Blogs

Blogs


1954 Inspection Tour

In December 1954, only a few months after becoming the director of JPL, Dr. William Pickering (in the light-colored suit) hosted a visit by Frank H. Higgins, assistant secretary of the Army, and several members of his military entourage. At that time, JPL was under contract to Army Ordnance to develop guided missiles. In this photo, the group is gathered in the control room of the 20-inch wind tunnel. Frank Goddard (in the dark suit), chief of the Supersonic Aerodynamics Division, assisted with the tour and Bud Schurmeier, manager of the Wind Tunnel Section, observed from the back of the room while technicians conducted a demonstration.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

TAGS: TECHNOLOGY, HISTORY, DEEP SPACE NETWORK

  • Julie Cooper
READ MORE

This view of Ceres shows some bright material that is not confined to “spots.”

Dear Descendawnts,

Flying on a blue-green ray of xenon ions, Dawn is gracefully descending toward dwarf planet Ceres. Even as Dawn prepares for a sumptuous new feast in its next mapping orbit, scientists are continuing to delight in the delicacies Ceres has already served. With a wonderfully rich bounty of pictures and other observations already secured, the explorer is now on its way to an even better vantage point.

Dawn Survey Orbit Image 31 This image, taken by NASA's Dawn spacecraft, shows dwarf planet Ceres from an altitude of 2,700 miles (4,400 kilometers). The image, with a resolution of 1,400 feet (410 meters) per pixel, was taken on June 25, 2015.
Dawn was in its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers) when it took this picture of Ceres. This area shows relatively few craters, suggesting it is younger than some other areas on Ceres. Some bright spots are visible, although they are not as prominent as the most famous bright spots. Scientists do not yet have a clear explanation for them, but you can register your vote here. Click on the picture (or follow the link to the full image) for a better view of some interesting narrow, straight features in the lower left. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

Dawn takes great advantage of its unique ion propulsion system to maneuver extensively in orbit, optimizing its views of the alien world that beckoned for more than two centuries before a terrestrial ambassador arrived in March. Dawn has been in powered flight for most of its time in space, gently thrusting with its ion engine for 69 percent of the time since it embarked on its bold interplanetary adventure in 2007. Such a flight profile is entirely different from the great majority of space missions. Most spacecraft coast most of the time (just as planets do), making only brief maneuvers that may add up to just a few hours or even less over the course of a mission of many years. But most spacecraft could not accomplish Dawn’s ambitious mission. Indeed, no other spacecraft could. The only ship ever to orbit two extraterrestrial destinations, Dawn accomplishes what would be impossible with conventional technology. With the extraordinary capability of ion propulsion, it is truly an interplanetary spaceship.

In addition to using its ion engine to travel to Vesta, enter into orbit around the protoplanet in 2011, break out of orbit in 2012, travel to Ceres and enter into orbit there this year, Dawn relies on the same system to fly to different orbits around these worlds it unveils, executing complex and graceful spirals around its gravitational master. After conducting wonderfully successful observation campaigns in its preantepenultimate Ceres orbit 8,400 miles (13,600 kilometers) high in April and May and its antepenultimate orbit at 2,700 miles (4,400 kilometers) in June, Dawn commenced its spiral descent to the penultimate orbit at 915 miles (1,470 kilometers) on June 30. (We will discuss this orbital altitude in more detail below.) A glitch interrupted the maneuvering almost as soon as it began, when protective software detected a discrepancy in the probe’s orientation. But thanks to the exceptional flexibility built into the plans, the mission could easily accommodate the change in schedule that followed. It will have no effect on the outcome of the exploration of Ceres. Let’s see what happened.

Survey orbit to HAMO
Dawn’s spiral descent from its second mapping orbit (survey), at 2,700 miles (4,400 kilometers), to its third (HAMO), at 915 miles (1,470 kilometers). The two mapping orbits are shown in green. The color of Dawn’s trajectory progresses through the spectrum from blue, when it began ion-thrusting in survey orbit, to red, when it arrives in HAMO. The red dashed sections show where Dawn is coasting for telecommunications. Compare this to the previous spiral. Image credit: NASA/JPL-Caltech

Control of Dawn’s orientation in the weightless conditions of spaceflight is the responsibility of the attitude control system. (To maintain a mystique about their work, engineers use the term “attitude” instead of “orientation.” This system also happens to have a very positive attitude about its work.) Dawn (and all other objects in three-dimensional space) can turn about three mutually perpendicular axes. The axes may be called pitch, roll and yaw; left/right, front/back and up/down; x, y and z; rock, paper and scissors; chocolate, vanilla and strawberry; Peter, Paul and Mary; etc., but whatever their names, attitude control has several different means to turn or to stabilize each axis. Earlier in its journey, the spacecraft depended on devices known as reaction wheels. As we have discussed in many Dawn Journals, that method is now used only rarely, because two of the four units have failed. The remaining two are being saved for the ultimate orbit at about 230 miles (375 kilometers), which Dawn will attain at the end of this year. Instead of reaction wheels, Dawn has been using its reaction control system, shooting puffs of hydrazine, a conventional rocket propellant, through small jets. (This is entirely different from the ion propulsion system, which expels high velocity xenon ions to change and control Dawn’s path through space. The reaction control system is used only to change and control attitude.)

Whenever Dawn is firing one of its three ion engines, its attitude control system uses still another method. The ship only operates one engine at a time, and attitude control swivels the mechanical gimbal system that holds that engine, thus imparting a small torque to the spacecraft, providing the means to control two axes (pitch and yaw, for example, or chocolate and strawberry). For the third axis (roll or vanilla), it still uses the hydrazine jets of the reaction control system.

On June 30, engine #3 came to life on schedule at 10:32:19 p.m. PDT to begin nearly five weeks of maneuvers. Attitude control deftly switched from using the reaction control system for all three axes to only one, and controlling the other two axes by tipping and tilting the engine with gimbal #3. But the control was not as effective as it should have been. Software monitoring the attitude recognized the condition but wisely avoided reacting too soon, instead giving attitude control time to try to rectify it. Nevertheless, the situation did not improve. Gradually the attitude deviated more and more from what it should have been, despite attitude control’s efforts. Seventeen minutes after thrusting started, the error had grown to 10 degrees. That’s comparable to how far the hour hand of a clock moves in 20 minutes, so Dawn was rotating only a little faster than an hour hand. But even that was more than the sophisticated probe could allow, so at 10:49:27 p.m., the main computer declared one of the “safe modes,” special configurations designed to protect the ship and the mission in uncertain, unexpected or difficult circumstances.

The spacecraft smoothly entered safe mode by turning off the ion engine, reconfiguring other systems, broadcasting a continuous radio signal through one of its antennas and then patiently awaiting further instructions. The radio transmission was received on a distant planet the next day. (It may yet be received on some other planets in the future, but we shall focus here on the response by Earthlings.) One of NASA’s Deep Space Network stations in Australia picked up the signal on July 1, and the mission control team at JPL began investigating immediately.

Engineers assessed the health of the spacecraft and soon started returning it to its normal configuration. By analyzing the myriad diagnostic details reported by the robot over the next few days, they determined that the gimbal mechanism had not operated correctly, so when attitude control tried to change the angle of the ion engine, it did not achieve the desired result.

