If you’re doing this in the 21st century (or later), there’s another trick you can employ, taking advantage of computer processing. Suppose you had a recording of the singing with the instruments and then obtained separate recordings of the instruments. You could subtract the musical sounds that constitute the noise, removing the contributions from both guitars, the drums, the harp, both ukuleles, the kazoo and all the theremins. And when you eliminate the noise of the instruments, what remains is the signal of the words, making them much more intelligible.

To obtain a better measure of the noise, Dawn needs to go to higher altitude, where GRaND will no longer detect Ceres. It will make detailed measurements of cosmic ray noise, which scientists then will subtract from their measurements at low altitude, where GRaND observed Ceres signal plus cosmic ray noise. The powerful capability to raise its orbit so much affords Dawn the valuable opportunity to gain greater insight into the atomic composition. Of course, it’s not quite that simple, but essentially this method will help Dawn hear Ceres’ nuclear song more clearly.

Ceres
Dawn took this photo on Oct. 17 at an altitude of 920 miles (1,480 kilometers). Above and to the right of center, part of the wall of a crater has collapsed, allowing material to flow into the larger crater. The area covered by the flow is less densely cratered than the surrounding terrain, because it is younger. We have seen how scientists use the number and size of craters to date geological features (no results are available yet in this area). The larger crater is Ghanan, one of the names of a Mayan maize god, although the devastating flow may not have been good for the maize harvest when the collapse occurred. Ghanan Crater, with an average diameter of 42 miles (68 kilometers), is on this map at 77°N, 31°E. Full image (with different picture adjustments) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To travel from one orbit to another, the sophisticated explorer has followed complex spiral routes. We have discussed the nature of these trajectories quite a bit, including how the operations team designs and flies them. But now they are using a slightly different method.

Those of you at Ceres who monitor the ship’s progress probably wouldn’t notice a difference in the type of trajectory. And the rest of you on Earth and elsewhere who keep track through our mission status updates also would not detect anything unusual in the ascent profile (to the extent that a spacecraft using ion propulsion to spiral around a dwarf planet is usual). But celestial navigators are now enjoying their use of a method they whimsically call local maximal energy spiral feedback control.

The details of the new technique are not as important for our discussion here as one of the consequences: Dawn’s next orbit will not be nearly as circular as any of its other orbits at Ceres (or at Vesta). Following the conclusion of this spiral ascent on Dec. 5, navigators will refine their computations of the orbit, and we will describe the details near the end of the month. We will see that as the spacecraft follows its elliptical loops around Ceres, each taking about a week, the altitude will vary smoothly, dipping below 4,700 miles (7,600 kilometers) and going above 5,700 miles (9,200 kilometers). Such a profile meets the mission’s needs, because as long as the craft stays higher than about 4,500 miles (7,200 kilometers), it can make the planned recordings of the cacophonous cosmic rays. We will present other plans for this next phase of the mission as well, including photography, in an upcoming Dawn Journal.

As Dawn continues its work at Ceres, the dwarf planet continues its stately 4.6-year-long orbit around the sun, carrying Earth’s robotic ambassador with it. Ceres follows an elliptical path around the sun (see, for example, this discussion, including the table). In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are describing how much each orbit deviates from a perfect circle.)

When Ceres tenderly took Dawn into its gravitational embrace in March 2015, they were 2.87 AU (267 million miles, or 429 million kilometers) from the sun. In January 2016, we mentioned that Ceres had reached its aphelion, or greatest distance from the sun, at 2.98 AU (277 million miles, or 445 million kilometers). Today at 2.85 AU (265 million miles, or 427 million kilometers), Ceres is closer to the sun than at any time since Dawn arrived, and the heliocentric distance will gradually decrease further throughout the extended mission. (If the number of numbers is overwhelming here, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles or kilometers. Ignore the other two scales so you can focus on the relative distances.)

Ceres
Dawn’s location in the solar system is shown on Nov. 7, 2016. On that day, the spacecraft and Ceres were at the same distance from the sun as when Dawn arrived last year. Now as Ceres advances counterclockwise in its elliptical orbit, they will move somewhat closer to the sun. We have plotted Dawn’s progress on this figure before, most recently in September. Image credit: NASA/JPL

Another consequence of orbiting the sun is the progression of seasons. Right on schedule, as we boldly predicted in August 2015, Nov. 13 was the equinox on Ceres, marking the beginning of northern hemisphere autumn and southern hemisphere spring. Although it is celebrated on Ceres with less zeal than on Earth, it is fundamentally the same: the sun was directly over the equator that day, and now it is moving farther south. It takes Ceres so long to orbit the sun that this season will last until Dec. 22, 2017.

A celebration that might occur on Ceres (and which you, loyal Dawnophile, are welcome to attend) would honor Dawn itself. Although the spacecraft completed its ninth terrestrial year of spaceflight in September, on Dec. 12, it will have been two Cerean years since Dawn left Earth for its interplanetary journey. Be sure to attend in order to learn how a dawnniversary is commemorated in that part of the solar system.

Although a year on Ceres lasts much longer than on Earth, 2016 is an unusually long year on our home planet. Not only was a leap day included, but a leap second will be added at the very end of the year to keep celestial navigators’ clocks in sync with nature. The Dawn team already has accounted for the extra second in the intricate plans formulated for the spacecraft. And at that second, on Dec. 31 at 23:59:60, we will be able to look back on 366 days and one second, an especially full and gratifying year in this remarkable deep-space expedition. But we needn’t wait. Even now, as mission control is bathed in a lovely glow, the members of the team as well as space enthusiasts everywhere are aglow with the thrill of new knowledge, the excitement of a daring, noble adventure and the anticipation of more to come.

Dawn is 3,150 miles (5,070 kilometers) from Ceres. It is also 2.08 AU (194 million miles, or 312 million kilometers) from Earth, or 770 times as far as the moon and 2.11 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PST November 28, 2016

TAGS: DAWN, CERES, ION ENGINE, DWARF PLANET

  • Marc Rayman
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Dawn photographed this scene in Yalode Crater on June 15, 2016

Dawn has just completed another outstandingly successful observation campaign at Ceres.

Far, far from Earth, the spacecraft has been making measurements at the alien world that were not even imagined until a few months ago. Once again, the experienced explorer has performed its complex assignments with distinction.

When Dawn arrived at Ceres in March 2015, becoming the first spacecraft to reach a dwarf planet, it was looking ahead to a very ambitious year of discovery from four different orbital altitudes. The great benefit of being able to enter orbit rather than fly by is that Dawn can scrutinize its subject over an extended period to develop a detailed, intimate portrait. Taking advantage of the ship’s ability to maneuver with its advanced ion propulsion system, mission planners had carefully selected the four orbits to enable a wide range of measurements.

By February of this year, Dawn had exceeded every one of its original mission objectives and was still going strong, accomplishing many new goals. Nevertheless, no one (at least, no one who was well informed) expected that the probe would complete its new assignments and yet still have the capability to maneuver to a fifth orbit and then undertake even more new observations. But that is exactly what occurred.

After more than eight months orbiting only 240 miles (385 kilometers) above the strange terrain of rock, ice and salt, Dawn ignited one of its ion engines on Sept. 2. By Oct. 6, when it had completed its graceful ascent, Dawn had made 93 spiral loops, reaching an orbit 920 miles (1,480 kilometers) high. From there, revolving once every 18.9 hours, the spacecraft has executed its new program of investigations.

With observations of Ceres from about the same altitude as a year ago in Dawn’s third mapping orbit, scientists will scour the expansive terrain, looking for changes. The most likely change is the presence of new, small craters. Everything in the solar system (including your planetary residence) is subject to strikes from rocks that orbit the sun. Ceres lives in the main asteroid belt between Mars and Jupiter, a particularly rough neighborhood, and being the largest resident there (by far) doesn’t give it any special protection or immunity. In fact, being the largest resident also makes Ceres the largest target.

Ceres
Dawn had this view on June 6, 2016, from an altitude of 240 miles (385 kilometers). The bright material at upper left is on the northwest rim of Kerwan Crater. Geologists have cataloged well over 130 locations on Ceres that are covered with reflective material. (The most famous deposits are in Occator Crater.) The brightness is because briny ice that had been on the surface sublimated, leaving behind salts, which reflect more sunlight than other minerals on the dwarf planet. Extending 174 miles (280 kilometers) across, Kerwan is the largest crater on Ceres. It is centered at 11°S, 124°E on the map shown last month. (Kerwan is a spirit of sprouting maize among the Hopi of Arizona in the US.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to remapping Ceres with all of the camera’s color filters, the flight team has given Dawn other tasks. Controlling a sophisticated interplanetary spacecraft conducting complex operations so very far from Earth is never easy (but it’s always incredibly cool). There have been many challenges throughout this ambitious mission, quite unlike any ever undertaken. One of the significant ones was observing specific targets of interest from low altitude. We have explained that orbiting so close to the ground, the spacecraft’s motion was quite difficult to predict with sufficient accuracy far enough in advance to guide the craft so that the instruments’ narrow fields of view would hit specific features. Dawn was designed to map uncharted worlds, not to conduct targeted observations.

The difficulty was compounded by the loss in 2010 and 2012 of two of the four reaction wheels, used for controlling the probe’s orientation. An important side effect of the nudges from the small hydrazine-fueled jets of the reaction control system (even in combination with the two operable reaction wheels in hybrid control mode) was tiny distortions in the spacecraft’s orbital trajectory. The cumulative effect of many jet firings over days and weeks was enough to make it quite challenging to ensure the sensors could spot the targets as Dawn sped around the rapidly rotating orb beneath it.

