As NASA's Dawn spacecraft orbits and explores its second target, dwarf planet Ceres, to provide scientists with a window into the dawn of the solar system, mission director and chief engineer Marc Rayman shares a monthly update on the mission's progress. Learn more about the Dawn mission on the JPL Missions database.
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.
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.)
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.
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.
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.
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.
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.
Dawn is concluding a remarkable year of exploring dwarf planet Ceres. At the beginning of 2016, the spacecraft was still a newcomer to its lowest altitude orbit (the fourth since arriving at Ceres in March 2015), and the flight team was looking forward to about three months of exciting work there to uncover more of the alien world’s mysteries.
As it turned out, Dawn spent more than eight months conducting an exceptionally rewarding campaign of photography and other investigations, providing a richly detailed, comprehensive look at the extraterrestrial landscapes and garnering an extraordinary bounty of data. In September, the craft took advantage of its advanced ion propulsion system to fly to a new orbit from which it performed still more unique observations in October. Last month, the ship took flight again, and now it is concluding 2016 in its sixth science orbit.
Dawn is in an elliptical orbit, sailing from about 4,670 miles (7,520 kilometers) up to up to almost 5,810 miles (9,350 kilometers) and back down. It takes nearly eight days to complete each orbital loop. Flying this high above Ceres allows Dawn to record cosmic rays to enhance the nuclear spectra it acquired at low altitude, improving the measurements of atomic constituents down to about a yard (meter) underground.
The spacecraft has been collecting cosmic ray data continuously since reaching this orbit (known to the Dawn team, imaginative readers of last month’s Dawn Journal and now you as extended mission orbit 3, or XMO3). These measurements will continue until the end of the extended mission in June. But there is more in store for the indefatigable adventurer than monitoring space radiation.
Based on studies of Dawn’s extensive inspections of Ceres so far, scientists want to see certain sites at new angles and under different illumination conditions. Next month, Dawn will begin a new campaign of photography and visible spectroscopy. All of Dawn’s five previous science orbits had different orientations from the sun. And now XMO3 will provide another unique perspective on the dwarf planet's terrain. The figure below shows what the orientation will be when the explorer turns its gaze once again on Ceres for the first set of new observations on Jan. 27, 2017.
We mentioned in the figure caption that the alignments are simplified. One of the simplifications is that some of the orbits covered a range of angles. There is a well-understood and fully predictable natural tendency for the angle to increase. In some phases of the mission, the flight team allows that, and in others they do not, depending on what is needed for the best scientific return. At the lowest altitude (orbit 4 in the diagram, and sometimes known as LAMO, XMO1 or "the lowest orbit"), navigators held the orbit at a fixed orientation. Had they not done so, it would have changed quite dramatically over the course of the eight months Dawn was there. For XMO3, the team has decided not to keep the angle constant. Therefore, later observations will provide still different views. We will return to this topic in a few months.
We have described before how places that remain shadowed throughout the Cerean year can trap water molecules. Dawn’s pictures have revealed well over 600 craters high in the northern hemisphere that are permanently in darkness, covering more than 800 square miles (more than 2,000 square kilometers). (It has not been possible to make as thorough a census of the southern hemisphere, because it has been fall and winter there during most of Dawn’s studies, so some areas were not lit well enough. Now that spring has come, new photography will tell us more.)
Dawn peered into craters to see what was hidden on the dark floors. Long exposures could reveal hints of the scenery using the faint light reflected from crater walls. In 10 of the craters, scientists found bright deposits. In one of those craters, the reflective material extends beyond the permanent shadow and so is occasionally illuminated, albeit still with the sun very low on the horizon. And sure enough, right there, Dawn’s infrared mapping spectrometer found the characteristic fingerprint of ice. These shadowed crater floors accumulate water that happens to land there, preserving it in a deep freeze that may be colder than -260°F (-163°C). Readers are invited to formulate their own business plans for how best to utilize that precious resource.
Jan. 1 is the anniversary of the discovery of Ceres. When Giuseppe Piazzi spotted the faint smudge of light in his telescope that night in 1801, he did not know that it would be known as a planet for almost two generations. (After all, he was an astronomer and mathematician, not a clairvoyant.) And he could never have imagined that more than two centuries later (by which time Ceres was known as a dwarf planet, reflecting progress in scientific knowledge), humankind would undertake an ambitious expedition to explore it, dispatching a sophisticated ship to take up residence at that distant and mysterious place. What Piazzi discovered was a lovely jewel set against the deep blackness of space and surrounded by myriad other gleaming stellar jewels. What Dawn has discovered is a unique and fascinating world of complex geology, composed of rock and ice and salt, with exotic and beautiful scenery. And as Dawn continues to build upon Piazzi’s legacy, unveiling Ceres’ secrets, everyone who has ever looked in wonder at the night sky, everyone who has ever hungered for new understanding, everyone who has ever felt the lure of a thrilling adventure far from home and everyone who has ever yearned to know the cosmos will share in the rewards.