Because Dawn had already accomplished more than 96 percent of the planned ion-thrusting for the entire mission (nearly 5.5 years so far), the remaining thrusting could easily be accomplished with only one of the ion engines. (Note that the 96 percent here is different from the 69 percent of the total time since launch mentioned above, simply because Dawn has been scheduled not to thrust some of the time, including when it takes data at Vesta and Ceres.) Similarly, of the ion propulsion system’s two computer controllers, two power units and two sets of valves and other plumbing for the xenon, the mission could be completed with only one of each. So although engineers likely could restore gimbal #3’s performance, they chose to switch to another gimbal (and thus another engine) and move on. Dawn’s goal is to explore a mysterious, fascinating world that used to be known as a planet, not to perform complex (and unnecessary) interplanetary gimbal repairs.

One of the benefits of being in orbit (besides it being an incredibly cool place to be) is that Dawn can linger at Ceres, studying it in great detail rather than being constrained by a fast flight and a quick glimpse. By the same principle, there was no urgency in resuming the spiral descent. The second mapping orbit was a perfectly fine place for the spacecraft, and it could circle Ceres there every 3.1 days as long as necessary. (Dawn consumed its hydrazine propellant at a very, very low rate while in that orbit, so the extra time there had a negligible cost, even as measured by the most precious resource.)

The operations team took the time to be cautious and to ensure that they understood the nature of the faulty gimbal well enough to be confident that the ship could continue its smooth sailing. They devised a test to confirm Dawn’s readiness to resume its spiral maneuvers. After swapping to gimbal #2 (and ipso facto engine #2), Dawn thrust from July 14 to 16 and demonstrated the excellent performance the operations team has seen so often from the veteran space traveler. Having passed its test with flying colors (or perhaps even with orbiting colors), Dawn is now well on its way to its third mapping orbit.

Artist’s concept of Dawn thrusting with ion engine #2.
Artist’s concept of Dawn thrusting with ion engine #2. The spacecraft captured the view of Ceres in June, and the intriguing cone described last month is visible on the limb at lower left. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Background image and caption

The gradual descent from the second mapping orbit to the third will require 25 revolutions. The maneuvers will conclude in about two weeks. (As always, you can follow the progress with your correspondent’s frequent and succinct updates here.) As in each mapping orbit, following arrival, a few days will be required in order to prepare for a new round of intensive observations. That third observing campaign will begin on August 17 and last more than two months.

Although this is the second lowest of the mapping orbits, it is also known as the high altitude mapping orbit (HAMO) for mysterious historical reasons. We presented an overview of the HAMO plans last year. Next month, we will describe how the flight team has built on a number of successes since then to make the plans even better.

The view of the landscapes on this distant and exotic dwarf planet from the third mapping orbit will be fantastic. How can we be so sure? The view in the second mapping orbit was fantastic, and it will be three times sharper in the upcoming orbit. Quod erat demonstrandum! To see the sights at Ceres, go there or go here.

Part of the flexibility built into the plans was to measure Ceres’ gravity field as accurately as possible in each mapping orbit and use that knowledge to refine the design for the subsequent orbital phase. Thanks to the extensive gravity measurements in the second mapping orbit in June, navigators were able not only to plot a spiral course but also to calculate the parameters for the next orbit to provide the views needed for the complex mapping activities.

This color-coded map from NASA's Dawn mission shows the highs and lows of topography on the surface of dwarf planet Ceres. It is labeled with names of features approved by the International Astronomical Union.
This map of Ceres depicts the topography ranging from 4.7 miles (7.5 kilometers) low in indigo to 4.7 miles (7.5 kilometers) high in white. (As a technical detail, the topography is shown relative to an ellipsoid of dimensions very close to those in the paragraph below.) The names of features have been approved by the International Astronomical Union following the system described in December. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

We have discussed some of the difficulty in describing the orbital altitude, including variations in the elevation of the terrain, just as a plane flying over mountains and valleys does not maintain a fixed altitude. As you might expect on a world battered by more than four billion years in the main asteroid belt and with its own internal geological forces, Ceres has its ups and downs. (The topographical map above displays them, and you can see a cool animation of Ceres showing off its topography here.) In addition to local topographical features, its overall shape is not perfectly spherical, as we discussed in May. Ongoing refinements based on Dawn’s measurements now indicate the average diameter is 584 miles (940 kilometers), but the equatorial diameter is 599 miles (964 kilometers), whereas the polar diameter is 556 miles (894 kilometers). Moreover, the orbits themselves are not perfect circles, and irregularities in the gravitational field, caused by regions of lower and higher density inside the dwarf planet, tug less or more on the craft, making it move up and down somewhat. (By using that same principle, scientists learn about the interior structure of Ceres and Vesta with very accurate measurements of the subtleties in the spacecraft’s orbital motions.) Although Dawn’s average altitude will be 915 miles (1,470 kilometers), its actual distance above the ground will vary over a range of about 25 miles (40 kilometers).

In March we summarized the four Ceres mapping orbits along with a guarantee that the dates would change. In addition to delivering exciting interplanetary adventures to thrill anyone who has ever gazed at the night sky in wonder, Dawn delivers on its promises. Therefore, we present the updated table here. With such a long and complex mission taking place in orbit around the largest previously uncharted world in the inner solar system, further changes are highly likely. (Nevertheless, we would consider the probability to be low that changes will occur for the phases in the past.)

Table showing Dawn's activities during the various mapping orbits
Find out more about Dawn's activities during these mapping orbits: RC3, survey, HAMO, LAMO

Click on the name of each orbit for a more detailed description. As a reminder, the last column illustrates how large Ceres appears to be from Dawn’s perspective by comparing it with a view of a soccer ball. (Note that Ceres is not only 4.4 million times the diameter of a soccer ball but it is a lot more fun to play with.)

Resolute and resilient, Dawn patiently continues its graceful spirals, propelled not only by its ion engine but also by the passions of everyone who yearns for new knowledge and noble adventures. Humankind’s robotic emissary is well on its way to providing more fascinating insights for everyone who longs to know the cosmos.

Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 1.95 AU (181 million miles, or 291 million kilometers) from Earth, or 785 times as far as the moon and 1.92 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
8:00 p.m. PDT July 29, 2015

› Learn more about the Dawn mission

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

  • Marc Rayman
READ MORE

The brightest spots on Ceres.

Dear Evidawnce-Based Readers,

Dawn is continuing to unveil a Ceres of mysteries at the first dwarf planet discovered. The spacecraft has been extremely productive, returning a wealth of photographs and other scientific measurements to reveal the nature of this exotic alien world of rock and ice. First glimpsed more than 200 years ago as a dot of light among the stars, Ceres is the only dwarf planet between the sun and Neptune.

Dawn has been orbiting Ceres every 3.1 days at an altitude of 2,700 miles (4,400 kilometers). As described last month, the probe aimed its powerful sensors at the strange landscape throughout each long, slow passage over the side of Ceres facing the sun. Meanwhile, Ceres turned on its axis every nine hours, presenting itself to the ambassador from Earth. On the half of each revolution when Dawn was above ground that was cloaked in the darkness of night, it pointed its main antenna to that planet far, far away and radioed its precious findings to eager Earthlings (although the results will be available for others throughout the cosmos as well). Dawn began this second mapping campaign (also known as "survey orbit") on June 5, and tomorrow it will complete its eighth and final revolution.

The spacecraft made most of its observations by looking straight down at the terrain directly beneath it. During portions of its first, second and fourth orbits, however, Dawn peered at the limb of Ceres against the endless black of space, seeing the sights from a different perspective to gain a better sense of the lay of the land.