This is not as difficult at higher altitude both because Dawn does not need to use its jets as often and because the instruments take in a wider area. As a result, the explorer has been better able to catch sight of preselected geological features, and it has acquired valuable new data.

Ceres
Dawn observed this area of craters, hills and canyons inside Urvara Crater on June 2, 2016, from an altitude of 240 miles (385 kilometers). The third largest crater on Ceres, it is 106 miles (170 kilometers) wide. We have seen Urvara several times before, and the crater on the right of this picture is visible in the northwestern part of Urvara shown here and here. Urvara is on the new map at 46°S, 249°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn also has studied selected sites at several times of the Cerean day. Mission planners may determine, for example, that if Dawn points not straight down on a particular orbit at a particular time but rather partially to the side, a certain crater could be spotted soon after Ceres’ nine-hour daily rotation has brought it into sunlight. In other words, it would be early in the morning at the crater when Dawn sees it, providing a nice dawn view. On another orbital revolution, Dawn might point in a different direction to see the same location longer after it has come into sunlight (that is, longer after sunrise), so from that same crater’s point of view, it is later in the day (albeit on a different day).

The spacecraft has done more than look at some special locations at different times of the Cerean day, corresponding to different lighting conditions. In taking pictures for a new map of Ceres this month, everywhere Dawn looked, the illumination was different from the photographs for the maps it compiled in its previous orbits. The orbit now is oriented at a different angle from the sun.

When the interplanetary adventurer was at Vesta, we described the orientation of the orbits in words. Thanks to changes in the Dawn Journal site since then, now we can present a picture showing that the scenery beneath Dawn has been illuminated from a different angle at each orbital altitude. And now in the fifth orbit, by seeing the sights from the same height as in the third mapping orbit but with different lighting, we gain a new perspective on the alien terrain.

Ceres
This illustrates (and simplifies) the relative size and alignment of Dawn’s five science orbits at Ceres. We are looking down on Ceres’ north pole. The spacecraft follows polar orbits, and seen edge-on here, each circular orbit looks like a line. (Orbits 1 and 2 extend off the figure to the right, on the night side. Like 3, 4 and 5, they are centered on Ceres.) The orbits are numbered chronologically. With the sun far to the left, the left side of Ceres is in daylight. Each time the spacecraft travels over the illuminated hemisphere in the different orbital planes, the landscape beneath it is lit from a different angle. Ceres rotates counterclockwise from this perspective (just as Earth does when viewed from the north). So higher numbers correspond to orbits that pass over ground closer to sunrise, earlier in the Cerean day. (Compare this diagram with this figure, which shows only the relative sizes of the orbits, with each one viewed face-on rather than edge-on.) Click on this image for a larger view. Image credit: NASA/JPL

In addition to all of its other work this month, the sophisticated robot has continued some specialized measurements it began at lower altitude. Being higher up does not cause as much of a reduction in the sharpness of some pictures as you might think. Held in a looser gravitational grip, Dawn’s orbital velocity is lower at higher altitude. As a result, observations that require a long exposure are not affected as much by the spacecraft’s movement. That’s helpful for some of the spectra and photographs. For example, Dawn has used its camera to peer into craters near the north and south poles that are in shadow continuously, every Cerean day of the Cerean year. These special locations might trap water molecules that escape from elsewhere on Ceres where it is too warm for them. With the benefits of a wider view from a higher altitude and a more predictable orbital path, Dawn’s coverage this month of these intriguing areas, faintly illuminated by sunlight reflected from crater walls, has been more complete than at lower altitude.

This fifth Ceres campaign was intricate and intensive, but it stayed right on the tight schedule. Dawn began collecting data as planned on Oct. 16 and finished transmitting its findings to Earth on Oct. 29. And it was exceedingly productive, yielding almost 3,000 photographs plus a great many infrared spectra and visible spectra containing a wealth of new information about Ceres.

This week controllers are going to check out the backup camera, as they do twice a year to confirm that it is still healthy and ready to take over should the primary camera develop a problem. Nevertheless, the primary camera remains fully functional. The team also is planning to switch to the backup set of reaction control system thrusters. Dawn has flown for so many years without a full complement of reaction wheels that these hydrazine thrusters have been used far more than anticipated when the ship was designed. They are healthy, but ever-cautious engineers do not want to overuse them.

Achita Crater
Dawn took this photo of Achita Crater on June 3, 2016, from an altitude of 240 miles (385 kilometers). Departing from what may seem to be the theme above of displaying interesting landscapes in the northwestern parts of the largest craters on Ceres, this scene includes most of the 25-mile (40-kilometer) Achita. Although many craters have a mountain peak in the center, this one has an extended ridge. (We have seen other craters on Ceres with central ridges, including Haulani and Urvara here and here.) Also note the bright material at the bottom of the southwest wall and a smaller deposit on the northeast rim. Achita Crater is at 26°N, 66°E on this map. (Achita is a god of agriculture in northern Nigeria.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn’s work in this fifth orbit is part of a comprehensive plan for exploring Ceres as thoroughly as possible. Surprising though it may be, we will see next month that scientists have determined that there is even more to learn about Ceres by flying to a higher altitude. So now that Dawn has accomplished all of its objectives for this phase of the mission, it is about to begin another month of maneuvering. On Nov. 4, the spaceship will once again power on ion engine #2 and start another spiral to a sixth orbital observing post.

As Earth and Ceres (accompanied by Dawn) follow their independent orbits around the sun, the distance between them is constantly changing. On Oct. 22, they were at their smallest separation in the 3.5 years from June 2014 to Dec. 2017. On that date, Dawn was a mere 1.900 AU (176.6 million miles, or 284.2 million kilometers) from its first solar system residence. Dawn never loses track of the rest of its team, still stationed on that faraway planet. But after many years of interplanetary travels and more than a year at Vesta, the denizen of deep space is now a devoted companion of Ceres, and that is where it focuses its attention. And it has more work to do as it seeks still greater insights into the nature of its mysterious and exotic home.

Dawn is 920 miles (1,480 kilometers) from Ceres. It is also 1.91 AU (178 million miles, or 286 million kilometers) from Earth, or 705 times as far as the moon and 1.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2016

P.S. Now that this Dawn Journal is complete, your correspondent can turn his attention to getting into costume for Halloween. This year, he will be disguised as someone who knew all along that Dawn would engage in a productive and innovative extended mission at Ceres. Just imagine what a great time the trick-or-treaters are going to have when they visit his home!

TAGS: DAWN, CERES, OBSERVATIONS

  • Marc Rayman
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Ceres

Dawn is actively continuing to add details to the intimate portrait it is creating of Ceres, a distant and exotic world.

The dwarf planet has been revealing many secrets to the companion she has held in her tender but firm gravitational embrace since early last year.

Following the conclusion of Dawn's ambitious 8.8-year prime mission on June 30, the spacecraft has been gathering a wealth of data with all sensors in its extended mission as it orbits closer to Ceres than the International Space Station is to Earth. When the adventurer descended to its current orbital altitude of 240 miles (385 kilometers) in December 2015, mission controllers had planned for only a few months of operations. Because of the prior failure of two reaction wheels, used for orienting the craft in space, Dawn had to rely on the creativity of the team to stretch the dwindling supply of hydrazine to keep the ship operating. No one on the team expected their efforts to be as productive as they turned out to be, allowing the mission to continue much longer. Now Dawn has completed more than eight months of virtually flawless activities at this altitude, over 1,100 orbital revolutions, returning far, far more data than ever anticipated.

We have recounted in recent months how Dawn has overachieved, and its extended mission has sustained that favorable trend. As just one example, since Ceres first showed up as a small, fuzzy blob in Dawn's camera in December 2014, the spacecraft has taken more than 51,000 photos of Ceres (and more than 51,000 more photos of Ceres than discoverer Giuseppe Piazzi took). More than 37,000 of those have been taken in this fourth and lowest orbit, providing exquisite resolution.

Dawn has achieved so much that it has been given new, special assignments not even envisioned at the beginning of this year. For example, scientists recently adjusted settings for the gamma ray spectrometer to search for the signature of atoms that were not part of the original program of inventorying Ceres' elements.

The reason for flying so low was to measure nuclear radiation and the variations in the gravity field. In fact, Dawn was not designed to map the vast territory with its other instruments from this tight orbit. All the pictures, infrared spectra and visible spectra have been bonuses of a successful mission. We have seen before how difficult it was to capture specific geological features on Ceres with the camera. It is even more challenging with the visible and infrared mapping spectrometers because they share a much narrower view than the camera. Nevertheless, with great effort, the team managed in the extended mission to obtain beautiful spectra of the famous bright region in Occator Crater, known from earlier spectra to be highly reflective deposits of salt left behind when briny ice covering the ground inside the crater sublimated. Dawn has been successful in tracking down other important sites with its visible and infrared spectrometers as well.

After photographing more than 99.9 percent of the dwarf planet at high resolution, the spacecraft took a great many more pictures at different angles, making stereo views to improve the topographical map it developed in the third mapping orbit. In addition, Dawn used the filters in its camera to take new, sharper color photos of some of the most geologically interesting locations.

The explorer has acquired other pictures of special scientific interest as well. Let's delve into one kind. We have described Dawn's findings about the location of the north and south poles and the tilt of Ceres' rotational axis. As we saw, Earth's axis is tilted more, so our planet experiences greater variation in the position of the sun during one heliocentric revolution (one year). On Ceres, the sun never moves far from the equator, which means it is always far from the poles. From the perspective of the high northern or southern latitudes, the sun is always near the horizon and is never high in the sky. As a result, the floors of some craters near the poles are in shadow continuously throughout the Cerean year (which lasts 4.6 terrestrial years). Without even brief warming rays of the distant sun, these locations must be especially cold.

