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 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.
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.
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.
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.)
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
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.
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.)
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.
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.
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.
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
Dear Dawnticipating Explorers,
Now orbiting high over the night side of a dwarf planet far from Earth, Dawn arrived at its new permanent residence on March 6. Ceres welcomed the newcomer from Earth with a gentle but firm gravitational embrace. The goddess of agriculture will never release her companion. Indeed, Dawn will only get closer from now on. With the ace flying skills it has demonstrated many times on this ambitious deep-space trek, the interplanetary spaceship is using its ion propulsion system to maneuver into a circular orbit 8,400 miles (13,500 kilometers) above the cratered landscape of ice and rock. Once there, it will commence its first set of intensive observations of the alien world it has traveled for so long and so far to reach.
For now, however, Dawn is not taking pictures. Even after it entered orbit, its momentum carried it to a higher altitude, from which it is now descending. From March 2 to April 9, so much of the ground beneath it is cloaked in darkness that the spacecraft is not even peering at it. Instead, it is steadfastly looking ahead to the rewards of the view it will have when its long, leisurely, elliptical orbit loops far enough around to glimpse the sunlit surface again.
Among the many sights we eagerly anticipate are those captivating bright spots. Hinted at more than a decade ago by Hubble Space Telescope, Dawn started to bring them into sharper focus after an extraordinary journey of more than seven years and three billion miles (nearly five billion kilometers). Although the spots are reflections of sunlight, they seem almost to radiate from Ceres as cosmic beacons, drawing us forth, spellbound. Like interplanetary lighthouses, their brilliant glow illuminates the way for a bold ship from Earth sailing on the celestial seas to a mysterious, uncharted port. The entrancing lights fire our imagination and remind us of the irresistible lure of exploration and the powerful anticipation of an adventure into the unknown.
As we describe below, Dawn’s extensive photographic coverage of the sunlit terrain in early May will include these bright spots. They will not be in view, however, when Dawn spies the thin crescent of Ceres in its next optical navigation session, scheduled for April 10 (as always, all dates here are in the Pacific time zone).
As the table here shows, on April 14 (and extending into April 15), Dawn will obtain its last navigational fix before it finishes maneuvering. Should we look forward to catching sight of the bright spots then? In truth, we do not yet know. The spots surely will be there, but the uncertainty is exactly where “there” is. We still have much to learn about a dwarf planet that, until recently, was little more than a fuzzy patch of light among the glowing jewels of the night sky. (For example, only last month did we determine where Ceres’north and south poles point.) Astronomers had clocked the length of its day, the time it takes to turn once on its axis, at a few minutes more than nine hours. But the last time the spots were in view of Dawn’s camera was on Feb. 19. From then until April 14, while Earth rotates more than 54 times (at 24 hours per turn), Ceres will rotate more than 140 times, which provides plenty of time for a small discrepancy in the exact rate to build up. To illustrate this, if our knowledge of the length of a Cerean day were off by one minute (or less than 0.2 percent), that would translate into more than a quarter of a turn during this period, drastically shifting the location of the spots from Dawn’s point of view. So we are not certain exactly what range of longitudes will be within view in the scheduled OpNav 7 window. Regardless, the pictures will serve their intended purpose of helping navigators establish the probe’s location in relation to its gravitational captor.
Dawn’s gradual, graceful arc down to its first mapping orbit will take the craft from the night side to the day side over the north pole, and then it will travel south. It will conclude its powered flight over the sunlit terrain at about 60 degrees south latitude. The spacecraft will finish reshaping its orbit on April 23, and when it stops its ion engine on that date, it will be in its new circular orbit, designated RC3. (We will return to the confusing names of the different orbits at Ceres below.) Then it will coast, just as the moon coasts in orbit around Earth and Earth coasts around the sun. It will take Dawn just over 15 days to complete one revolution around Ceres at this height. We had a preview of RC3 last year, and now we can take an updated look at the plans.