And what marvels Dawn has beheld! How can you not be mesmerized by the luminous allure of the famous bright spots? They are not, in fact, a source of light, but for a reason that remains elusive, the ground there reflects much more sunlight than elsewhere. Still, it is easy to imagine them as radiating a light all their own, summoning space travelers from afar, beckoning the curious and the bold to venture closer in return for an attractive reward. And that is exactly what we will do, as we seek the rewards of new knowledge and new insights into the cosmos.

Although scientists have not yet determined what minerals are there, Dawn will gather much more data. As summarized in this table, our explorer will map Ceres again from much closer during the course of its orbital mission. New bright areas have shown up in other locations too, in some places as relatively small spots, in others as larger areas (as in the photo below), and all of them will come into sharper focus when Dawn descends further.

limb with crater and bright materials inside and out
There is bright material easily visible inside and around the crater near the upper right. Did the powerful impact that excavated the crater deposit bright material that it brought from elsewhere in space, excavate bright material from underground or create the conditions that subsequently caused some material to become bright? The reason for the greater reflectivity is not yet known. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

In the meantime, you can register your opinion for what the bright spots are. Join more than 100 thousand others who have voted for an explanation for this enigma. Of course, Ceres will be the ultimate arbiter, and nature rarely depends upon public opinion, but the Dawn project will consider sending the results of the poll to Ceres, courtesy of our team member on permanent assignment there.

In addition to the bright spots, Dawn's views from its present altitude have included a wide range of other intriguing sights, as one would expect on a world of more than one million square miles (nearly 2.8 million square kilometers). There are myriad craters excavated by objects falling from space, inevitable scars from inhabiting the main asteroid belt for more than four billion years, even for the largest and most massive resident there.

The craters exhibit a wide range of appearances, not only in size but also in how sharp and fresh or how soft and aged they look. Some display a peak at the center. A crater can form from such a powerful punch that the hard ground practically melts and flows away from the impact site. Then the material rebounds, almost as if it sloshes back, while already cooling and then solidifying again. The central peak is like a snapshot, preserving a violent moment in the formation of the crater. By correlating the presence or absence of central peaks with the sizes of the craters, scientists can infer properties of Ceres' crust, such as how strong it is. Rather than a peak at the center, some craters contain large pits, depressions that may be a result of gasses escaping after the impact. (Craters elsewhere in the solar system, including on Vesta and Mars, also have pits.)

crater with terraced walls, a central peak and ridge, smooth areas at top of picture and more rugged terrain at bottom
Several craters here have central peaks. The largest also has a ridge at the center. Note other intriguing geological structures, including the terraced walls of that crater and the contrast between the smooth area in the top half of the picture and the more rugged terrain at the bottom. The picture below overlaps the top of this view. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

Dawn also has spied many long, straight or gently curved canyons. Geologists have yet to determine how they formed, and it is likely that several different mechanisms are responsible. For example, some might turn out to be the result of the crust of Ceres shrinking as the heat and other energy accumulated upon formation gradually radiated into space. When the behemoth slowly cooled, stresses could have fractured the rocky, icy ground. Others might have been produced as part of the devastation when a space rock crashed, rupturing the terrain.

Bright spots on the limb plus canyons
Several long canyons are evident in this view. The large crater that extends off the bottom of the picture is in the center of the picture above. Also notice the bright spots, just visible on the limb at upper left. The first picture above shows them from overhead. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.Full image and caption

Ceres shows other signs of an active past rather than that of a static chunk of inert material passing the eons with little notice. Some areas are less densely cratered than others, suggesting that there are geological processes that erase the craters. Indeed, some regions look as if something has flowed over them, as if perhaps there was mud or slush on the surface.

In addition to evidence of aging and renewal, some powerful internal forces have uplifted mountains. One particularly striking structure is a steep cone that juts three miles (five kilometers) high in an otherwise relatively smooth area, looking to an untrained (but transfixed) eye like a volcanic cone, a familiar sight on your home planet (or, at least, on mine). No other isolated, prominent protuberance has been spotted on Ceres.

limb with conical mountain above and to the right of center plus a few other bright areas
The conical mountain is above and to the right of center. With the solar illumination from the top of the picture, note how crater walls are brighter on the bottom (facing the sun) and darker on the top (shaded by the ground they sink into). The cone stands out because it is brighter on the top (facing the sun), and the opposite side is in the shade. (In addition, the material in some places on the cone is brighter than in other places on the same structure.) This view also show several bright spots and larger areas. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

another view of the limb with the same conical mountain and a few other bright areas.
The conical feature in the previous picture is visible here on the limb at bottom center. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

It is too soon for scientists to understand the intriguing geology of this ancient world, but the prolific adventurer is providing them with the information they will use. The bounty from this second mapping phase includes more than 1,600 pictures covering essentially all of Ceres, well over five million spectra in visible and infrared wavelengths and hundreds of hours of gravity measurements.

The spacecraft has performed its ambitious assignments quite admirably. Only a few deviations from the very elaborate plans occurred. On June 15 and 27, during the fourth and eighth flights over the dayside, the computer in the combination visible and infrared mapping spectrometer (VIR) detected an unexpected condition, and it stopped collecting data. When the spacecraft's main computer recognized the situation, it instructed VIR to close its protective cover and then power down. The unit dutifully did so. Also on June 27, about three hours before VIR's interruption, the camera's computer experienced something similar.

Most of the time that Dawn points its sensors at Ceres, it simultaneously broadcasts through one of its auxiliary radio antennas, casting a very wide but faint signal in the general direction of Earth. (As Dawn progresses in its orbit, the direction to Earth changes, but the spacecraft is equipped with three of these auxiliary antennas, each pointing in a different direction, and mission controllers program it to switch antennas as needed.) The operations team observed what had occurred in each case and recognized there was no need to take immediate action. The instruments were safe and Dawn continued to carry out all of its other tasks.

When Dawn subsequently flew to the nightside of Ceres and pointed its main antenna to Earth, it transmitted much more detailed telemetry. As engineers and scientists continue their careful investigations, they recognize that in many ways, these events appear very similar to ones that have occurred at other times in the mission.

Four years ago, VIR's computer reset when Dawn was approaching Vesta, and the most likely cause was deemed to be a cosmic ray strike. That's life in deep space! It also reset twice in the survey orbit phase at Vesta. The camera reset three times in the first three months of the low altitude mapping orbit at Vesta.

Even with the glitches in this second mapping orbit, Dawn's outstanding accomplishments represent well more than was originally envisioned or written into the mission's scientific requirements for this phase of the mission. For those of you who have not been to Ceres or aren't going soon (and even those of you who want to plan a trip there of your own), you can see what Dawn sees by going to the image gallery.

Although Dawn already has revealed far, far more about Ceres in the last six months than had been seen in the preceding two centuries of telescopic studies, the explorer is not ready to rest on its laurels. It is now preparing to undertake another complex spiral descent, using its sophisticated ion propulsion system to maneuver to a circular orbit three times as close to the dwarf planet as it is now. It will take five weeks to perform the intricate choreography needed to reach the third mapping altitude, starting tomorrow night. You can keep track of the spaceship's flight as it propels itself to a new vantage point for observing Ceres by visiting the mission status page or following it on Twitter @NASA_Dawn.