Ceres
Dawn looked down from 240 miles (385 kilometers) on May 27, 2016, at this scene at 73 degrees north latitude. From this location, the sun (which is off the picture, far to the right) never gets high above the horizon. More recently, Dawn has taken long exposures to see into some of the craters that are in persistent shadow. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Thanks to Dawn, we know ice has been on the ground in some places in the past and is there even now in Oxo Crater. (We also know there is a tremendous amount of ice underground.) When ice on the surface is heated enough by the sun, it sublimates, the water molecules receiving enough energy to escape from the solid, becoming a gas. Some of them leave with so much energy, they break free of Ceres' gravitational pull and go far into space. But many of the molecules follow a familiar parabolic arc, landing elsewhere on the dwarf planet, just as a ball thrown on Earth will come back down. If the landing spot is similarly warm enough for ice to sublimate (as most places on Ceres are), eventually the molecule will be lofted again, having a chance of landing in a new, random location. But molecules that happen to fall in the deep cold of a crater in persistent shadow will be trapped. As a result, these "cold traps" may harbor ice that has accumulated over thousands of years (or even longer).

Dawn has peered into craters that might be cold traps. Of course, sunlight doesn't illuminate them directly. But faint reflections from other parts of the crater may be just barely bright enough that with long exposures and special care in analyzing the pictures, new insights might come to light.

Dawn could continue operating in this orbit, but it has already squeezed nearly as much out of its suite of sophisticated sensors as it can, and it soon would reach the point of diminishing returns. In addition, its lifetime here is now very limited. Although the hydrazine has lasted longer than expected, the gauge on the tank is dropping relentlessly as the robotic ship uses the propellant to counter the strong gravitational torque at this low altitude. Even if the two functioning reaction wheels continue to run correctly in hybrid control, the hydrazine would be exhausted early next year, and the mission would come to an immediate end. And given the premature death of the other two wheels, Dawn might not last even that long. If one more wheel fails, Dawn's remaining lifetime would be cut in half. At this point, how can we get the most out of Earth's deep-space ambassador?

Ceres
Dawn observed this tortuous landscape at 70 degrees north latitude on Feb. 4, 2016, from its current mapping orbit at an altitude of 240 miles (385 kilometers). The impact that formed the lower crater partially obliterated the older one above. As in the previous picture, sunlight comes from the right. Look carefully, especially in the newer crater, to see large boulders, which are bright on the right, as described in more detail here. You can also see streaks of bright material on the crater wall. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We have explained before that Dawn will never go closer to Ceres. There are several reasons. The rate at which hydrazine is consumed depends quite strongly on the altitude, so if the probe ventured lower, its lifetime would be significantly shorter. (Similarly, at higher altitude, it uses less hydrazine and so its lifetime would be longer.) Ceres has water (albeit mostly frozen, although perhaps some as liquid), energy (both from the distant sun and from radioactive elements incorporated when Ceres formed more than 4.5 billion years ago), and some of the other important ingredients for the development of life. We want to protect this astrobiologically interesting environment from the spacecraft's terrestrial contamination, so we cannot risk going low enough that it might crash, even long after the mission concludes. (And a controlled landing is not possible.) Also, at lower altitudes Dawn would orbit so fast that pictures and other measurements would be smeared, reducing the benefit of being closer. There are other reasons as well, but the bottom line is that this orbit is where Dawn draws its bottom line.

Ever creative, the team has found new ways to increase the mission's scientific productivity. Once again, the strategy involves changes never anticipated and that may be contrary to what your intuition would suggest. For more than two years, your correspondent has been emphasizing that this would be Dawn's final orbit. Now, on Sept. 2, Dawn will begin flying to a higher altitude.

The prospect of raising the orbit also raises several natural questions about what will happen in the coming months, including how, why and what kind of cake will be served at the team's celebration on Sept. 27 of the ninth anniversary of Dawn's launch. This month, let's look at how, and as the team refines its plans for the other key questions, we will discuss the answers in future Dawn Journals.

To gain altitude, Dawn will take advantage of its remarkable ion propulsion system. Ion propulsion has enabled many bold missions from Star Trek to Star Wars to NASA's unique expedition to orbit Vesta and Ceres, which would have been not simply difficult but impossible with conventional propulsion. And like the spaceships that in science fiction fly wherever they want to go, now Dawn will use its xenon ions to maneuver to an orbit it would not otherwise be able to reach. (Despite the similarity, there are some ways in which Dawn differs from the fictional ships: our craft uncompromisingly obeys all the laws of physics and carries relatively few systems designed to destroy other ships in battle.)

Dantu Crater
Dawn took this photo of peaks in the center of Dantu Crater on June 3, 2016, while orbiting at 240 miles (385 kilometers). We have seen other intriguing parts of this 78-mile (126-kilometer) crater before, both from this distance and from farther away (showing the entire crater). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To climb higher, Dawn will essentially reverse the spiral route it took down to its current orbit, much as it did when it ascended from Vesta. (There are some interesting technical differences in the nature of this trajectory from all of the other spirals. The design incorporates clever new ideas from Dawn's celestial navigators. But to the casual interplanetary observer, it will appear the same.) As with all of Dawn's activities, you can follow its progress upward with the mission status updates.

After five weeks of ion thrusting, looping higher and higher, the spacecraft will stop at about 910 miles (1,460 kilometers). Readers with eidetic memories (or who reread past Dawn Journals) may note that that is very close to the altitude of the third mapping orbit. However, the orientation of the orbit will be different. The spaceship will still circle in a polar orbit. It will travel over the north pole, then fly south above the face of Ceres lit by the sun. After it passes over the south pole, it will streak to the north over terrain hidden in the dark of night. But the plane of this orbit will be rotated from that of the third mapping orbit. The angle to the sun will be larger, so Dawn will pass over a different part of the sunlit hemisphere, gaining new perspectives on the extraterrestrial landscapes.

At its current low altitude, Dawn is now completing a truly extraordinary phase of its exploration of Ceres. But there is still much more to come, with new scientific investigations, new discoveries and new adventures at higher altitudes. Now that we have seen a little of the how, be sure to look for upcoming Dawn Journals to learn more about the why (and about the anniversary cake).

Dawn is 240 miles (385 kilometers) from Ceres. It is also 2.24 AU (208 million miles, or 335 million kilometers) from Earth, or 855 times as far as the moon and 2.22 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 37 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT August 31, 2016

TAGS: CERES, DAWN

  • Marc Rayman
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Dawn trajectory illustration

Humankind dispatched Dawn on an extraordinary extraterrestrial expedition in 2007.

It visited Mars briefly in 2009 and spent 14 months orbiting protoplanet Vesta in 2011-2012, revealing fascinating details of that uncharted, alien world. After traveling for another two and a half years through the interplanetary void, the spacecraft arrived at Ceres in March 2015. It has now conducted an outstandingly successful exploration of the only dwarf planet in the inner solar system. Dawn greatly surpassed its objectives at both Vesta and Ceres, accomplishing well more than was envisioned when NASA decided to undertake this ambitious mission. Having realized its raison d'être, the official end of Dawn's prime mission was June 30.

Following the conclusion of the prime mission, the adventurer began its "extended mission" of performing more Ceres observations without missing a beat. We described in April some of what Dawn can do as it continues investigating many of the mysteries there. Dawn's extension allows for even better measurements with the gamma ray and neutron detector of the nuclear radiation emanating from Ceres. This is like taking a longer exposure of the very faint nuclear glow, yielding a brighter, sharper picture that reveals more about the atomic constituents down to about a yard (meter) underground. The spacecraft is taking more stereo photos, continuing to improve the topographical map it created from four times higher. Scientists also are taking advantage of this opportunity to study more geological features with the visible and infrared mapping spectrometers, providing important insight into Ceres' mineralogical inventory.

Dawn has already made extraordinary discoveries at Ceres, some of which we have described in recent months. But on a dwarf planet of 1.1 million square miles (2.8 million square kilometers), there is a great deal to see. That, after all, is the benefit of being in orbit, lingering long enough to make a richly detailed portrait of the exotic expanse. Indeed, Ceres has 36 percent of the land area of the contiguous United States, or the combined land areas of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. In such a vast territory, there are innumerable mysteries to unravel. And that is only the surface.

Dawn also is continuing its studies of the gravitational field to discover more about the interior structure of the largest body between Mars and Jupiter.

Occator Crater
Dawn captured this view inside Occator Crater on March 26 from an altitude of 240 miles (385 kilometers). We have explained that the bright areas are salts, which reflect much more sunlight than typical materials on Ceres. Recent analysis of Dawn's infrared spectra shows the salt is mostly sodium carbonate. (This is the brightest region on Ceres, but you can see another of the many reflective deposits in one of the pictures below of two adjoining craters.) Occator Crater formed 80 million years ago. Another part of this geologically young crater is shown immediately below. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Occator Crater
Dawn observed the edge of Occator Crater from an altitude of 240 miles (385 kilometers) on January 26. The crater is 57 miles (92 kilometers) in diameter. Note the detail on the steep walls and the fractures both inside and outside the crater that generally are parallel to the rim. Look carefully to spot some very large boulders (as described here), especially near the top center and left. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In the coming months we will discuss other intriguing activities and how Dawn will make measurements never even considered before. But for now, let's look at how this extension came about.