The dwarf planet is around 590 miles (950 kilometers) in diameter (like Earth and other planets, however, it is slightly wider at the equator than from pole to pole). At the spacecraft’s orbital altitude, it will appear to be the same size as a soccer ball seen from 10 feet (3 meters) away. Part of the basis upon which mission planners chose this distance for the first mapping campaign is that the visible disc of Ceres will just fit in the camera’s field of view. All the pictures taken at lower altitudes will cover a smaller area (but will be correspondingly more detailed). The photos from RC3 will be 3.4 times sharper than those in RC2.
There will be work to do before photography begins however. The first order of business after concluding ion thrusting will be for the flight team to perform a quick navigational update (this time, using only the radio signal) and transmit any refinements (if necessary) in Dawn’s orbital parameters, so it always has an accurate knowledge of where it is. (These will not be adjustments to the orbit but rather a precise mathematical description of the orbit it achieved.) Controllers will also reconfigure the spacecraft for its intensive observations, which will commence on April 24 as it passes over the south pole and to the night side again.
As at Vesta, even though half of each circular orbit will be over the night side of Ceres, the spacecraft itself will never enter the shadows. The operations team has carefully designed the orbits so that at Dawn’s altitude, it remains illuminated by the sun, even when the land below is not.
It may seem surprising (or even be surprising) that Dawn will conduct measurements when the ground directly beneath it is hidden in the deep darkness of night. To add to the surprise, these observations were not even envisioned when Dawn’s mission was designed, and it did not perform comparable measurements during its extensive exploration of Vesta in 2011-2012.
The measurements on the night side will serve several purposes. One of the many sophisticated techniques scientists use to elucidate the nature of planetary surfaces is to measure how much light they reflect at different angles. Over the course of the next year, Dawn will acquire tens of thousands of pictures from the day side of Ceres, when, in essence, the sun is behind the camera. When it is over the night side in RC3, carefully designed observations of the lit terrain (with the sun somewhat in front of the camera, although still at a safe angle) will significantly extend the range of angles.
In December, we described the fascinating discovery of an extremely diffuse veil of water vapor around Ceres. How the water makes its way from the dwarf planet high into space is not known. The Dawn team has devised a plan to investigate this further, even though the tiny amount of vapor was sighted long after the explorer left Earth equipped with sensors designed to study worlds without atmospheres.
It is worth emphasizing that the water vapor is exceedingly tenuous. Indeed, it is much less dense than Earth’s atmosphere at altitudes above the International Space Station, which orbits in what most people consider to be the vacuum of space. Our hero will not need to deploy its umbrella. Even comets, which are miniscule in comparison with Ceres, liberate significantly more water.
There may not even be any water vapor at all now because Ceres is farther from the sun than when the Herschel Space Observatory saw it, but if there is, detecting it will be very challenging. The best method to glimpse it is to look for its subtle effects on light passing through it. Although Dawn cannot gaze directly at the sun, it can look above the lit horizon from the night side, searching intently for faint signs of sunlight scattered by sparse water molecules (or perhaps dust lofted into space with them).
For three days in RC3 after passing over the south pole, the probe will take many pictures and visible and infrared spectra as it watches the slowly shrinking illuminated crescent and the space over it. When the spacecraft has flown to about 29 degrees south latitude over the night side, it will no longer be safe to aim its sensitive instruments in that direction, because they would be too close to the sun. With its memory full of data, Dawn will turn to point its main antenna toward distant Earth. It will take almost two days to radio its findings to NASA’s Deep Space Network. Meanwhile, the spacecraft will continue northward, gliding silently high over the dark surface.
On April 28, it will rotate again to aim its sensors at Ceres and the space above it, resuming measurements when it is about 21 degrees north of the equator and continuing almost to the north pole on May 1. By the time it turns once again to beam its data to Earth, it will have completed a wealth of measurements not even considered when the mission was being designed.
Loyal readers will recall that Dawn has lost two of its four reaction wheels, gyroscope-like devices it uses to turn and to stabilize itself. Although such a loss could be grave for some missions, the operations team overcame this very serious challenge. They now have detailed plans to accomplish all of the original Ceres objectives regardless of the condition of the reaction wheels, even the two that have not failed (yet). It is quite a testament to their creativity and resourcefulness that despite the tight constraints of flying the spacecraft differently, the team has been able to add bonus objectives to the mission.