As Dawn moves closer to Ceres, Earth will be moving closer as well. Earth and Ceres travel on independent orbits around the sun, the former completing one revolution per year (indeed, that's what defines a year) and the latter completing one revolution in 4.6 years (which is one Cerean year). (We have discussed before why Earth revolves faster in its solar orbit, but in brief it is because being closer to the sun, it needs to move faster to counterbalance the stronger gravitational pull.) Of course, now that Dawn is in a permanent gravitational embrace with Ceres, where Ceres goes, so goes Dawn. And they are now and forever more so close together that the distance between Earth and Ceres is essentially equivalent to the distance between Earth and Dawn.

On July 22, Earth and Dawn will be at their closest since June 2014. As Earth laps Ceres, they will be 1.94 AU (180 million miles, or 290 million kilometers) apart. Earth will race ahead on its tight orbit around the sun, and they will be more than twice as far apart early next year.

trajectory
Earth's and Ceres' orbits will bring them to their minimum separation on July 22. Earth's orbit is shown in green and Ceres' is in purple. Dawn's interplanetary trajectory is in blue. Compare this figure with the ones depicting Dawn and Earth on opposite sides of the sun in December 2014 and showing Dawn equidistant from Earth and the sun in April 2015. Credit: NASA/JPL-Caltech

Although Dawn communicates regularly with Earth, it left that planet behind nearly eight years ago and will keep its focus now on its new residence. With two very successful mapping campaigns complete, its next priority is to work its way down through Ceres' gravitational field to an altitude of about 900 miles (less than 1,500 kilometers). With sharper views and new kinds of observations (including stereo photography), the treasure trove obtained by this intrepid extraterrestrial prospector will only be more valuable. Everyone who longs for new understandings and new perspectives on the cosmos will grow richer as Dawn continues to pioneer at a mysterious and distant dwarf planet.

Dawn is 2,700 miles (4,400 kilometers) from Ceres. It is also 2.01 AU (187 million miles, or 301 million kilometers) from Earth, or 785 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
10:00 p.m. PDT June 29, 2015

› Learn more about the Dawn mission

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

  • Marc Rayman
READ MORE

Earth climate visualization videoYoutube video

“Do we think about the aerosol propellant in our underarm deodorant every day?” Gavin Schmidt, climatologist and director of The Goddard Institute for Space Studies (GISS), asked me. “I don’t think we even have aerosols anymore,” I answered, wondering where he was going with this.

“That’s the point,” he continued, “and nobody cares. Nobody cares where your energy comes from; nobody cares whether your car is electric or petrol. People confuse energy supply with where the energy is supplied from.” He was trying to make the point that as long as people have the things they want, it doesn’t matter, to the vast majority of us, how we get them. This means that as long as the light switch still turns on the lights, most people would barely notice if we were to shift from burning fossil fuels to energy sources with less impact on Earth’s climate (just as people don’t notice that ozone-depleting propellants aren’t used in aerosol cans any more).

I was eager to speak with Dr. Schmidt because of his passion for communicating climate science to public audiences on top of his work as a climatologist. Schmidt is a co-founder and active blogger at Real Climate and was also awarded the inaugural Climate Communications Prize, by the American Geophysical Union (AGU) in 2011. “My goal in communicating,” he explained, “is a totally futile effort to raise the level of the conversation so that we actually discuss the things that matter.”

Since the mere mention of a computer model can cause an otherwise normal person’s face to glaze over, I thought Schmidt, a leader in climate simulations and Earth system modeling, would be the ideal candidate to explain one of the most important, yet probably one of the most misunderstood, instruments scientists have for studying Earth’s climate. See, people commonly confuse climate and weather, and this confusion is perhaps most pronounced when it comes to understanding the difference between a weather forecast and a climate simulation.

Numerical laboratory

Schmidt’s work routine is much like that of any other scientist. He spends a few months preparing experiments, then a few more months conducting the experiments, then a few more months refining and improving the experiments, then a few more months going back and looking at fine details, then a few more months … you get the idea. Climate scientists use complex computer simulations as numerical laboratories to conduct experiments because we don’t have a bunch of spare Earths just lying around. These simulations model Earth’s conditions as precisely as possible. “A single run can take three months on up on super computers,” Schmidt said. “For really long runs, it can take a year.” NASA scientists can reserve time for High-End Computing Capability at the NASA Advanced Supercomputing facility and/or the NASA Center for Climate Simulation to run simulations. Like an astronomer who reserves time on a large telescope to run her experiments, Schmidt books time on these computers to run his.

Schmidt asks the computer to calculate the weather in 20-minute time steps and see how it changes. Every 20 minutes it updates its calculation over hundred-year or even thousand-year periods in the past or the future. “The models that we run process about three to four years of simulation, going through every 20 minute time step, every real day.”

A typical climate simulation code is large, as in 700,000 lines of computer code large. For comparison, the Curiosity Rover required about 500,000 lines of code to autonomously descend safely on Mars, a planet 140 million miles away with a signal time delay of about 14 minutes. The size of a typical app, such as our Earth Now mobile app, is just over 6,000 lines of code. Climate simulations require such a large quantity of code because Earth’s climate is so extraordinarily complex. And, according to Schmidt, “Complexity is quite complex.”

Like a scientist who runs an experiment in a science lab, climate modelers want code that’s consistent from one experiment to another. So they spend most of their time developing that code, looking at code, improving code and fixing bugs.

The model output is compared to data and observations from the real world to build in credibility. “We rate the predictions on whether or not they’re skillful; on whether we can demonstrate they are robust.” When models are tested against the real world, we get a measure of how skillful the model is at reproducing things that have already happened. Then we can be more confident about the accuracy in predicting what’s going to happen.

Schmidt wants to find out where the models have skill and where they provide useful information. For example, they’re not very useful for tornado statistics, but they're extremely useful on global mean temperature. According to Schmidt, the credible and consistently reliable predictions include ones that involve adding carbon dioxide to the atmosphere. “You consistently get increases in temperature and those increases are almost always greater over land than they are in the ocean. They’re always larger in the Arctic than in the mid-latitudes and always more in the northern hemisphere than the southern, particularly Antarctica. Those are very, very robust results.”

Lately, his team has been working on improving the code for sea ice dynamics to include the effects of brine pockets (very salty fluid within the ice matrix) as well as the wind moving the ice around. For example, to understand the timeline for Arctic sea ice loss, his team has to work on the different bits of code for the wind, the temperature, the ocean and the water vapor and include the way all these pieces intersect in the real world. After you improve the code, you can see the impact of those improvements.

I asked Schmidt what people’s behavior would look like “if they understood that burning fossil fuels produces carbon dioxide, which causes global warming.” He replied, “People would start focusing on policies and processes that would reduce the amount of fossil fuels without ruining the economy or wrecking society.” Then he added, “I think, I hope! that people will get it before it’s too late.”

I hope so, too.

› Visit Earth Right Now to comment on this post

TAGS: EARTH, CLIMATE, CLIMATE MODELS

READ MORE

Illustration of NASA's ocean-observing satellites orbiting Earth

Bring out the trumpeters! We’re preparing for another satellite launch. Woohoo! This time it’s Jason-3, an altimetry mission that will observe sea surface topography from space. It’s a legacy mission that continues the 23-year record of global sea level measurements started by TOPEX/Poseidon and carried on by Jason-1 and -2.

When I think of the word “legacy,” I normally become all melancholy ‘n’ stuff, because the word reminds me of what I was doing when those previous satellites went up and of all the people, science and stories that have influenced my life since. So this morning, on my drive to work, I tuned in to a '70s music radio station to funky disco my way out of my melancholy funkiness. (Take that, '80s music heads!)