Dawn is 5,640 miles (9,070 kilometers) from Ceres. It is also 2.43 AU (226 million miles, or 364 million kilometers) from Earth, or 915 times as far as the moon and 2.48 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PST December 29, 2016
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.
in ft (m)
distance of a soccer ball
|1||RC3||04.23.15 – 05.09.15||8,400
|3||HAMO||08.17.15 – 10.23.15||915
|12.16.15 – 09.02.16||240
|5||XMO2||10.16.16 – 11.04.16||920
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.
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.
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.)
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
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.
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.
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.
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.
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!
Nine years ago today, Dawn set sail on an epic journey of discovery and adventure.
The intrepid explorer has sailed the cosmic seas and collected treasures that far exceeded anything anticipated or even hoped for. It began its voyage at Earth with a fiery ascent atop a Delta rocket. After escaping from its home planet’s gravitational grasp, it flew through the solar system perched on a pillar of blue-green xenon ions that enabled the probe to accomplish a mission that would have been impossible with conventional propulsion. In 2009, with its sights set on more distant lands, Dawn swept past Mars, taking some of the planet’s orbital energy for its own. By its fourth anniversary, Dawn was conducting an extensive orbital investigation of protoplanet Vesta, the second most massive resident of the main asteroid belt. Dawn found it to be quite unlike typical asteroids. Rather than a big chunk of rock, Vesta is like a small planet, and scientists recognize it as being more closely related to the rocky planets of the inner solar system (including Earth) than to the much smaller asteroids. Vesta’s nearer brethren are the blue and white planet where Dawn began its mission nine years ago and the red one it flew by 17 months later. By its fifth anniversary of leaving Earth, the interplanetary spaceship was on its way to yet another distant, alien world. Under the careful guidance of its human colleagues, Dawn completed its 2.5-year journey from Vesta to Ceres last year. Now a perpetual companion of the first dwarf discovered, the veteran space traveler will spend all future anniversaries in orbit around Ceres, even after its operational lifetime has concluded.
By February of this year, the spacecraft had exceeded all of its original objectives established by NASA. Doing so involved orbiting Vesta for 14 months and, at that time, Ceres for almost a year. On June 30, Dawn’s prime mission concluded, and on July 1, its "extended mission" began.
One year ago today, the ship was in its third Ceres mapping orbit, scrutinizing the exotic landscapes 915 miles (1,470 kilometers) beneath it. Less than four weeks later, it started powering its way down through the uncharted depths of Ceres gravitational field to undertake the final planned observations of its long mission.
When ion thrusting concluded on Dec. 13, 2015, Dawn was orbiting closer to Ceres than the International Space Station is to Earth. From its vantage point only 240 miles (385 kilometers) high, the probe used its suite of sophisticated sensors to develop a richly detailed portrait of the only dwarf planet in the inner solar system. Dawn’s reason for venturing to its fourth mapping orbit was to collect about 35 days of neutron spectra, 35 days of gamma-ray spectra and 20 days of gravity measurements. Given the complexity of operating in the low, tight orbit, mission planners expected it could take about three months to acquire these precious data and transmit them to Earth. Operations turned out to be essentially flawless, and by the time Dawn left that orbit on Sept. 2, it had accumulated 183 days of neutron spectra, 183 days of gamma-ray spectra and 165 days of gravity measurements. In addition, the spacecraft amassed a sensational bonus of 38,000 high resolution photos (including stereo and color) as well as more than 11 million infrared spectra and 12 million spectra in visible wavelengths. The original plan was not to take any pictures or visible or infrared spectra at the lowest altitude.
For such an overachiever, it’s fitting that now, on its ninth anniversary, the spacecraft is engaged in activities entirely unimagined on its eighth. With the critical loss of two of the four reaction wheels used to orient and stabilize the ship in space, the flight team (and your correspondent) considered it unlikely Dawn would survive long enough to celebrate a ninth anniversary. And everyone was confident that whether it was operating or not, it would still be in the fourth mapping orbit. There was a clear intent never to go anywhere else. But as we explained last month, with the extraordinary wealth of information Dawn gleaned, the team has been developing plans for new and previously unforeseen work at higher altitudes. Next month, we will detail the first set of new observations from an orbital perch of about 920 miles (1,480 kilometers).
For now, Dawn is using its ion engine #2 to gradually raise its orbit. We have seen how the spacecraft’s uniquely capable propulsion system leads to intriguing spiral trajectories. Right now, on the ninth anniversary of the last moment Dawn’s rocket stood motionless at Cape Canaveral’s Space Launch Complex 17B, Dawn is 660 miles (1,060 kilometers) above Ceres. With its signature combination of exceptional gentleness and exceptional efficiency, the ion engine will propel Dawn to an altitude 20 miles (35 kilometers) higher by the end of the day today. (In contrast, by the end of the day it launched nine years ago, Dawn had gained about 175,000 miles, or 280,000 kilometers, in altitude. The Delta rocket provided a much stronger thrust at much lower efficiency. We will discuss this further below.)