Dawn will finish transmitting its data after its orbit takes it over the north pole and to the day side of Ceres again. For three periods during its gradual flight of more than a week over the illuminated landscape, it will take pictures (in visible and near-infrared wavelengths) and spectra. Each time, it will look down from space for a full Cerean day, watching for more than nine hours as the dwarf planet pirouettes, as if showing off to her new admirer. As the exotic features parade by, Dawn will faithfully record the sites.
It is important to set the camera exposures carefully. Most of the surface reflects nine percent of the sunlight. (For comparison, the moon reflects 12 percent on average, although as many Earthlings have noticed, there is some variation from place to place. Mars reflects 17 percent, and Vesta reflects 42 percent. Many photos seem to show that your correspondent’s forehead reflects about 100 percent.) But there are some small areas that are significantly more reflective, including the two most famous bright spots. Each spot occupies only one pixel (2.7 miles, or 4.3 kilometers across) in the best pictures so far. If each bright area on the ground is the size of a pixel, then they reflect around 40 percent of the light, providing the stark contrast with the much darker surroundings. When Dawn’s pictures show more detail, it could be that they will turn out to be even smaller and even more reflective than they have appeared so far. In RC3, each pixel will cover 0.8 miles (1.3 kilometers). To ensure the best photographic results, controllers are modifying the elaborate instructions for the camera to take pictures of the entire surface with a wider range of exposures than previously planned, providing high confidence that all dark and all bright areas will be revealed clearly.
Dawn will observe Ceres as it flies from 45 degrees to 35 degrees north latitude on May 3-4. Of course, the camera’s view will extend well north and south of the point immediately below it. (Imagine looking at a globe. Even though you are directly over one point, you can see a larger area.) The territory it will inspect will include those intriguing bright spots. The explorer will report back to Earth on May 4-5. It will perform the same observations between 5 degrees north and 5 degrees south on May 5-6 and transmit those findings on May 6-7. To complete its first global map, it will make another full set of measurements for a Cerean day as it glides between 35 degrees and 45 degrees south on May 7.
By the time it has transmitted its final measurements on May 8, the bounty from RC3 may be more than 2,500 pictures and two million spectra. Mission controllers recognize that glitches are always possible, especially in such complex activities, and they take that into account in their plans. Even if some of the scheduled pictures or spectra are not acquired, RC3 should provide an excellent new perspective on the alien world, displaying details three times smaller than what we have discerned so far.
Dawn activated its gamma ray spectrometer and neutron spectrometer on March 12, but it will not detect radiation from Ceres at this high altitude. For now, it is measuring space radiation to provide context for later measurements. Perhaps it will sense some neutrons in the third mapping orbit this summer, but its primary work to determine the atomic constituents of the material within about a yard (meter) of the surface will be in the lowest altitude orbit at the end of the year.
Dawn will conduct its studies from three lower orbital altitudes after RC3, taking advantage of the tremendous maneuverability provided by ion propulsion to spiral from one to another. We presented previews last year of each phase, and as each approaches, we will give still more up-to-date details, but now that Dawn is in orbit, let’s summarize them here. Of course, with complicated operations in the forbidding depths of space, there are always possibilities for changes, especially in the schedule. The team has developed an intricate but robust and flexible plan to extract as many secrets from Ceres as possible, and they will take any changes in stride.
Each orbit is designed to provide a better view than the one before, and Dawn will map the orb thoroughly while at each altitude. The names for the orbits – rotation characterization 3 (RC3); survey; high altitude mapping orbit (HAMO); and low altitude mapping orbit (LAMO) – are based on ancient ideas, and the origins are (or should be) lost in the mists of time. Readers should avoid trying to infer anything at all meaningful in the designations. After some careful consideration, your correspondent chose to use the same names the Dawn team uses rather than create more helpful descriptors for the purposes of these blogs. That ensures consistency with other Dawn project communications. After all, what is important is not what the different orbits are called but rather what amazing new discoveries each one enables.
The robotic explorer will make many kinds of measurements with its suite of powerful instruments. As one indication of the improving view, this table includes the resolution of the photos, and the ever finer detail may be compared with the pictures during the approach phase. For another perspective, we extend the soccer ball analogy above to illustrate how large Ceres will appear to be from the spacecraft’s orbital vantage point.