But no, really, TOPEX/Poseidon was totally cool. (The acronym “TOPEX” comes from “TOPography EXperiment” and Poseidon is the Greek god of the sea; “Jason” is from Jason and the Argonauts, also from Greek mythology.) It launched in 1992 and was the first revolutionary precision oceanography satellite. It transformed the way we study the ocean, because the view from space is the only way to truly observe the vastness of the ocean on a global scale. Since TOPEX/Poseidon began collecting data in 1993, global sea level has risen 80 millimeters. That’s 3 inches in just over two decades. Holy moly! Now you see why it’s so important to have an uninterrupted stream of satellite data that extends far into the future.

Before TOPEX/Poseidon, there was Seasat, which was designed to find out if global satellite monitoring of Earth’s ocean was even feasible. It launched in 1978 and operated for just over 100 days. And yupity do dah, now we know that satellite observations are feasible.

Just like its predecessors, Jason-3 is a radar altimeter, an instrument that measures sea surface height by precisely knowing the satellite’s position in its orbit and by measuring the distance between itself and the top of the ocean. You see, the ocean surface is constantly changing, from waves, to tides, to El Niño, to sea level rise. As increased global warming causes more and more glacial ice and ice sheets to melt into Earth’s ocean, and as this same warming heats the ocean surface, causing the warmer water to expand and sea levels to rise higher, it’s absolutely crucial to have highly accurate, continuous global sea level measurements.

Furthermore, since the majority of Planet Earth is covered by ocean, and since water is exceptionally good at storing heat, the ocean will continue to play an enormous role in Earth’s long-term climate.

Jason-3 will launch from Vandenberg Air Force Base in July or August. I’ll be keeping you up-to-date on all of the goings on about the launch and its preparation. Meanwhile, you can find out more about Jason-3 and all of NASA's ocean surface topography missions here:

http://sealevel.jpl.nasa.gov/
http://sealevel.jpl.nasa.gov/missions/jason3/

I look forward to your comments.

- Laura

› Visit Earth Right Now to comment on this post

Jason-3 is an international partnership led by the National Oceanic and Atmospheric Administration with participation from NASA, France's Centre Nationale d'Etudes Spatiales (the French space agency) and EUMETSAT, the European Organisation for the Exploitation of Meteorological Satellites. JPL built Jason-3's radiometer, GPS and laser reflector; is procuring the launch; and will help oversee the science team, which is responsible for ensuring the quality of the data.

TAGS: EARTH, OCEANS, JASON-3, SATELLITES, CLIMATE CHANGE

READ MORE

Animated gif of Ceres rotating

Dear Emboldawned Readers,

A bold adventurer from Earth is gracefully soaring over an exotic world of rock and ice far, far away. Having already obtained a treasure trove from its first mapping orbit, Dawn is now seeking even greater riches at dwarf planet Ceres as it maneuvers to its second orbit.

The first intensive mapping campaign was extremely productive. As the spacecraft circled 8,400 miles (13,600 kilometers) above the alien terrain, one orbit around Ceres took 15 days. During its single revolution, the probe observed its new home on five occasions from April 24 to May 8. When Dawn was flying over the night side (still high enough that it was in sunlight even when the ground below was in darkness), it looked first at the illuminated crescent of the southern hemisphere and later at the northern hemisphere.

When Dawn traveled over the sunlit side, it watched the northern hemisphere, then the equatorial regions, and finally the southern hemisphere as Ceres rotated beneath it each time. One Cerean day, the time it takes the globe to turn once on its axis, is about nine hours, much shorter than the time needed for the spacecraft to loop around its orbit. So it was almost as if Dawn hovered in place, moving only slightly as it peered down, and its instruments could record all of the sights as they paraded by.

We described the plans in much more detail in March, and they executed beautifully, yielding a rich collection of photos in visible and near infrared wavelengths, spectra in visible and infrared, and measurements of the strength of Ceres' gravitational attraction and hence its mass.

To gain the same view Dawn had, simply build your own ion-propelled spaceship, voyage deep into the main asteroid belt between Mars and Jupiter, take up residence at the giant orb and look out the window. Or go to the image gallery here.

Either way, the sights are spectacular. And they have already gotten even better. As Dawn has been descending to its second mapping orbit, it paused ion-thrusting on May 16 and May 22 to take more pictures, helping navigators get a tight fix on its orbital location. We explained this technique of optical navigation earlier, but now it is slightly different. Dawn is so close to Ceres that the behemoth fills the camera's field of view. No longer charting Ceres' location relative to background stars, navigators now use distinctive features on Ceres itself. It was an indistinct, fuzzy little blob just a few months ago, but now the maps are becoming detailed and accurate. Mathematical analyses of the locations of specific landmarks in each picture allow navigators to determine where Dawn was when the picture was taken.

Let's see how this works. Suppose I gave you a picture I had taken in your house. (The last time I was there, I opted for the cover of darkness rather than a more visible demonstration of optical navigation, but we can still imagine.) Because you know the positions of the doors, windows, furniture, impact craters, paintings, etc., you could establish where I had been when I took the photo. Now that they have charted the positions of the features at Dawn's new home, navigators can do virtually the same thing.

In addition to aiding in celestial navigation, the photos provided still better views of the world Dawn traveled so long and so far to explore. Greater and greater detail is visible as Dawn orbits closer, and a tremendous variety of intriguing sights are coming into view. It may well be that the most interesting discoveries have not even been made yet, but for now, what captivates most people (and other readers as well) are the bright spots.

We have discussed them here and there in recent months, and their luminous power continues to dazzle us. What appeared initially as one fuzzy spot proved to be two smaller spots and now many even smaller regions as the focus has become sharper. Why the ground there reflects so much sunlight remains elusive. Dawn's finer examinations with its suite of sophisticated instruments in the second, third and then final mapping orbits will provide scientists with data they need to unravel this marvelous mystery. For now, the enigmatic lights present an irresistible cosmic invitation to go closer and to scrutinize this strange and wonderful world, and we are eager to accept. After all, we explore to learn, to know the unknown, and the uniquely powerful scientific method will reveal the nature of the bright areas and what they can tell us about the composition and geology of this complex dwarf planet.

Close-up of the bright spots on Ceres
This was Dawn's view on May 16, as it flew from its first mapping orbit to its second. This OpNav 8 photo was taken at an altitude of 4,500 miles (7,200 kilometers). Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› Full image and caption

After having been viewed as little more than a smudge in telescopes for more than two centuries since its discovery, Ceres now is seen as a detailed, three-dimensional world. As promised, measurements from Dawn have revised the size to be about 599 miles (963 kilometers) across at the equator. Like Earth and other planets, Ceres is oblate, or slightly wider at the equator than from pole to pole. The polar diameter is 554 miles (891 kilometers). These dimensions are impressively close to what astronomers had determined from telescopic observations and confirm Ceres to be the colossus we have described.

Before Dawn, scientists had estimated Ceres' mass to be 1.04 billion billion tons (947 billion billion kilograms). Now it is measured to be 1.03 billion billion tons (939 billion billion kilograms), well within the previous margin of error. It is an impressive demonstration of the success of science that astronomers had been able to determine the heft of that point of light so accurately. Nevertheless, even this small change of less than one percent is important for planning the rest of Dawn’s mission as it orbits closer and closer, feeling the gravitational tug ever more strongly.