As readers of the Dawn Journals know (and as you will be reminded below), there has been very good reason in recent years to believe the spacecraft would not operate beyond the end of its prime mission. However, the veteran explorer is in very good health. It is one of Earth's most experienced and capable ambassadors to the cosmos, we want to squeeze as much out of this mission as we can. Ever resourceful, the Dawn team recognized in March 2016 that the probe had the capability to do yet more and decided to give NASA Headquarters a unique choice: remain at Ceres (as always expected) or go elsewhere.

It is worth pondering how extraordinary this is. Most spacecraft can only make minor adjustments to their trajectories, so at the end of their prime missions, they generally go wherever they were already headed. If a spacecraft is in orbit around some planetary body, it remains in orbit. If a spacecraft is not in orbit, having previously flown past one or more bodies that orbit the sun, its course is largely determined by the targeting for the last encounter. A planet's gravity may have redirected it, but otherwise its propulsion system has to do the work, and that usually can produce only a tiny change in direction. If a spacecraft is not already in orbit around a planetary body, it won't be able to enter orbit.

Dawn is different. With its uniquely capable ion propulsion system, Dawn is the only spacecraft ever to travel to a distant destination, orbit it, later break out of orbit, then travel to another faraway destination, and orbit it. And even while in orbit around Vesta and Ceres, Dawn maneuvered extensively, optimizing its orbits for its scientific investigations. And yet this remarkable ship can do still more. It has the capability to leave its second destination and continue its travels.

Dawn's brilliant and creative navigators analyzed possible missions to more than 68,000 known objects. That alone is a nice illustration of the powerful potential.

The project team very quickly narrowed the list to the most interesting body Dawn could reach after leaving Ceres, a large asteroid named Adeona. That mission offered the best alternative to further studies of the dwarf planet.

Ceres
Dawn had this view of two adjoining craters on Ceres on March 26 from an altitude of 240 miles (385 kilometers). Reflective material, most likely salt left after ice sublimated (as in Occator Crater, shown above), is easily visible. Look carefully inside both craters to see many large boulders (light on the right and dark on the left). Also note what appears to be the remnants of material that flowed to near the middle of the larger crater. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Ceres
Dawn observed this pair of craters on January 25 from an altitude of 240 miles (385 kilometers). The upper crater is the younger of the two, as indicated by its sharper features, which have not been eroded as much by the gradual but persistent rain of interplanetary debris falling on Ceres. (In May we discussed how scientists quantify the ages, although the dates these craters formed have not yet been computed.) The wall where the craters meet has partially collapsed into the older one. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

But how to decide between these two attractive possibilities? Some members of the Dawn team preferred continuing the exploration of Ceres and others preferred going to Adeona. Similarly, some people prefer cake and some prefer pie. (That's not a perfect example, because it's obvious cake is better, but you get the idea.)

NASA thoroughly evaluated the scientific potential and other aspects of the options. Part of this was an assessment by an independent group of esteemed scientists. The conclusion was that either would be valuable but that studying Ceres further was preferable.

From the perspective of your correspondent -- passionate about space exploration since the age of four, a professional scientist (as well as a scientist at heart), an engineer and a taxpayer -- this is a wonderful outcome. How could one want anything other than such a well-considered decision?

But how is it even possible that the team could have offered to NASA the option of visiting Adeona for the extended mission? We have emphasized for several years that Ceres would be Dawn's final home. If you had asked even as recently as early this year whether the spacecraft could leave Ceres (and many of you did), we would have responded that such a prospect was unrealistic and inconceivable (and we did). We have described in great detail how the failure of two of Dawn's four reaction wheels was so serious that it was only with heroic effort that the distant robot was able to complete its original assignments. We have explained repeatedly that the spacecraft will soon expend the last of its hydrazine propellant, then immediately lose the ability to point its solar arrays at the sun, its antenna at Earth, its scientific sensors at Ceres or its ion engine in the direction needed to fly elsewhere. Why the change now, and how could Dawn operate for a multiyear journey?

We have discussed in recent months how remarkably well the flight team has done in conserving hydrazine, significantly exceeding any reasonable expectations and thereby extending Dawn's functional lifetime. Moreover, mission controllers know that the probe consumes less hydrazine at higher altitudes. Contrary to many people's notions, the dwarf planet's gravity is appreciable, and operating so close to it requires a very high rate of hydrazine consumption. Dawn is circling only 240 miles (385 kilometers) above Ceres, closer than the International Space Station is to Earth. But during the long deep-space journey to Adeona, Dawn would use the precious propellant much more sparingly. So despite the loss of the two reaction wheels, under the expert guidance of its terrestrial colleagues, the ship could set sail once again for a new and distant land beyond the horizon.

Ceres
Dawn had this scenic view on June 13 while orbiting 240 miles (385 kilometers) above Ceres. This is one of the occasional photographs of the landscape reaching to the horizon, in this case near Kirnis Crater. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Isn't it incredibly cool that humankind has the capability to fire up the ion engine on a distant interplanetary spaceship and pilot it out of orbit around a dwarf planet to fly more than halfway around the sun on a bold expedition of 900 million miles (1.5 billion kilometers) to investigate a huge asteroid? (Hint: the answer is yes.)

Exciting as such a voyage might seem, it is gratifying that a thoughtful, rationale decision was made that yields an even better outcome. Rather than terminate the present mission after it has exceeded all of its original objectives, and rather than embark on that new mission, the best possible use of Dawn is to do what it is doing right now: extracting secrets from dwarf planet Ceres. And now we can look forward to more, as Dawn pursues new objectives. As the extended mission progresses, we will describe marvelous new findings from the rich trove of data Dawn is returning, and we will see how the team plans to take advantage of this unique opportunity to learn more about the nature of the solar system.

If you share in the passion for exploration, if you thrill to new discoveries and new knowledge or even if you just want to see how many more silly Dawn Journal greetings your correspondent can concoct, stay onboard as Dawn's adventure at Ceres continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 2.69 AU (250 million miles, or 403 million kilometers) from Earth, or 1,090 times as far as the moon and 2.65 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 45 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT July 27, 2016

TAGS: CERES, DAWN

  • Marc Rayman
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Ezinu Crater

Dawn is continuing to record the extraordinary sights on dwarf planet Ceres. The experienced explorer is closer to the alien world than the International Space Station is to Earth.

Dawn has completed more than 1,000 orbital revolutions since entering into Ceres' gentle but firm gravitational grip in March 2015. The probe is healthy and performing its ambitious assignments impeccably. In the last few months, we have described how Dawn has greatly exceeded all of its original objectives at Ceres and the excellent progress it has been making in collecting bonus data. On schedule on May 25, the spacecraft completed the mapping campaign it began on April 11, in which it took photographs with the camera pointed to the left and forward as it circled Ceres. Now it is looking to the right and forward to get another stereo view.

In January we mentioned that, having already acquired far more measurements with the visible and infrared mapping spectrometer than anticipated, scientists were devoting further observations to infrared rather than visible. Now Dawn is operating both spectrometers again. Having seen much more of Ceres in the infrared from this low altitude than planned, mission controllers now can afford to allocate some of the spacecraft's data storage and interplanetary radio transmissions to visible spectra in exchange for limiting the infrared to a few select targets. In addition, a device in the infrared spectrometer that lowers the sensor's temperature to -307 degrees Fahrenheit (-188 degrees Celsius) is showing signs of age. (We saw here that the sensor can detect heat. So to avoid interference from its own heat, it needs to be cooled.) Its symptoms are not a surprise, given that the instrument has acquired far, far more data at Vesta and Ceres than it was designed for. It is continuing to function quite productively, but now its use is being curtailed.

One of the mission's objectives was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. Dawn has now photographed nearly the entirety (99.9 percent) with a resolution of 120 feet (35 meters) per pixel. The adventurer has shown us 25 percent more terrain than planned with 5.7 times the clarity. We can see detail 830 times sharper than the Hubble Space Telescope revealed.

What is the value of that much detail? The more detailed the portrait, the better understanding geologists can obtain. Imagine the difference (not only visually but also emotionally and socially) between seeing a person at the opposite end of a soccer field and seeing them from five inches (12 centimeters) away.

The pictures speak quite eloquently (and succinctly) for themselves, but let's take a look at one of the many uses of these sharp photographs: determining the age of geological features.

In December, we gave an approximate age of 80 million years for Occator Crater, site of the famous "bright spots" (or famously bright spots). It takes more than an experienced geological eye to estimate such an age.

Occator Crater
Occator Crater is shown in this mosaic of photos Dawn took at its lowest altitude of 240 miles (385 kilometers). The crater is 57 miles (92 kilometers) in diameter. Go to the full image to see exquisite details of the bright areas as well as fractures in the crater floor and other intriguing features. Note how few craters are within Occator or the area around it. Scientists can translate the number and size of craters into an age. From pictures taken at higher altitudes, they estimate Occator is 80 million years old, as explained below. That age will be refined with these sharper pictures, which reveal smaller craters. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Now don't forget that we are trying to ascertain the age, but we are going to get there on a long and winding path, mostly because it's an opportunity to touch on some fun and interesting topics.

To begin, we go back in time, not quite 80 million years, to the Apollo program. Astronauts returned from the moon with many treasures, including 842 pounds (382 kilograms) of lunar material collected on six missions. In addition, three Soviet robotic Luna spacecraft came back with a total of 11 ounces (0.3 kilograms).

Earth's total inventory of lunar samples is larger. By comparing the chemical composition of that material with a great many meteorites, scientists have identified nearly 120 pounds (54 kilograms) of meteorites that were blasted from the moon by asteroid impacts and then landed on our planet.