As Dawn orbits Ceres, together they orbit the sun. Closer to the master of the solar system, Earth (with its own retinue, including the moon and many artificial satellites) travels faster in its heliocentric orbit because of the sun’s stronger gravitational pull at its location. In December, Earth was on the opposite side of the sun from Dawn, and now the planet’s higher speed is causing their separation to shrink. Earth will get closer and closer until July 22, when it will pass on the inside track, and the distance will increase again.
In the meantime, on April 12, Dawn will be equidistant from the sun and Earth. The spacecraft will be 2.89 AU or 269 million miles (433 million kilometers) from both. At the same time, Earth will be 1.00 AU or 93.2 million miles (150 million kilometers) from the sun.
It will be as if Dawn is at the tip of a giant celestial arrowhead, pointing the way to a remarkable solar system spectacle. The cosmos should take note! Right there, a sophisticated spaceship from Earth is gracefully descending on a blue-green beam of xenon ions. Finally, the dwarf planet beneath it, a remote remnant from the dawn of the solar system, is lonely no more. Almost 4.6 billion years after it formed, and 214 years after inquisitive creatures on a distant planet first caught sight of it, a mysterious world is still welcoming the new arrival. And as Dawn prepares to settle into its first close orbit, ready to discover secrets Ceres has kept for so long, everyone who shares in the thrill of this grand and noble adventure eagerly awaits its findings. Together, we look forward to the excitement of new knowledge, new insight and new fuel for our passionate drive to explore the universe.
Dawn is 35,000 miles (57,000 kilometers) from Ceres, or 15 percent of the average distance between Earth and the moon. It is also 3.04 AU (282 million miles, or 454 million kilometers) from Earth, or 1,120 times as far as the moon and 3.04 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 p.m. PDT March 31, 2015
Dear Unprecedawnted Readers,
Since its discovery in 1801, Ceres has been known as a planet, then as an asteroid, and later as a dwarf planet. Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls it “home.”
Earth’s robotic emissary arrived at about 4:39 a.m. PST today. It will remain in residence at the alien world for the rest of its operational life, and long, long after.
Before we delve into this unprecedented milestone in the exploration of space, let’s recall that even before reaching orbit, Dawn started taking pictures of its new home. Last month we presented the updated schedule for photography. Each activity to acquire images (as well as visible spectra and infrared spectra) has executed smoothly and provided us with exciting and tantalizing new perspectives.
While there are countless questions about Ceres, the most popular now seems to be what the bright spots are. It is impossible not to be mesmerized by what appear to be glowing beacons, shining out across the cosmic seas from the uncharted lands ahead. But the answer hasn’t changed: we don’t know. There are many intriguing speculations, but we need more data, and Dawn will take photos and myriad other measurements as it spirals closer and closer during the year. For now, we simply know too little.
For example, some people ask if those spots might be lights from an alien city. That’s ridiculous! At this early stage, how could Dawn determine what kinds of groupings Cereans live in? Do they even have cities? For all we know, they may live only in rural communities, or perhaps they only have large states.
What we already know is that in more than 57 years of space exploration, Dawn is now the only spacecraft ever to orbit two extraterrestrial destinations. A true interplanetary spaceship, Dawn left Earth in Sep. 2007 and traveled on its own independent course through the solar system. It flew past Mars in Feb. 2009, robbing the red planet of some of its own orbital energy around the sun. In July 2011, the ship entered orbit around the giant protoplanet Vesta, the second most massive object in the main asteroid belt between Mars and Jupiter. (By the way, Dawn’s arrival at Vesta was exactly one Vestan year ago earlier this week.) It conducted a spectacular exploration of that fascinating world, showing it to be more closely related to the terrestrial planets (including Earth, home to many of our readers) than to the typical objects people think of as asteroids. After 14 months of intensive operations at Vesta, Dawn climbed out of orbit in Sep. 2012, resuming its interplanetary voyage. Today it arrived at its final destination, Ceres, the largest object between the sun and Pluto that had not previously been visited by a spacecraft. (Fortunately, New Horizons is soon to fly by Pluto. We are in for a great year!)