Let's put this change in context. Dawn has now refined the mass, making a proportionally small adjustment of about 0.01 billion billion tons (eight billion billion kilograms). Although no more than a tweak on the overall value, it is still significantly greater than the combined mass of all asteroids visited by all other spacecraft. Ceres is so immense, so massive that even if all those asteroids were added to it, the difference would hardly even have been noticeable. This serves as another reminder that the dwarf planet really is quite unlike the millions of small asteroids that constitute the main asteroid belt. This behemoth contains about 30 percent of all the mass in that entire vast region of space. Vesta, the protoplanet Dawn orbited and studied in 2011-2012, is the second most massive resident there, holding about 8 percent of the asteroid belt's mass. Dawn by itself is exploring around 40 percent of the asteroid belt's mass!

Upon concluding its first mapping orbit, Dawn powered on its remarkable ion propulsion system on May 9 to fly down to a lower altitude where it will gain a better view. We examined the nature of the spiral paths between mapping orbits last year (and at Vesta in 2011-2012).

Dawn's orbits about Ceres
Dawn's spiral descent from its first mapping orbit (RC3) to its second (survey). The two mapping orbits are shown in green. The color of Dawn's trajectory progresses through the spectrum from blue, when it began ion-thrusting on May 9, to red, when ion-thrusting concludes on June 3. The red dashed sections show where Dawn is coasting, mostly for telecommunications. The first two coast periods include OpNav 8 and 9. Image credit: NASA/JPL-Caltech
› Larger image

In its first mapping orbit, Dawn was 8,400 miles (13,600 kilometers) high, revolving once in 15.2 days at a speed of 150 mph (240 kilometers per hour). By the time it completes this descent, the probe will be at an altitude of 2,700 miles (4,400 kilometers), orbiting Ceres every 3.1 days at 254 mph (408 kilometers per hour). (All of the mapping orbits were summarized in this table.) We have discussed that lower orbits require greater velocity to counterbalance the stronger gravitational hold.

Dawn's uniquely capable ion propulsion system, with its extraordinary combination of efficiency and gentleness, propels the ship to its new orbital destination in just under four weeks. The descent requires five revolutions, each one faster than the one before. The flight profile is complicated, and sometimes Dawn even dips below the final, planned altitude and then rises to greater heights as it flies on a path that is temporarily elliptical. The overall trend, of course, is downward. As Dawn heads for its targeted circular orbit, its maneuvering is also generally reducing the orbit period, the time required to make one complete revolution around Ceres. Indeed, if Dawn stopped thrusting now, its orbit period would be about 83 hours, or 3.5 days.

Dawn will complete ion-thrusting on June 3, but it will not be ready to begin its next science observations then. Rather, as in the other new mapping orbits, the first order of business will be for navigators to measure the new orbital parameters accurately. The flight team then will install in Dawn's main computer the details of the orbit it achieved so it will always know its location.

In addition, the intensive campaign of observations is planned to begin when the robotic explorer travels from the night side to the day side over the north pole. With the three-day orbit period, that will next occur on June 5. Controllers will take advantage of the intervening time to conduct other activities, including routine maintenance of the two reaction wheels that remain operable, although they are powered off most of the time. (Two of the four failed years ago. Dawn no longer relies on these devices to control its orientation, and it is remarkable that the mission can accomplish all of its original objectives without them. But if two do function in the final mapping orbit later this year, they will help extend the spacecraft's lifetime for bonus studies.)

We have already presented the ambitious plans for this second mapping orbit, sometimes known as "the second mapping orbit" and sometimes more succinctly and confusingly as "survey orbit." As with all four of Dawn's mapping orbits, it is designed to take the spacecraft over the poles, ensuring the best possible coverage. The ship will fly from the north pole to the south over the side of Ceres facing the sun, and then loop back to the north over the side hidden in the deep dark of night. On the day side, Dawn will aim its camera and spectrometers at the lit ground, filling its memory to capacity with the readings. On the night side, it will point its main antenna to distant Earth in order to radio its findings home. At Dawn's altitude, Ceres will appear twice as wide as the camera's view. (As illustrated in this table, it will look about the size of a soccer ball seen from a yard, or a meter, away.) But as the dwarf planet rotates on its axis and Dawn sails around in its more leisurely orbit, eventually all of the landscape will come within sight of the instruments.

Only one noteworthy change has been made in the intricate plans for survey orbit since May 2014's shocking exposé. With the observations starting on June 5, the subsequent complex orbital flight to the third mapping orbit (also known as HAMO) would have begun on June 27. As we have seen, the rapidly changing orbit in the spiral descents requires a great deal of effort by the small operations team on a rigid schedule. The capable men and women flying Dawn accomplished the maneuvers flawlessly at Vesta and are well prepared for the challenges at Ceres. The work is very demanding, however, and so, just as at Vesta, the team has built into the strategy the capability to make adjustments to align most of the tasks with a conventional work schedule. The technical plans (even including the exquisitely careful husbanding of hydrazine following the loss of the two reaction wheels) fully account for such human factors. It turns out that leaving survey orbit three days later shifts a significant amount of the following work off weekends, making it more comfortable for the team members. Three days is one complete revolution, and always extracting as much from the mission as possible, they have devised another full set of observations for an eighth orbit. As a result, survey orbit may be even more extensive and productive than originally anticipated.

What awaits Dawn in the next mapping phase? The views will be three times as sharp as in the previous orbit, and exciting new discoveries are sure to come. What answers will be revealed? And what new questions (besides this one) will arise? We will know soon, as we all share in the thrill of this grand adventure. To help you keep track of Dawn's progress as it powers its way down and then conducts further observations, your correspondent writes brief (hard to believe, isn't it?) mission status updates. And although in space no one can hear you tweet, terrestrial followers can get even more frequent updates with information he provides for Twitter @NASA_Dawn.

Dawn is 3,400 miles (5,500 kilometers) from Ceres. It is also 2.30 AU (214 million miles, or 345 million kilometers) from Earth, or 855 times as far as the moon and 2.27 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.

Dr. Marc D. Rayman
12:00 p.m. PDT May 28, 2015

TAGS: DAWN, CERES, VESTA, BRIGHT SPOTS, SPACECRAFT, MISSION

  • Marc Rayman
READ MORE

Video showing a solar filament eruptionYoutube video

“I knew the names of the planets in order before I went to kindergarten," Joan Feynman, the younger sister of the famous physicist, told me. "My father was delighted by science. My brother, of course, was Richard Feynman—gifted as hell. When I was about three or four, he taught me to add numbers. I’d add them and if I got them right, he’d give me a reward. The reward was allowing me to pull his hair. As soon as I pulled his hair he’d make a terrible face.”

Joan Feynman (left) and her brother Richard (right)
Joan Feynman (left) and her brother Richard (right). Richard Feynman image courtesy of the National Nanotechnology Initiative.

Joan and I were supposed to be discussing her work on sunspots, aurora, geomagnetism and solar winds but we kept veering off on tangents, and I’m certain it was all my fault. I was charmed by her Long Island accent and wacky sense of humor, and the fact that at 88, she holds a research position at NASA’s Jet Propulsion Laboratory.

But Joan is much more than a little sister and a funny storyteller: She’s a legit solar physicist who spent a long career analyzing the patterns of sunspot frequencies and their relationship to aurora. You see, people who live meaningful lives look deeply at the world around them—deeply enough to be moved, and sometimes deeply enough to move others. So in one afternoon, she managed to transfer a bit of her passion to me. I became mesmerized, mind-blown and in awe of solar physics.