Other meteorites are known to have originated on Mars. The principal method by which that connection was made was comparison of gasses trapped in the meteorites with the known constituents of the Martian atmosphere as measured by the two Viking spacecraft that landed there 40 years ago. Scientists thus have 276 pounds (125 kilograms) of Martian material.

Of course, unlike the Apollo and Luna samples, the lunar and Martian meteorites were selected for us by nature's randomness from arbitrary locations that are not easy to determine.

The moon and Mars are two of only three (extant) extraterrestrial bodies that are clearly established as the source of specific meteorites. The third is Vesta, the fascinating protoplanet Dawn explored in 2011-2012. That world is farther away even than Mars, and yet we have 3,090 pounds (1,402 kilograms) from Vesta, or more than 11 times as much as from the red planet and more than three times as much as from the moon. We reflected on these meteorites during our travel from Vesta to Ceres.

It is thanks to Dawn's detailed measurements of the composition of Vesta that scientists were able to clinch the connection with the meteorites that were under study in terrestrial laboratories. The impact of an asteroid perhaps 20 to 30 miles (30 to 50 kilometers) in diameter more than one billion years ago excavated Vesta's Rheasilvia Crater. It left behind a yawning basin more than 300 miles (500 kilometers) across, a mountain more than twice the height of Mt. Everest, and a network of about 90 canyons with dimensions rivaling those of the Grand Canyon. And it launched a tremendous amount of material into space. Some of it settled back onto Vesta, resurfacing much of the southern hemisphere, but some of it departed with so much energy that it escaped Vesta's gravitational hold. Some of the biggest pieces liberated by that tremendous impact are now visible as small asteroids known as vestoids. And some of the small pieces eventually made their way to the part of the solar system where many of our readers (perhaps including you) reside. After Earth's gravity took hold of any of those wandering interplanetary rocks and pulled them in, they became meteors upon entering the atmosphere, meteorites upon hitting the ground, and keys to studying the second largest object in the main asteroid belt upon entering laboratories. One esteemed scientist on the Dawn team opined that with Dawn's detailed data and our Vestan samples, Vesta joined the ranks of the moon and Mars as the only extraterrestrial bodies that have been geologically explored in a rigorous way.

With so many meteorites from Vesta, why have we not linked any to Ceres? Is it because the rocks didn't get blasted away in the first place, or they didn't make it to the vicinity of Earth or to the ground, or we have not recognized that they are in our collections? While there are some ideas, the answer is not clear. For that matter, although Vesta and Ceres are the two largest residents of the main asteroid belt, why have we not tied meteorites to any of the smaller but still sizable bodies there? We will return to this question in a future Dawn Journal, but for now, let's get back to the question of how Dawn's pictures help with measuring the ages of features on Ceres.

Crater on Ceres
Dawn took this picture on March 22 from an altitude of 240 miles (385 kilometers). The impact that formed the crater in the upper left deposited material outside the crater, partially covering the smaller craters that were already there. The area on the lower right of the picture, including the other large crater in this scene, has many more small craters and so must be older. Sunlight in this photograph is coming from the right, so all the craters are dark on the right side where their walls descend into shadow. The crater walls on the left face the sun and so are illuminated. Look closely around the young crater and on its floor to see many very small features with the opposite lighting: they are bright on the right and dark on the left. Unlike all the craters, they are not depressions but rather are very large boulders, catching sunlight on the right side. (Each pixel in this picture is 120 feet, or 35 meters.) The tremendous punch that excavated the young crater must have produced these boulders. The Dawn project does not recommend doing the same thing at home. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists have measured the relative abundance of different atomic species in the Apollo and Luna samples from different locations. Elements with known radioactive decay rates serve as clocks, providing a record of how old a sample is. This process enabled scientists to pin down the ages of many craters on the moon, and from that, they developed a history of the rate at which craters of different sizes formed.

During some periods in the moon's history, it was pelted with more interplanetary debris, forming more craters, than at other times. This uneven history is a reflection of solar-system-wide events. For example, it seems that the giant planets of the outer solar system jockeyed for their orbital positions around the sun about four billion years ago. Their gravitational jostling over the course of about 300 million years may have sent a flurry of material into the inner solar system, where the moon recorded the bombardments.

The moon lives at one astronomical unit (1 AU, which is 93 million miles or 150 million kilometers) from the sun (because that's where Earth is). Scientists can extrapolate the cratering history the moon experienced to other locations in the solar system, so they can calculate what other bodies should have been subjected to. Ceres lives between 2.6 and 3.0 AU from the sun.

Azacca Crater
Dawn observed this scene on March 28 from an altitude of 240 miles (385 kilometers). The prominent crater on the left lies on the western rim of Azacca Crater, which goes vertically through the center of the picture. (Azacca is a Haitian god of agriculture.) With a diameter of 31 miles (50 kilometers) Azacca, is too large to fit in a single picture from this low altitude. Note the many deposits of bright material, which is likely some kind of salt. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists count the number and size of craters in an area of interest, like inside Occator Crater and on the blanket of ejected material surrounding it. (See the picture above.) With their mathematical description of how many impacts should have occurred over time, they can estimate how long the surface has been exposed and accumulating craters. Although the ages have not been computed yet, compare the third and fourth pictures presented in April for a clear illustration of areas that are of very different ages.

The method of determining the age involves many subtleties we did not touch on here, and there are many complicating factors that limit the accuracy. But the dating results are improved substantially by including smaller craters in the count.

It is readily apparent in pictures of Ceres, Vesta, the moon, and elsewhere that small craters are more prevalent than large ones. There has simply been more small stuff than large stuff flying around in the solar system and crashing into surfaces to make craters. There are more bits like sand grains than pebbles, more pebbles than boulders, more small boulders than big boulders, etc.

Extending Dawn's photographic documentation of the Cerean landscapes to finer resolution provides the means to develop a better census of the population of craters, yielding a better measure of the age.

Dawn's bonus observations thus give us not only a sharper view of the dwarf planet beneath it today but also a more accurate view of the mysterious world's past. As this extraordinary journey through space and time continues, next month, we will look to the future.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.42 AU (318 million miles, or 512 million kilometers) from Earth, or 1,400 times as far as the moon and 3.38 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.

Dr. Marc D. Rayman
3:30 p.m. PDT May 31, 2016

TAGS: CERES, DAWN, EZINU CRATER

  • Marc Rayman
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Haulani Crater in color

Dear Glutdawnous Readers,
The distant dwarf planet that Dawn is circling is full of mystery and yet growing ever more familiar.

Ceres, which only last year was hardly more than a fuzzy blob against the stars, is now a richly detailed world, and our portrait grows more elaborate every day. Having greatly surpassed all of its original objectives, the reliable explorer is gathering still more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.

One of the major objectives of the mission was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet (35 meters) per pixel.


Dawn captured this picture of Haulani crater in cycle 6 of its third mapping orbit at 915 miles (1,470 kilometers). The crater is shown in a new false-color version above. Its well-defined shape indicates it is relatively young, the impact that formed it having occurred in recent geological times. It displays a substantial amount of bright material, which scientists have identified as some form of salt. The same crater as viewed by Dawn from three times higher altitude is here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

This example of Dawn's extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.

Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.


Dawn captured this view of Oxo Crater on Jan. 16 from an altitude of 240 miles (385 kilometers). Although it is a modest six miles (10 kilometers) across, it is a particularly interesting crater. This is the only location (so far) on Ceres where Dawn has clearly detected water. Oxo is the second brightest area on Ceres. Only Occator Crater is brighter. Oxo also displays a uniquely large "slump" in its rim, where a mass of material has dropped below the surface. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons. We have discussed before how the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres' nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.

In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength of the dwarf planet's gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.


Dawn took this picture on Feb. 8 at an altitude of 240 miles (385 kilometers). Prominent in the center is part of a crater wall, which shows many scars from subsequent impacts, indicating it is old. Two sizable younger craters with bright material, which is likely some kind of salt, are evident inside the larger crater. Compare the number and size of craters in this scene with those in the younger scene below showing an area of the same size. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn has dramatically overachieved in acquiring spectra at both visible and infrared wavelengths. We have previously delved into how these measurements reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January that VIR has such a narrow view that it will never see all of Ceres from this close, so it is programmed to observe features that have caught scientists' interest based on the broad coverage from higher altitudes.


Dawn took this picture on Feb. 16 (eight days after the picture above) at an altitude of 240 miles (385 kilometers). It shows a region northwest of Occator Crater, site of the famous bright region (which may become one of the most popular tourist destinations on Ceres). (You can locate this area in the upper right of the mosaic shown last month.) Compare the number and size of craters in this scene with those in the older scene above showing an area of the same size. There are fewer craters here, because the material ejected from the impact that excavated Occator resurfaced the area nearby, erasing the craters that had formed earlier. Because Occator is relatively young (perhaps 80 million years old), there has not been enough time for as many new craters to form as in most other areas on Ceres, including the one shown in the previous picture, that have been exposed to pelting from interplanetary debris for much longer. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn's remarkable success at Ceres was not a foregone conclusion. Of course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices aren’t actually lost. We know precisely where they are. But given that one stopped functioning in 2010 and the other in 2012, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Without three of these units to control its orientation in space, the robot has relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.

In the meantime, Dawn is continuing its intensive campaign to reveal the dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would have been impossible without ion propulsion.) The spacecraft provided us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.

For an interplanetary traveler, terrestrial days have little meaning. They are merely a memory of how long a faraway planet takes to turn on its axis. Dawn left that planet long ago, and as one of Earth's ambassadors to the cosmos, it is an inhabitant of deep space. But for those who keep track of its progress yet are still tied to Earth, on May 3 the journey will be pi thousand days long. (And for our nerdier friends and selves, it will be shortly after 6:47 p.m. PDT.)