What was the scene like at JPL for Dawn’s historic achievement? It’s easy to imagine the typical setting in mission control. The tension is overwhelming. Will it succeed or will it fail? Anxious people watch their screens, monitoring telemetry carefully, frustrated that there is nothing more they can do now. Nervously biting their nails, they are thinking of each crucial step, any one of which might doom the mission to failure. At the same time, the spacecraft is executing a bone-rattling, whiplash-inducing burn of its main engine to drop into orbit. When the good news finally arrives that orbit is achieved, the room erupts! People jump up and down, punch the air, shout, tweet, cry, hug and feel the tremendous relief of overcoming a huge risk. You can imagine all that, but that’s not what happened.
If you had been in Dawn mission control, the scene would have been different. You would mostly be in the dark. (For your future reference, the light switches are to the left of the door.) The computer displays would be off, and most of the illumination would be from the digital clock and the string of decorative blue lights that indicate the ion engine is scheduled to be thrusting. You also would be alone (at least until JPL Security arrived to escort you away, because you were not cleared to enter the room, and, for that matter, how did you get past the electronic locks?). Meanwhile, most of the members of the flight team were at home and asleep! (Your correspondent was too, rare though that is. When Dawn entered orbit around Vesta, he was dancing. Ceres’ arrival happened to be at a time less conducive to consciousness.)
Why was such a significant event treated with somnolence? It is because Dawn has a unique way of entering orbit, which is connected with the nature of the journey itself. We have discussed some aspects of getting into orbit before (with this update to the nature of the approach trajectory). Let’s review some of it here.
It may be surprising that prior to Dawn, no spacecraft had even attempted to orbit two distant targets. Who wouldn’t want to study two alien worlds in detail, rather than, as previous missions, either fly by one or more for brief encounters or orbit only one? A mission like Dawn’s is an obvious kind to undertake. It happens in science fiction often: go somewhere, do whatever you need to do there (e.g., beat someone up or make out with someone) and then boldly go somewhere else. However, science fact is not always as easy as science fiction. Such missions are far, far beyond the capability of conventional propulsion.
Deep Space 1 (DS1) blazed a new trail with its successful testing of ion propulsion, which provides 10 times the efficiency of standard propulsion, showing on an operational interplanetary mission that the advanced technology really does work as expected. (This writer was fortunate enough to work on DS1, and he even documented the mission in a series of increasingly wordy blogs. But he first heard of ion propulsion from the succinct Mr. Spock and subsequently followed its use by the less logical Darth Vader.)
Dawn’s ambitious expedition would be truly impossible without ion propulsion. (For a comparison of chemical and ion propulsion for entering orbit around Mars, an easier destination to reach than either Vesta or Ceres, visit this earlier log.) So far, our advanced spacecraft has changed its own velocity by 23,800 mph (38,400 kilometers per hour) since separating from its rocket, far in excess of what any other mission has achieved propulsively. (The previous record was held by DS1.)
Dawn is exceptionally frugal in its use of xenon propellant. In this phase of the mission, the engine expends only a quarter of a pound (120 grams) per day, or the equivalent of about 2.5 fluid ounces (75 milliliters) per day. So although the thrust is very efficient, it is also very gentle. If you hold a single sheet of paper in your hand, it will push on your hand harder than the ion engine pushes on the spacecraft at maximum thrust. At today’s throttle level, it would take the distant explorer almost 11 days to accelerate from zero to 60 mph (97 kilometers per hour). That may not evoke the concept of a drag racer. But in the zero-gravity, frictionless conditions of spaceflight, the effect of this whisper-like thrust can build up. Instead of thrusting for 11 days, if we thrust for a month, or a year, or as Dawn already has, for more than five years, we can achieve fantastically high velocity. Ion propulsion delivers acceleration with patience.
Most spacecraft coast most of the time, following their repetitive orbits like planets do. They may use the main engine for a few minutes or perhaps an hour or two throughout the entire mission. With ion propulsion, in contrast, the spacecraft may spend most of its time in powered flight. Dawn has flown for 69% of its time in space emitting a cool blue-green glow from one of its ion engines. (With three ion engines, Dawn outdoes the Star Wars TIE (twin ion engine) fighters.)