You can follow Dawn’s ascent to its new orbit by flying right behind it as it loops around Ceres or by checking the frequent mission status reports.
Nine years after launch, as Dawn maneuvers in orbit around a distant dwarf planet in order to conduct new observations, it is convenient to look back over its long trek through deep space. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the ninth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to reflect upon Dawn’s ambitious journey may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh and eighth anniversaries.
In its nine years of interplanetary travels, the spacecraft has thrust for a total of 2,044 days (5.6 years), or 62 percent of the time (and 0.000000041 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 890 pounds (404 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 68 of the 71 gallons (256 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.
The thrusting since then has achieved the equivalent of accelerating the probe by 24,800 mph (39,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided 97 percent of the change in speed that the entire Delta rocket provided.
Since launch, our readers who have remained on or near Earth have completed nine revolutions around the sun, covering 56.6 AU (5.3 billion miles, or 8.5 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 38.6 AU (3.6 billion miles, or 5.8 billion kilometers). As it climbed away from the sun, up the solar system hill, to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the nine years since Dawn began its voyage, Vesta has traveled only 36.6 AU (3.4 billion miles, or 5.5 billion kilometers), and the even more sedate Ceres has gone 34.0 AU (3.2 billion miles, or 5.1 billion kilometers). (To develop a feeling for the relative speeds, 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 differences in distance among Earth, Dawn, Vesta and Ceres over the nine years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Vesta, Ceres and Dawn) follow their paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.
from the Sun (AU)
from the Sun (AU)
|Dawn’s orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn’s orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn’s orbit on Sept. 27, 2011||2.15||2.57||7.1°|
|Dawn’s orbit on Sept. 27, 2012||2.17||2.57||7.3°|
|Dawn’s orbit on Sept. 27, 2013||2.44||2.98||8.7°|
|Dawn’s orbit on Sept. 27, 2014||2.46||3.02||9.8°|
|Dawn’s orbit on Sept. 27, 2015||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2016||2.56||2.98||10.6°|
For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that five years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. But now, Dawn has gone even beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind in 2012. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. Dawn devoted another 2.5 years to reshaping and tilting its orbit even more so that now it is identical to Ceres’. Once again, that was essential to the intricate celestial choreography in March 2015, when the behemoth tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.
Its ion propulsion system has allowed Dawn to do even more than orbit two distant and fascinating bodies. At each one, the spacecraft has changed its orbits extensively, optimizing its views to conduct detailed studies, something it would not have been able to do with conventional propulsion.
Dawn passed a coincidental pair of milestones in its orbital mission at Ceres last week. The dwarf planet reached out to take Earth’s emissary into a gentle but permanent gravitational embrace on March 6, 2015. Sept. 23, 2016, was 1,500 Cerean days later. (Ceres turns on its axis in 9 hours, 4 minutes, considerably faster than Earth, although not all that different from the giant planet Jupiter, which takes 9 hours, 56 minutes). Interestingly, on Sept. 22, Dawn completed its 1,500th orbital revolution around Ceres.
Given the equality between the number of orbits and the number of Cerean days, you may be tempted to conclude that Dawn orbits at the same rate that Ceres rotates. Please resist this temptation! Dawn’s early orbits took weeks to complete, and as the spacecraft maneuvered to lower altitudes, eventually they took days and then hours. In its lowest altitude, the spacecraft circled Ceres in only 5.4 hours. (For a reminder of the details of the orbits, see this table and this diagram depicting preliminary orbit sizes.) So, it truly is a coincidence that the average has worked out so that Dawn has revolved as many times as Ceres has rotated. And now that Dawn is raising its altitude and thus increasing the time required to complete an orbit, such a coincidence will not occur again. Ceres is very stubborn and will keep rotating at the same rate. Dawn, much nimbler and more flexible, is currently in a 13-hour orbit. By the time it completes ion thrusting next week, the orbit period will be almost 19 hours.
Now in the 10th year of its deep-space expedition, Dawn is not satisfied simply to rest on its laurels. The explorer (along with its support team on distant Earth) is committed to remaining as prolific and profitable at Ceres as it was during earlier years of its extraordinary and innovative mission of discovery. The largest body between Mars and Jupiter is a relict from the dawn of the solar system, a strange and fascinating world of rock, ice and salt that likely has been geologically active for more than 4.5 billion years. Ceres was first glimpsed from Earth more than 200 years ago but held her secrets close until Earth finally answered her cosmic invitation. Now, after so very long, Ceres is whispering those wondrous secrets to her permanent companion. Dawn is listening carefully!
Dawn is 660 miles (1,060 kilometers) from Ceres. It is also 1.99 AU (185 million miles, or 297 million kilometers) from Earth, or 760 times as far as the moon and 1.98 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2016
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.
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?
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.)
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
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.
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.
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.
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
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.
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.
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.
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
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.
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 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 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'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.
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.
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.
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.
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.
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.
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.