By any measure, Dawn has already accomplished an extraordinary mission, and there is more to look forward to as its ambitious expedition continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346 million miles, or 558 million kilometers) from Earth, or 1,455 times as far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.

TAGS: CERES, DAWN, MISSION, SPACECRAFT, VESTA, DWARF PLANET

  • Marc Rayman
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Occator Crater

Dear Resplendawnt Readers,
Scientists are still working on refining their understanding of this striking region.

One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of only 240 miles (385 kilometers).

Even as we marvel at intriguing pictures and other discoveries, scientists are still in the early stages of putting together the pieces of the big puzzle of how (and where) Ceres formed, what its subsequent history has been, what geological processes are still occurring on this alien world and what all that reveals about the solar system.

For many readers who have not visited Ceres on their own, Occator Crater is the most mysterious and captivating feature. (To resolve the mystery of how to pronounce it, listen to the animation below.) As Dawn peered ahead at its destination in the beginning of 2015, the interplanetary traveler observed what appeared to be a bright spot, a shining beacon guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation by venturing in for a closer look, entering into Ceres' gravitational embrace. The latest pictures are one thousand times sharper than those early views. What was not so long ago a single bright spot has now come into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.


Dawn took these pictures of Occator Crater on March 16. This is the most reflective area on Ceres. The exposure was optimized for the brightest part of the scene, revealing details that were indiscernible in longer exposures and in photos from higher altitudes. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.

While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.


The high resolution photo of the central feature of Occator Crater is combined here with color data from the third mapping orbit. With enhanced color to highlight subtle variations, this illustrates the red tinge that we described in December. (The scene would not look this colorful to your eye, even if you and your eye were fortunate enough to be in a position to see it.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI/LPI

These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.

We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.


This is an accelerated excerpt from this complete animation showing Dawn's accumulated photographic coverage of Ceres during the lowest altitude mapping campaign from December 16 to March 11. To ensure that it can see all latitudes, Dawn travels in a polar orbit, flying from the north pole to the south pole over the illuminated hemisphere and back to the north over the nighttime hemisphere. Each orbital revolution takes 5.4 hours. Meanwhile, Ceres rotates from east to west, completing one Cerean day in just over nine hours. The combined motion causes the spacecraft's path over the landscape to follow these graceful curves. Consecutive orbits pass over widely separated regions because Ceres continues to rotate beneath Dawn while the spaceship glides over the hidden terrain of the night side. The swaths that don't fit the typical pattern are the extra pictures Dawn took as it turned away from the scenery below it, as described in January. The spacecraft does not take pictures on every orbit, because sometimes it performs other functions (such as pointing its main antenna to Earth), so that causes gaps that are filled in later. Note that the center of the popular Occator Crater (slightly above and to the right of center), just happened to be one of the last places to be imaged as Dawn progressively built its high-resolution map. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.

We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn's neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron's energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.

GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.


This map, centered over the northern hemisphere, uses color to depict the rate at which GRaND detected neutrons of a particular energy from an altitude of 240 miles (385 kilometers). (The underlying image of Ceres is based on pictures Dawn took with its camera at a higher altitude.) Red indicates more neutrons than blue. The relative deficiency of neutrons near the north pole (and near the south pole, although not shown here) is because hydrogen is more abundant there. The hydrogen atoms rob the neutrons of energy, so GRaND does not find as many at the special energy used for this study. (It does find them at other energies.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.

Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.

And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.

At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn's first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.

Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.

As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels, which are used for controlling the ship's orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.

As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.


This is an excerpt from an animation showing some of the highlights of Dawn's exploration of Ceres so far, including Occator and Oxo craters, both of which are discussed above. You can also hear your correspondent's pronunciation of the names of those and other features on Ceres. Full animation and transcript. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)

The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn's charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.

TAGS: CERES, DAWN, MISSION, SPACECRAFT, VESTA, DWARF PLANET

  • Marc Rayman
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Sekhet Crater

Dear Indawnbitably Successful Readers,

A story of intense curiosity about the cosmos, passionate perseverance and bold ingenuity, a story more than two centuries in the making, has reached an extraordinary point. It begins with the discovery of dwarf planet Ceres in 1801 (129 years before its sibling Pluto; each was designated a planet for a time). Protoplanet Vesta was discovered in 1807. Following 200 years of telescopic observations, Dawn's daring mission was to explore these two uncharted worlds, the largest, most massive residents of the main asteroid belt between Mars and Jupiter. And now, as of February 2016, the spacecraft has accomplished all of the objectives that NASA defined for it in 2004, even before construction began (and before the very first Dawn Journal, nearly a decade ago).

More than eight years after leaving its erstwhile planetary home behind for an ambitious deep space adventure, Dawn has now collected all of the data originally planned. Indeed, even prior to this third intercalary day of its expedition, the probe had already actually sent back a great deal more data for all investigations, significantly exceeding not only the original goals but also new ones added after the ship had set sail on the interplanetary seas. While scientists have a great deal of work still ahead to translate the bounty of data into knowledge, which is the greatest joy of science, the spacecraft can continue its work with the satisfaction that it has fulfilled its purpose and achieved an outstandingly successful mission.

Dawn took this picture of the rim of Datan crater on Jan. 7 in its fourth mapping orbit at 240 miles (385 kilometers). It flew over the same location on Oct. 2, 2015, in its third mapping orbit at 915 miles (1,470 kilometers). To see the improvement in detail, compare this with the earlier image (presented fully in November but reproduced in part below to make comparison easier). The bright material to the right of the crater rim here may help you locate this area within the wider image. Full image and captionImage credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA



Dawn took this picture in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit. The prominent triplet of overlapping craters nicely displays relative ages, which are apparent by which ones affect others and hence which ones formed later. The largest crater, Geshtin, is 48 miles (77 kilometers) across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian goddess of the vine.) A subsequent impact that excavated Datan crater, which is 37 miles (60 kilometers) in diameter, obliterated a large section of Geshtin's rim and made its own crater wall in Geshtin's interior. (Datan is one of the Polish gods who protect the fields but apparently not this crater.) Still later, Datan itself was the victim of a sizable impact on its rim (although not large enough to have merited an approved name this early in the geological studies of Ceres). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, which would have been impossible without its advanced ion propulsion system. It is the only spacecraft ever to orbit an object in the main asteroid belt. It is also the only spacecraft ever to orbit massive bodies (apart from the sun and Earth) that had not been visited first by a flyby spacecraft to characterize the gravity and other properties. (By the way, Ceres is one of eight solar system bodies that operating spacecraft are orbiting now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko, Mars and Saturn.)

Now in its fourth and final mapping orbit at Ceres, at an altitude of 240 miles (385 kilometers), Dawn is closer to the exotic terrain than the International Space Station is to Earth. The benefit of being in orbit is that the probe can linger rather than take only a brief look during a fast flyby. Even though Dawn has met its full list of objectives at Ceres, it continues to return new, valuable pictures and other measurements to provide even greater insight into this relict from the dawn of the solar system. For example, it is acquiring more nuclear spectra with its gamma ray and neutron detector, sharpening its picture of some atomic elements on Ceres. In addition, taking advantage of its unique vantage point, Dawn is collecting more infrared spectra of locations that are of special interest and soon will also take color photos and stereo photos (as it did in the third mapping orbit) of selected areas.

Dawn has completed more than 600 revolutions since taking up residence one year ago. The first few orbits took several weeks each, but as the spacecraft descended and Ceres' gravitational embrace grew more firm, its orbital velocity increased and the orbital period decreased. Now circling in less than five and a half hours, Dawn has made 370 orbits since reaching this altitude on Dec. 7.


On Jan. 1, Dawn observed this scene at 78 degrees south latitude. This deep in the southern hemisphere, the sun is low on the horizon (it is three degrees north of the equator). The long shadows emphasize the topography in this densely cratered (and therefore old) region. Landslides are evident in the large crater wall on the left. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The pace of observations here is higher than in the previous mapping orbits, where the orbital periods were longer. The spacecraft flies over the landscape faster now, and being closer to the ground, its instruments discern much more detail but capture a smaller area. Mission controllers have developed intricate plans for observing Ceres, but those plans depend on the spacecraft being at the right place at the right time. As we will see below, however, sometimes it may not be.

Suppose, for example, the intent is to observe a particular feature, perhaps the bright center of Occator crater, the lonely, towering mountain Ahuna Mons, the fractures in Dantu crater or artificial structures that definitively prove the existence of extraterrestrial intelligence, utterly transforming our understanding of the cosmos and shattering our naive perspectives on life in the universe. Trajectory analysis indicates when Dawn will fly over the designated location, and engineers will program it to take pictures or infrared spectra at that time. They will also include some margin, so they may program it to start 10 minutes before and end 10 minutes after. But they can't afford to put in too much margin. Data storage on the spacecraft is limited, so other geological features could not be observed. Also, transmitting data to Earth requires pointing the main antenna at that distant planet instead of pointing sensors at Ceres, so it would be unwise to collect much more than is necessary.

Even if devoting additional time (and data) to trying to observe the desired place were feasible, it wouldn't necessarily solve the problem. Dawn travels in a polar orbit, which is the only way to ensure that it passes over all latitudes. While Dawn soars from north to south over the sunlit hemisphere making its observations, the dwarf planet itself rotates on its axis, so the ground moves from east to west. If the spacecraft arrives at the planned orbital location a little early or a little late, the feature of interest may not even be beneath it but rather could be too far east or west, out of view of the instruments. In that case, increasing the duration of the observation period doesn't help.