The robotic probe uses its gentle thrust to gradually reshape its path through space rather than simply following the natural course that a planet would. After it escaped from Vesta’s gravitational clutches, it slowly spiraled outward from the sun, climbing the solar system hill, making its heliocentric orbit more and more and more like Ceres’. By the time it was in the vicinity of the dwarf planet today, both were traveling around the sun at more than 38,600 mph (62,100 kilometers per hour). Their trajectories were nearly identical, however, so the difference in their speeds was only 100 mph (160 kilometers per hour), or less than 0.3 percent of the total. Flying like a crackerjack spaceship pilot, Dawn elegantly used the light touch of its ion engine to be at a position and velocity that it could ease gracefully into orbit. At a distance of 37,700 miles (60,600 kilometers), Ceres reached out and tenderly took the newcomer from Earth into its permanent gravitational embrace.
If you had been in space watching the event, you would have been cold, hungry and hypoxic. But it would not have looked much different from the 1,885 days of ion thrust that had preceded it. The spacecraft was perched atop its blue-green pillar of xenon ions, patiently changing its course, as it does for so much of quiet cruise. But now, at one moment it was flying too fast for Ceres’ gravity to hang on to it, and the next moment it had slowed just enough that it was in orbit. Had it stopped thrusting at that point, it would have continued looping around the dwarf planet. But it did not stop. Instead, it is working now to reshape its orbit around Ceres. As we saw in November, its orbital acrobatics first will take it up to an altitude of 47,000 miles (75,000 kilometers) on March 19 before it swoops down to 8,400 miles (13,500 kilometers) on April 23 to begin its intensive observations in the orbit designated RC3.
In fact, Dawn’s arrival today really is simply a consequence of the route it is taking to reach that lower orbit next month. Navigators did not aim for arriving today. Rather, they plotted a course that began at Vesta and goes to RC3 (with a new design along the way), and it happens that the conditions for capture into orbit occurred this morning. As promised last month, we present here a different view of the skillful maneuvering by this veteran space traveler.
If Dawn had stopped thrusting before Ceres could exert its gravitational control, it wouldn’t have flown very far away. The spacecraft had already made their paths around the sun very similar, and the ion propulsion system provides such exceptional flexibility to the mission that controllers could have guided it into orbit some other time. This was not a one-time, all-or-nothing event.
So the flight team was not tense. They had no need to observe it or make a spectacle out of it. Mission control remained quiet. The drama is not in whether the mission will succeed or fail, in whether a single glitch could cause a catastrophic loss, in whether even a tiny mistake could spell doom. Rather, the drama is in the opportunity to unveil the wonderful secrets of a fascinating relict from the dawn of the solar system more than 4.5 billion years ago, a celestial orb that has beckoned for more than two centuries, the first dwarf planet discovered.
Dawn usually flies with its radio transmitter turned off (devoting its electricity instead to the power-hungry ion engine), and so it entered orbit silently. As it happened, a routine telecommunications session was scheduled about an hour after attaining orbit, at 5:36 a.m. PST. (It’s only coincidence it was that soon. At Vesta, it was more than 25 hours between arrival and the next radio contact.) For primary communications, Dawn pauses thrusting to point its main antenna to Earth, but other times, as in this case, it is programmed to use one of its auxiliary antennas to transmit a weaker signal without stopping its engine, whispering just enough for engineers to verify that it remains healthy.
The Deep Space Network’s exquisitely sensitive 230-foot (70-meter) diameter antenna in Goldstone, Calif., picked up the faint signal from across the solar system on schedule and relayed it to Dawn mission control. One person was in the room (and yes, he was cleared to enter). He works with the antenna operator to ensure the communications session goes smoothly, and he is always ready to contact others on the flight team if any anomalies arise. In this case, none did, and it was a quiet morning as usual. The mission director checked in with him shortly after the data started to trickle in, and they had a friendly, casual conversation that included discussing some of the telemetry that indicated the spacecraft was still performing its routine ion thrusting. The determination that Dawn was in orbit was that simple. Confirming that it was following its flight plan was all that was needed to know it had entered orbit. This beautifully choreographed celestial dance is now a pas de deux.
As casual and tranquil as all that sounds, and as logical and systematic as the whole process is, the reality is that the mission director was excited. There was no visible hoopla, no audible fanfare, but the experience was powerful fuel for the passionate fires that burn within.