“People have been observing aurora and drawing pictures of them for thousands of years,” she explained. “We have records of them from the year 450 C.E. to 1450 C.E. The Swedes and Norwegians had made lists of what days there were aurora and the Chinese made lists. So they knew that when there was a big aurora in China, there was a big aurora in Sweden.”

Aurora over Yukon
An aurora shines over Whitehorse, Yukon. Image courtesy of David Cartier, Sr.

Obviously I know that counting sunspot and aurora frequencies sounds completely dorky. But trust me, it feels like diving into a whole other reality when you spend time talking with Joan.

She went on to describe how “aurora, the geomagnetic currents and solar flares are part of the same process. Solar flares put out huge disturbances in the magnetic field of the solar wind and in the kind of particles that come out, and they cause the aurora.” Solar geomagnetic activity varies over 11-year and 88-year cycles. The 88-year Gleissberg cycle, a variation in amplitude of the 11-year solar cycle, is what’s captivated Joan throughout her career. “The whole point of science is to understand the mysteries you see around you,” she said. And solar activity has been part of that mystery for Joan her entire life.

Check out this story about the first time she saw an aurora as a very young child:

“One night I had already gone to bed. I was supposed to stay in bed, but my brother got permission to come in and wake me up because there was an aurora in the sky over the golf course near our home in Long Island. It looked to me like marvelous lights in the sky moving back and forth. It was very impressive to me. My brother said to me that nobody knows what that’s from, which was true in 1930.”

“I was fascinated by that aurora,” she continued. “They’re gigantic, they’re impressive; everybody runs out to see them. They were mysteries. The sky is lit up red and gold and yellow and shooting.”

When I told her that I didn’t even know they had aurora in Long Island, she said, “Auroras come down to lower latitudes when they’re very big.”

Are you starting to see how I got all jacked up just listening to her? I’ve never even seen a real live aurora. It was like sitting next to a kid with a better toy. She was taunting me, describing colors, movement and wild sky. I was hooked and jealous.

I want to see one now!

Although today we understand that aurora are caused by the interaction between the Earth’s magnetosphere and the magnetic particles in the solar wind, there are still plenty of solar mysteries to keep Joan occupied. “How does the sun do that?” She wants to know. “How does the sun manage to get a cycle of 88 years?”

For me, the question is clear: “If there were lots of them in the 1930s, and there’s an 88-year cycle, does that mean I’m going to get to see them soon?”

She answered: “It’s time for me to write the next paper on this: Are they back?”

I look forward to your comments.

- Laura

› Visit Earth Right Now to comment on this post

TAGS: AURORA, EARTH, JOAN FEYNMAN, SUN, SOLAR FLARES

READ MORE

Rainbow

When you read my blog, you’re either going to say “no way” or “finally.” And honestly, I’m kind of saying both, but definitely leaning more toward the latter.

See, normally I’m weighed down by the epic crisis known as global climate change (ugh). But this morning, I’m thinking: "finally." At last, Earth’s climate is going to get the attention it deserves. Finally, enough people are showing they care and it’s going to make a difference. Finally.

True, it’s hard to believe—even for me. But deep in my bones, I feel the tide starting to turn. (And by using that expression, I do not mean higher tides are flooding low-lying regions. That’s been going on for a while.)

What I mean is that everywhere I turn I see people paying attention to Earth’s climate. Not enough attention yet, mind you, but at least people are talking about it. And for the first time in a long time, I feel hopeful.

Maybe this optimism has something to do with the rainbow I saw while walking out in the rain over the weekend. Maybe it’s because NASA’s Global Climate Change website won both the People’s Voice and the juried Webby Awards for Best Green Site, and our Earth Now mobile app won in the Education and Reference category.

Whatever it is, I hope you’ll join me in my moment of optimism, because together—you and me—we are responsible for making this planet the kind of world we want to live in.

Bring it.

- Laura

› Visit Earth Right Now to comment on this post

TAGS: EARTH, CLIMATE CHANGE, RAINBOW

READ MORE

Goldstone Antenna Construction Model

In October 1963, the Advanced Antenna System, also known as the 210-foot (64-meter) Mars antenna, was under construction at the Goldstone Deep Space Instrumentation Facility. The site was being cleared and a foundation dug, an access road was nearing completion, and a reservoir was built to provide water during construction. Assembly of the antenna required a 200-ton guy derrick, used to lift large pieces into place. In preparation for this stage of construction, scale models of the antenna and the guy derrick were built, showing how the derrick would be anchored to the desert floor by long cables.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

TAGS: HISTORY, DSN, GOLDSTONE, ANTENNA

  • Julie Cooper
READ MORE

Anaglyph of Ceres

Let's get Dawn to business, Dear Readers,

Dawn's assignment when it embarked on its extraordinary extraterrestrial expedition in 2007 can be described quite simply: explore the two most massive uncharted worlds in the inner solar system. It conducted a spectacular mission at Vesta, orbiting the giant protoplanet for 14 months in 2011-2012, providing a wonderfully rich and detailed view. Now the sophisticated probe is performing its first intensive investigation of dwarf planet Ceres. Dawn is slowly circling the alien world of rock and ice, far from Earth and far from the sun, executing its complex operations with the prowess it has demonstrated throughout its ambitious journey.

Following an interplanetary trek of 7.5 years and 3.1 billion miles (4.9 billion kilometers), Earth's ambassador arrived in orbit on March 6, answering Ceres' two-century-old celestial invitation. With its advanced ion propulsion system and ace piloting skills, it has maneuvered extensively in orbit. Traveling mostly high over the night side of Ceres, arcing and banking, thrusting and coasting, accelerating and decelerating, climbing and diving, the spaceship flew to its first targeted orbital altitude, which it reached on April 23.

Dawn is at an altitude of about 8,400 miles (13,600 kilometers) above the mysterious terrain. This first mapping orbit is designated RC3 by the Dawn team and is a finalist in the stiff competition for the coveted title of Most Confusing Name for a Ceres Mapping Orbit. (See this table for the other contestants.) Last month we described some of the many observations Dawn will perform here, including comprehensive photography of the alien landscapes, spectra in infrared and visible wavelengths, a search for an extremely tenuous veil of water vapor and precise tracking of the orbit to measure Ceres' mass.

Illustration of Dawn’s four mapping orbits
Dawn’s four mapping orbits, shown to scale in altitude with the size of Ceres, which is about 590 miles (950 kilometers) in diameter. See this table for descriptions of the orbits and links to the activities scheduled for each. Image credit: NASA/JPL-Caltech

On the way down to this orbit, the spacecraft paused ion thrusting twice earlier this month to take pictures of Ceres, as it had seven times before in the preceding three months. (We presented and explained the schedule for photography during the three months leading up to RC3 here.) Navigators used the pictures to measure the position of Ceres against the background of stars, providing crucial data to guide the ship to its intended orbit. The Dawn team also used the pictures to learn about Ceres to aid in preparing for the more detailed observations.

We described last month, for example, adjusting the camera settings for upcoming pictures to ensure good exposures for the captivating bright spots, places that reflect significantly more sunlight than most of the dark ground. Scientists have also examined all the pictures for moons of Ceres (and many extra pictures were taken specifically for that purpose). And thanks to Dawn's pictures, everyone who longs for a perspective on the universe unavailable from our terrestrial home has been transported to a world one million times farther away than the International Space Station.