All of that is why, as we saw last month, it requires more pictures to fully map Ceres than you might expect. Many pictures may have to be taken in order to fill in gaps, and quite a few of the pictures overlap with others. Nevertheless, Dawn has done an excellent job. The spacecraft has photographed 99.6 percent of the dwarf planet from this low altitude. (If you aren't regularly visiting the image gallery, you are missing out on some truly out-of-this-world scenes.)

Dawn LAMO Image 33

Dawn photographed this scene on Jan. 4 as it was looking toward the horizon (as explained last month). Fluusa, the large crater from the center to the upper left is 37 miles (60 kilometers) in diameter. (Fluusa was a goddess of flowers for the Oscans of southern Italy who honored her to make plants bloom and bear fruit.) Its degraded features and dense cratering show it is old. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The flight team devises very detailed plans that tell the spacecraft what to do every second, including where to point and what data to collect with each sensor. When the observation plans are developed, they are checked and double-checked. Then they are translated into the appropriate software that the robotic ship will understand, and these instructions are checked and double-checked. That is integrated with all the other software that will be beamed to the spacecraft covering the same period of time, any conflicts are resolved and then the final version is checked and, well, you know.

This process is very involved, and it is usually well over a month between the formulation and the execution of the plan. During that time, Dawn's orbit can deviate slightly from the expert navigators' mathematical predictions, preventing the spacecraft from flying over the desired targets. There are several reasons the actual orbit may differ from the orbit used for developing the plan. (We have seen related examples of this, including as Dawn approached Mars, when it orbited Vesta and when it spiraled from one mapping orbit to another.) Let's briefly consider two.

One reason is that we do not have perfect knowledge of the variations in the strength of Ceres' gravitational pull from one location to another. We have discussed before that measuring these tiny irregularities in the gravity field provides insight into the distribution of mass within the dwarf planet that gives rise to them. The team has mapped the hills and valleys of the field quite well and even better than expected. Still, the remaining small uncertainty can lead to slight differences between what navigators calculate Dawn's motion will be and what its actual motion will be as it is buffeted by the gravitational currents.

A second source of discrepancy is that Dawn's own activities distort its orbit. Every time the reaction control system expels a tiny burst of hydrazine to control the spacecraft's orientation, keeping it pointed at its target, the force not only affects the orientation but also nudges the probe in its orbit, slowing it down or speeding it up very slightly. It's up to the spacecraft to decide exactly when to make these small adjustments, and it is not possible for controllers to predict their timing. (In a similar way, when you are driving, you occasionally move the steering wheel to keep going the direction you want, even if is straight ahead. It would be impossible to forecast each tiny movement, because they all depend on what has already happened plus the exact conditions at the moment.) The details of the reaction control system activity also depend on the use of the novel hybrid control scheme, which the joint Orbital/JPL team developed because of the failure of two of the spacecraft's four reaction wheels. The effect of each small firing of hydrazine is very small, but they can add up.

Dawn LAMO Image 20

Dawn had this view of two unnamed craters on Jan. 1. The craters are about 10 miles (16 kilometers) and 3 miles (5 kilometers) in diameter. The distinct features show these are relatively young craters, not yet degraded by subsequent impacts or geological processes intrinsic to Ceres. The lighting in the craters shows that the sun is to the right, illuminating the left side of the depressions and missing the right side. Click on the image (or follow the link to the full image) and look carefully inside and around the larger crater. There are many small features that are light on the right and dark on the left. Therefore, they aren't depressions like these two craters. Rather, they rise up, catching the light as it comes in from the right, and their left sides are in shadow. These are large blocks from the impact that excavated the crater. Each pixel in this picture is 120 feet (35 meters). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It took about a month in this mapping orbit to discover many of the subtleties of the gravity field and gain experience with how hybrid control affects the orbit. But even before descending to this altitude, the operations team understood the nature of these effects and was well prepared to deal with them.

They devised several strategies, all of which are being used to good effect. One of the ways to account for Dawn's actual orbit differing from its planned orbit is simply to change the orbit. Simply? Well, not really. It turns out to that to analyze the orbit and then maneuver to correct it in a timely way is a surprisingly complicated process, but, come to think of it, what isn't complicated when flying a spaceship around a distant, alien world? Nevertheless, every three weeks, the flight team makes a careful assessment of the orbit and determines whether a small refinement with the ion propulsion system is in order. For technical reasons, if maneuvers are needed, they will be executed in pairs, so mission planners have scheduled two windows (each 12 hours long and separated by eight days) about every 23 days.

Adjustments to resynchronize the actual orbit with the predicted orbit that formed the basis of the exploration plan are known as “orbit maintenance maneuvers.” Succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as if it were a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)

For both Vesta and Ceres, trajectory analyses long in advance determined that OMMs would not be needed in the higher orbits, so no windows were included in those schedules. There have been three OMM opportunities since arriving at the lowest altitude above Ceres, but only the first was needed. Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously efficient ion engine. The orbit was good enough the next two times that OMMs were deemed unnecessary. It is certain that some future OMMs will be required. Your faithful correspondent provides frequent (and uncharacteristically concise) reports on Dawn's day-to-day activities, including OMMs.

By the end of the Jan. 8 OMM, Dawn's ion propulsion system had accumulated 2,019 days of operation in space, more than 5.5 years. During that time, the effective change in speed was 24,600 mph (39,600 kilometers per hour). (We have discussed in detail that this is not Dawn's current speed but rather the amount by which the ion engines have changed it.) This is uniquely high for a spacecraft to accomplish with its own propulsion system and validates our description of ion propulsion as delivering acceleration with patience. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

The effect of Dawn's gentle ion thrusting during its mission has been nearly the same as that of the entire Delta II 7925H-9.5 rocket, with its nine external rocket engines, first stage, second stage and third stage. To get started on its interplanetary adventure, Dawn's rocket boosted it from Cape Canaveral to out of Earth orbit with only four percent higher velocity than Dawn subsequently added on its own with its ion engines.

As Dawn and Earth follow their own independent orbits around the sun (Dawn's now tied permanently to its gravitational master, Ceres), next month they will reach their greatest separation of the entire mission. On March 4 (about one Earth year after Ceres took hold of Dawn), on opposite sides of the solar system, they will be 3.95278 AU (367.434 million miles, or 591.328 million kilometers) from each other. (For those of you with full schedules, note that the maximum separation will be 5:40 a.m. PST.) They won't be this far apart again until Feb. 6, 2025, long after Dawn has ceased operating (as discussed below). The figure below depicts the arrangement next month.

March Geometry

Earth's and Ceres' orbits will bring them to their maximum separation on March 4. Earth's orbit is shown in green and Ceres' is in purple. Dawn's interplanetary trajectory is in blue. Compare this figure with the ones depicting Dawn and Earth on opposite sides of the sun in December 2014, Dawn equidistant from Earth and the sun in April 2015, and Dawn and Earth at their minimum separation in July 2015. Also note that Earth has completed one full loop around the sun in the year since March 2015, when Dawn arrived at Ceres. During the same period, Ceres, traveling in a higher heliocentric orbit, has completed only about a fifth of a revolution. Credit: NASA/JPL-Caltech

Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship's remaining lifetime is very limited.

A year ago, the team couldn't count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.

NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.

Regardless of when Dawn's end comes, it will not be a time for regret. The mission has realized its raison d'être and is reaping rewards even beyond those envisioned when it was conceived. It has taken us all on a marvelous interplanetary journey and allowed us to behold previously unseen sights of distant lands. The conclusion of the mission will be a time for gratitude that it was so successful. And until then, every new picture or other measurement adds to the richly detailed portrait of a faraway, exotic world. There is plenty more still to do before this remarkable story draws to a close.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367 million miles, or 591 million kilometers) from Earth, or 1,475 times as far as the moon and 3.99 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and six minutes to make the round trip.

TAGS: DAWN, MISSION, SPACECRAFT, CERES, DWARF PLANET

  • Marc Rayman
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Animated flight over dwarf planet CeresYoutube video

Dear Spellbindawngs,

A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition. In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.

Kupalo Crater from LAMO

Dawn captured this view of Kupalo crater on Dec. 20, shortly after beginning the observations from its current low altitude mapping orbit at 240 miles (385 kilometers). (Kupalo is a Slavic harvest deity associated with love and fertility.) This is a relatively young crater, as seen by its sharp, clear features and the paucity of overlying smaller impact craters, which would have formed later. Bright material on the rim and walls may be salts, as explained last month. The crater is 16 miles (26 kilometers) across. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours. It flies from the north pole to the south over the side of Ceres facing the sun. Then when it heads north, the ground beneath it is cloaked in the deep dark of night on a world without a moon (save Dawn itself). As we discussed last month, Dawn's primary measurements do not depend on illumination. It can sense the nuclear radiation (specifically, gamma rays and neutrons) and the gravity field regardless of the lighting. This month, let's take a look at the other measurements our explorer is performing, most of which do depend on sunlight.

Of course the photographs do. Dawn had already mapped Ceres quite thoroughly from higher altitudes. The spacecraft acquired an extensive set of stereo and color pictures in its third mapping orbit. But now that Dawn is only about 240 miles (385 kilometers) high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.

Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area. Dawn’s camera sees a square about 23 miles (37 kilometers) on a side, less than one twentieth of one percent of the more than one million square miles (nearly 2.8 million square kilometers). In an ideal world (which is not the one Dawn is in or at), it would take just over two thousand photos from this altitude to see all the sights. However, as we will discuss in more detail next month, it is not possible to control the orbital motion and the pointing of the camera accurately enough to manage without more photos than that.

Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at that distant planet, home not only to its controllers but also to many others (including you, loyal reader) who share in the thrill of a bold cosmic adventure. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radios its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.

Mission planners have devised an intricate plan that should allow nearly complete coverage in about six weeks. To accomplish this, they guided Dawn to a carefully chosen orbit, and it has been doing an exceptionally good job there executing its complex activities.

Floor of Dantu Crater from LAMO
On Dec. 21, in its lowest orbit, at about 240 miles (385 kilometers), Dawn flew over Dantu crater and obtained pictures with four times the clarity of the third mapping orbit, where we saw the entire crater. (Dantu is a timekeeper god who initiates the cycle of planting rites among the Ga people of the Accra Plains of southeastern Ghana.) The bright material here is at the 4 o'clock position, half way from the center to the rim, in the picture shown in November. The network of fractures may have formed when the ground cooled after being heated by the crater-forming impact, or perhaps later when other geological processes caused the crater floor to be uplifted. The crater is about 78 miles (126 kilometers) in diameter. The next picture below shows detail of another part of Dantu. The animation above includes Dantu (as seen from farther away). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Last month, we marveled at a stunning view that was not the typical perspective of peering straight down from orbit. Sometimes controllers now program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera's sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn't feel the attraction of the horizon and long to know what lies beyond?

Another of Dawn's scientific devices is two different sensors combined into one instrument. Like the camera, the visible and infrared mapping spectrometers (VIR) look at the sunlight reflected from the ground. (As we'll see below, however, VIR also can detect something more.) A spectrometer breaks up light into its constituent colors, just as a prism or a droplet of water does when revealing, quite literally, all the colors of the rainbow. Dawn's visible spectrometer would have a view very much like that. The infrared spectrometer, of course, looks at wavelengths of light our limited eyes cannot see, just as there are wavelengths of sound our limited ears cannot hear (consult with your dog for details).

A spectrometer does more than simply disperse the light into its components, however. It measures the intensity of that light at the different wavelengths. The materials on the surface leave their signature in the sunlight they reflect, making some wavelengths relatively brighter and some dimmer. That characteristic pattern is called a spectrum. By comparing these spectra with spectra measured in laboratories, scientists can infer the nature of the minerals on the ground. We described some of the intriguing conclusions last month.

On Dec. 19, Dawn's orbit took it over a different part of Dantu crater, showing more reflective material on the walls and floor. (This scene is from the right side of the crater as pictured in November.) More of the fractures visible in the picture above are in the upper left of this picture. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

VIR does still more. Rather than record the visible spectrum and the infrared spectrum from a single region, it takes spectra at 256 adjacent locations simultaneously. This would be like taking one column of 256 pixels in a picture and having a separate spectrum for each. By stitching columns together, you could construct the two dimensional picture but with the added dimension of an extensive spectrum at every location. (Because the extra information provides a sort of depth that flat pictures don't have, the result is sometimes called an “image cube.”) This capability to build up an image with spectra everywhere is what makes it a mapping spectrometer. VIR produces a remarkably rich view of its targets!

VIR's spectra contain much finer measurements of the colors and a wider range of wavelengths than the camera's images. In exchange, the camera has sharper vision and so can discern smaller geological features. In more technical terms, VIR achieves better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.

VIR generates a very large volume of data in each snapshot. As a result, Dawn can only capture and store relatively small areas of the dwarf planet with the mapping spectrometers, especially at this low altitude. Scientists have recognized from the first design of the mission that it would not be possible to cover all of Ceres (or Vesta) with VIR from the closer orbits. Nevertheless, Dawn has far exceeded expectations, returning a great many more spectra than anticipated. Still, as long as the spacecraft operates in this final mapping orbit, there will continue to be interesting targets to study with VIR.

Based on the nearly 20 million spectra of Ceres that VIR acquired from higher altitudes, the team has determined that new infrared spectra will provide more insight into the dwarf planet's character than the visible spectra. Because of their composition, the minerals display more salient signatures in infrared wavelengths than visible. The excellent visible spectra from the first three mapping orbits are deemed more than sufficient. Therefore, to make the best use of our faithful probe and to dedicate the resources to what is most likely to yield new knowledge about Ceres, VIR is devoting its share of the mission data in this final orbit to its infrared mapping spectrometer. We have many more exciting discoveries to look forward to!

Crater with Scarps in LAMO
Dawn photographed this unnamed crater on Dec. 23. It is 20 miles (32 kilometers) in diameter and is between Dantu and Rao craters. (See the map here.) Part of this crater is shown at the bottom left of the photo of Dantu we saw in November. The many ridges and steep slopes here may be the result of the crater partially collapsing during its formation. The complex geology evokes an image of a flower (at least for this writer). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The infrared light Ceres reflects from the sun can tell scientists a great deal about the composition, but they can learn even more from analyzing VIR's measurements. The sun isn't the only source of infrared. Ceres itself is. Many people correctly associate infrared with heat, because warm objects emit infrared light, and the strength at different wavelengths depends on the temperature. That calls for measuring the spectrum! Distant from the sun though it is, Ceres is warmed slightly by the brilliant star, so it has a very faint infrared glow of its own. Scientists can distinguish in VIR's observations between the reflected infrared sunlight and the infrared light Ceres radiates. In essence, VIR can function as a remote thermometer.

Last month, in one of Dawn's best photos yet of Ceres, we considered planning a hike across a breathtaking landscape. In case we do, VIR has shown we should be prepared for chilly conditions. Observed temperatures (all rounded to the nearest multiple of five) during the day on the dwarf planet range from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit (-35 degrees Celsius). (It is so cold in some locations and times, especially at night, that Ceres produces too little infrared light for VIR to measure. Temperatures below the coldest reported here actually don't register.) This finding provides compelling support for this writer's frequent claim that Ceres is really cool. In addition, knowing the temperatures will be very important for understanding geological processes on this icy, rocky world, just as we know the movement of terrestrial glaciers depends on temperature.

Your loyal correspondent can't -- or, at least, won't -- help but indulge his nerdiness with a brief tangent. The range of temperatures above represent the warmest on Ceres, given that VIR cannot measure lower values. It's amusing, if you have a similar weird sense of humor, that Ceres' average temperature apparently is not that far from what it would be for a black hole of the same mass. We won't delve into the physics here, but such a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). OK, enough hilarity. Back to Dawn and Ceres...

Ever creative, scientists are attempting another clever method to gain insight into the nature of this exotic orb. When Dawn is at just the right position in its orbit on the far side of Ceres, so that a straight line to Earth passes very close to the limb of Ceres itself, the spacecraft's radio signal will actually hit the dwarf planet. The radio waves interact with the materials on the surface, which can induce an exquisitely subtle distortion. After bouncing off the ground at a grazing angle, the radio signal continues on its way, heading toward Earth. The effect on the signal is much too small to affect the normal communications at all, but specialized equipment at NASA's Deep Space Network designed for this purpose might still be able to detect the tiny changes. The fantastically sensitive antennas measure the properties of the radio waves, and by studying the details, scientists may be able to learn more about the properties of the surface of the distant world. For example, this could help them distinguish between different types of materials (such as ice, rocks, sand, etc.) as well as reveal how rough or smooth the ground is at scales far, far smaller than the camera can discern. This is an extremely challenging measurement, and no small distortions have been detected so far, but always making the best possible use of the resources, scientists continue to look for them.

In addition to those bonus measurements, Dawn remains very productive in acquiring infrared spectra, photographs, gamma ray spectra and neutron spectra plus conducting measurements of the massive body's gravitational field, all of which contribute to unlocking the mysteries of the first dwarf planet ever discovered or explored. The venerable adventurer is in good condition and is operating flawlessly.

Dawn LAMO Image 5

Dawn observed Victa crater on Dec. 19. (Victa was a Roman goddess of food and nourishment.) The crater is 20 miles (32 kilometers) in diameter and so is the same size as the unnamed one shown above. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We have discussed extensively the failures of two of the four reaction wheels, devices Dawn used to depend on to control its orientation in space. Without three healthy reaction wheels, the probe has had to rely instead on hydrazine propellant expelled from the small jets of the reaction control system. (When Dawn uses its ion engine, that remarkable system does double duty, reducing the need for the hydrazine.)

For most of the time since escaping from Vesta's gravitational clutches in 2012, Dawn has kept the other two reaction wheels in reserve so any remaining lifetime from those devices could offset the high cost of hydrazine propellant to turn and point in this current tight orbit. Those two wheels have been on and functioning flawlessly since Dec. 14, 2015, and every day they operate, they keep the expenditure of the dwindling supply of hydrazine to half of what it would be without them. (Next month we will offer some estimates of how long Dawn might continue to operate.) But the ever-diligent team recognizes another wheel could falter at any moment, and they remain ready to continue the mission with pure hydrazine control after only a short recovery operation. If a third failure is at all like the two that have occurred already, the hapless wheel won't give an indication of a problem until it's too late. A reaction wheel failure evidently is entirely unpredictable. We'll know about it only after it occurs in the remote depths of space where Dawn resides at an alien world.

Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.

Coincidentally, both reached extremes earlier this month in their elliptical heliocentric orbits. Earth, in its annual journey around our star, was at perihelion, or the closest point to the sun, on Jan. 2, when it was 0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which takes 4.6 years (one Cerean year) for each loop, attained its aphelion, or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 AU (277 million miles, or 445 million kilometers) from the gravitational master of the solar system.

Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.

TAGS: DAWN, MISSION, SPACECRAFT, CERES, DWARF PLANET

  • Marc Rayman
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