As soundlessly as a spacecraft gliding through the void, the realization emerges …
Dawn made it!!
It is in orbit around a distant world!!
Yes, it’s clear from the technical details, but it is more intensely reflected in the silent pounding of a heart that has spent a lifetime yearning to know the cosmos. Years and years of hard work devoted to this grand undertaking, constant hopes and dreams and fears of all possible futures, uncounted challenges (some initially appearing insurmountable) and a seeming infinitude of decisions along the way from early concepts through a real interplanetary spacecraft flying on an ion beam beyond the sun.
And then, a short, relaxed chat over a few bits of routine data that report the same conditions as usual on the distant robot. But today they mean something different.
They mean we did it!!
Everyone on the team will experience the news that comes in a congratulatory email in their own way, in the silence and privacy of their own thoughts. But it means the same to everyone.
We did it!!
And it’s not only the flight team. Humankind!! With our relentless curiosity, our insatiable hunger for knowledge, our noble spirit of adventure, we all share in the experience of reaching out from our humble home to the stars.
Together, we did it!!!
It was a good way to begin the day. It was Dawn at Ceres.
Let’s bring into perspective the cosmic landscape on which this remarkable adventure is now taking place. Imagine Earth reduced to the size of a soccer ball. On this scale, the International Space Station would orbit at an altitude of a bit more than one-quarter of an inch (seven millimeters). The moon would be a billiard ball almost 21 feet (6.4 meters) away. The sun, the conductor of the solar system orchestra, would be 79 feet (24 meters) across at a distance of 1.6 miles (2.6 kilometers). But even more remote, Dawn would be 5.3 miles (8.6 kilometers) away. (Just a few months ago, when the spacecraft was on the opposite side of the sun from Earth, it would have been more than six miles, or almost 10 kilometers, from from the soccer ball.) Tremendously far now from its erstwhile home, it would be only a little over a yard (a meter) from its new residence. (By the end of this year, Dawn will be slightly closer to it than the space station is to Earth, a quarter of an inch, or six millimeters.) That distant world, Ceres, the largest object between Mars and Jupiter, would be five-eighths of an inch (1.6 centimeters) across, about the size of a grape. Of course a grape has a higher water content than Ceres, but we can be sure that exploring this intriguing world of rock and ice will be much sweeter!
As part of getting to know its new neighborhood, Dawn has been hunting for moons of Ceres. Telescopic studies had not revealed any, but if there were a moon smaller than about half a mile (one kilometer), it probably would not have been discovered. The spacecraft’s unique vantage point provides an opportunity to look for any that might have escaped detection. Many pictures have been taken specifically for this purpose, and scientists scrutinize them and all of the other photographs for any indication of moons. While the search will continue, so far, no picture has shown evidence of companions orbiting Ceres.
And yet we know that as of today, Ceres most certainly does have one. Its name is Dawn!
Dawn is 37,800 miles (60,800 kilometers) from Ceres, or 16 percent of the average distance between Earth and the moon. It is also 3.33 AU (310 million miles, or 498 million kilometers) from Earth, or 1,230 times as far as the moon and 3.36 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 a.m. PST March 6, 2015
Dear Fine and Dawndy Readers,
The Dawn spacecraft is performing flawlessly as it conducts the first exploration of the first dwarf planet. Each new picture of Ceres reveals exciting and surprising new details about a fascinating and enigmatic orb that has been glimpsed only as a smudge of light for more than two centuries. And yet as that fuzzy little blob comes into sharper focus, it seems to grow only more perplexing.
Dawn is showing us exotic scenery on a world that dates back to the dawn of the solar system, more than 4.5 billion years ago. Craters large and small remind us that Ceres lives in the rough and tumble environment of the main asteroid belt between Mars and Jupiter, and collectively they will help scientists develop a deeper understanding of the history and nature not only of Ceres itself but also of the solar system.
Even as we discover more about Ceres, some mysteries only deepen. It certainly does not require sophisticated scientific insight to be captivated by the bright spots. What are they? At this point, the clearest answer is that the answer is unknown. One of the great rewards of exploring the cosmos is uncovering new questions, and this one captures the imagination of everyone who gazes at the pictures sent back from deep space.