The final pictures before reaching RC3 certainly provide a unique perspective. (You can see Dawn's pictures of Ceres here.) On April 10 and April 14-15, Dawn peered down over the northern hemisphere and watched for two hours each time as Ceres turned on its axis, part of the unfamiliar cratered terrain bathed in sunlight, part in the deep dark of night. This afforded a very different view from what we are accustomed to in looking at other planets, as most depictions of planetary rotations are from nearer the equator to show more of the surface. (Indeed, Dawn acquired views like that in its February "rotation characterizations.") The latest animations of Ceres rotating beneath Dawn are powerful visual reminders that this capable interplanetary explorer really is soaring around in orbit about a distant, alien world. Following the complex flight high above the dark hemisphere, where there was nothing to see, the pictures also show us that the long night's journey into day has ended.

Animated sequence of images from NASA's Dawn spacecraft showing the northern terrain on the sunlit side of dwarf planet Ceres
This animated sequence of images from NASA's Dawn spacecraft shows northern terrain on the sunlit side of dwarf planet Ceres. Dawn took these images on April 14 and 15 from a vantage point 14,000 miles (22,000 kilometers) above Ceres' northern hemisphere. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA | › Full image and caption

Gradually descending atop its blue-green beam of high velocity xenon ions, Dawn crossed over the terminator -- the boundary between the dark side and the lit side -- on April 15 almost directly over the north pole. On April 20, on final approach to RC3, it flew over the equator at an altitude of about 8,800 miles (14,000 kilometers).

The spacecraft completed its ion thrusting shortly after 1:00 a.m. PDT on April 23. What an accomplishment this was! From the time Dawn left its final mapping orbit at Vesta in July 2012, this is where it has been headed. The escape from Vesta's gravitational clutches in September 2012, the subsequent two and a half years of interplanetary travel and entering into orbit around Ceres on March 6, as genuinely exciting and important as it was, all really occurred as consequences of targeting this particular orbit.

In September 2014, the aftereffects of being struck by cosmic radiation compelled the operations team to rapidly develop a complex new approach trajectory because they still wanted to achieve this very orbit, where Dawn is now. And the eidetic reader will note that even when the innovative flight profile was presented five months ago (with many further details in subsequent months), we explained that it would conclude on April 23. And it did! Here we are! All the descriptions and figures plus a cool video elucidated a pretty neat idea, but it's also much more than an idea: it's real!! A probe from Earth is in a mapping orbit around a faraway dwarf planet.

When it had accomplished the needed ion thrusting, the veteran space traveler turned to point its main antenna to Earth so mission controllers could prepare it for the intensive mapping observations. The first task was to measure the orbital parameters so they could be transmitted to the spacecraft.

A few readers (you and I both know who you are) may have noted that in Dawn Journals during the last year, we have described the altitude of RC3 as 8,400 miles and 13,500 kilometers. Above, however, it is 13,600 kilometers. This is not a mistake. (It would be a mistake if the previous sentence were written, "Above, howevr, it is 13,600 kilometers.") This subtle difference belies several important issues about the orbits at Ceres. Let's take a further look.

As we explained when Dawn resided at Vesta, the orbital altitude we present is always an average (and rounded off, to avoid burdening readers with too many unhelpful digits). Vesta, Ceres, Earth and other planetary bodies are not perfect spheres, so even if the spacecraft traveled in a perfect circle, its altitude would change. They all are somewhat oblate, being wider across the equator than from pole to pole. In addition, they have more localized topography. Think of flying in a plane over your planet. If the pilot maintains a constant altitude above sea level, the distance above the ground changes because the elevation of the ground itself varies, coming closer to the aircraft on mountains and farther in valleys. In addition, as it turns out, orbits are not perfect circles but tend to be slightly elliptical, as if the plane flies slightly up and down occasionally, so the altitude changes even more.

In their exquisitely detailed planning, the Dawn team has had to account for the unknown nature of Ceres itself, including its mass and hence the strength of its gravitational pull. Dawn is the only spacecraft to orbit large, massive planetary bodies that were not previously visited by flyby spacecraft. Mercury, Venus, the moon, Mars, Jupiter and Saturn all were studied by spacecraft that flew past them before subsequent missions were sent to orbit them. The first probes to each provided an initial measurement of the mass and other properties that were helpful for the arrival of the first orbiters. At Vesta and Ceres, Dawn has had to discover the essential characteristics as it spirals in closer and closer. For each phase, engineers make the best measurements they can and then use them to update the plans for the subsequent phases. As a result, however, plans are based on impressive but nevertheless imperfect knowledge of what will be encountered at lower altitudes. So even if the spacecraft executes an ion thrust flight profile perfectly, it might not wind up exactly where the plan had specified.

There are other reasons as well for small differences between the predicted and the actual orbit. One is minor variations in the thrust of the ion propulsion system, as we discussed here. Another is that every time the spacecraft fires one of its small rocket thrusters to rotate or to stabilize its orientation in the zero-gravity conditions of spaceflight, that also nudges the spacecraft, changing its orbit a little. (See here for a related example of the effect of the thrusters on the trajectory.)

The Dawn flight team has a deep understanding of all the sources of orbit discrepancies, and they always ensure that their intricate plans account for them. Even if the RC3 altitude ended up more than 300 miles (500 kilometers) higher or lower than the specified value, everything would still work just fine to yield the desired pictures and other data. In fact, the actual RC3 orbit is within 25 miles (40 kilometers), or less than one tenth what the plan was designed to accommodate, so the spacecraft achieved a virtual cosmic bullseye!

In the complex preparations on April 23, one file was not radioed to Dawn on time, so late that afternoon when the robot tried to use this file, it could not find it. It responded appropriately by running protective software, stopping its activities, entering "safe mode" and beaming a signal back to distant Earth to indicate it needed further instructions. After the request arrived in mission control at JPL, engineers quickly recognized what had occurred. That night they reconfigured the spacecraft out of safe mode and back to its normal operational configuration, and they finished off the supply of ice cream in the freezer just outside the mission control room. Although Dawn was not ready to begin its intensive observation campaign in the morning of April 24, it started later that same day and has continued to be very productive.

Dawn is a mission of exploration. And rather than be constrained by a fast flight by a target for a brief glimpse, Dawn has the capability to linger in orbit for a very long time at close range. The probe will spend more than a year conducting detailed investigations to reveal as much as possible about the nature of the first dwarf planet discovered, which we had seen only with telescopes since it was first glimpsed in 1801. The pictures Dawn has sent us so far are intriguing and entrancing, but they are only the introduction to this exotic world. They started transforming it from a smudge of light into a real, physical place and one that a sophisticated, intrepid spacecraft can even reach. Being in the first mapping orbit represents the opportunity now to begin developing a richly detailed, intimate portrait of a world most people never even knew existed. Now, finally, we are ready to start uncovering the secrets Ceres has held since the dawn of the solar system.

Dawn is 8,400 miles (13,600 kilometers) from Ceres. It is also 2.66 AU (247 million miles, or 398 million kilometers) from Earth, or 985 times as far as the moon and 2.64 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 44 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 p.m. PDT April 29, 2015

P.S. Our expert outreach team has done a beautiful job modernizing the website, and blog comments are no longer included. I appreciate all the very kind feedback, expressions of enthusiasm, interesting questions and engaging discussions that the community of Dawnophiles has posted over the last year or so. Now I will devote the time I had been spending responding to comments to providing more frequent mission status updates as well as fun and interesting tidbits you can follow on Twitter @NASA_Dawn.

TAGS: DAWN, CERES, BRIGHT SPOT, SPACECRAFT, MISSION

  • Marc Rayman
READ MORE