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Other intriguing features newly visible on the unfamiliar landscape further assure us that there will be much more to see and to learn -- and probably much more to puzzle over -- when Dawn flies in closer and acquires new photographs and myriad other measurements. Over the course of this year, as the spacecraft spirals to lower and lower orbits, the view will continue to improve. In the lowest orbit, the pictures will display detail well over one hundred times finer than the RC2 pictures returned a few days ago (and shown below). Right now, however, Dawn is not getting closer to Ceres. On course and on schedule for entering orbit on March 6, Earth's robotic ambassador is slowly separating from its destination.
"Slowly" is the key. Dawn is in the vicinity of Ceres and is not leaving. The adventurer has traveled more than 900 million miles (1.5 billion kilometers) since departing from Vesta in 2012, devoting most of the time to using its advanced ion propulsion system to reshape its orbit around the sun to match Ceres' orbit. Now that their paths are so similar, the spacecraft is receding from the massive behemoth at the leisurely pace of about 35 mph (55 kilometers per hour), even as they race around the sun together at 38,700 mph (62,300 kilometers per hour). The probe is expertly flying an intricate course that would be the envy of any hotshot spaceship pilot. To reach its first observational orbit -- a circular path from pole to pole and back at an altitude of 8,400 miles (13,500 kilometers) -- Dawn is now taking advantage not only of ion propulsion but also the gravity of Ceres.
On Feb. 23, the spacecraft was at its closest to Ceres yet, only 24,000 miles (less than 39,000 kilometers), or one-tenth of the separation between Earth and the moon. Momentum will carry it farther away for a while, so as it performs the complex cosmic choreography, Dawn will not come this close to its permanent partner again for six weeks. Well before then, it will be taken firmly and forever into Ceres' gentle gravitational hold.
The photographs Dawn takes during this approach phase serve several purposes. Besides fueling the fires of curiosity that burn within everyone who looks to the night sky in wonder or who longs to share in the discoveries of celestial secrets, the images are vital to engineers and scientists as they prepare for the next phase of exploration.
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The primary purpose of the pictures is for "optical navigation" (OpNav), to ensure the ship accurately sails to its planned orbital port. Dawn is the first spacecraft to fly into orbit around a massive solar system world that had not previously been visited by a spacecraft. Just as when it reached its first deep-space target, the fascinating protoplanet Vesta, mission controllers have to discover the nature of the destination as they proceed. They bootstrap their way in, measuring many characteristics with increasing accuracy as they go, including its location, its mass and the direction of its rotation axis.
Let's consider this last parameter. Think of a spinning ball. (If the ball is large enough, you could call it a planet.) It turns around an axis, and the two ends of the axis are the north and south poles. The precise direction of the axis is important for our mission because in each of the four observation orbits (previews of which were presented in February, May, June and August), the spacecraft needs to fly over the poles. Polar orbits ensure that as Dawn loops around, and Ceres rotates beneath it every nine hours, the explorer eventually will have the opportunity to see the entire surface. Therefore, the team needs to establish the location of the rotation axis to navigate to the desired orbit.
We can imagine extending the rotation axis far outside the ball, even all the way to the stars. Current residents of Earth, for example, know that their planet's north pole happens to point very close to a star appropriately named Polaris (or the North Star), part of an asterism known as the Little Dipper in the constellation Ursa Minor (the Little Bear). The south pole, of course, points in exactly the opposite direction, to the constellation Octans (the Octant), but is not aligned with any salient star.
With their measurements of how Ceres rotates, the team is zeroing in on the orientation of its poles. We now know that residents of (and, for that mater, visitors to) the northern hemisphere there would see the pole pointing toward an unremarkable region of the sky in Draco (the Dragon). Those in the southern hemisphere would note the pole pointing toward a similarly unimpressive part of Volans (the Flying Fish). (How appropriate it is that that pole is directed toward a constellation with that name will be known only after scientists advance their understanding of the possibility of a subsurface ocean at Ceres.)
The orientation of Ceres'; axis proves convenient for Dawn's exploration. Earthlings are familiar with the consequences of their planet's axis being tilted by about 23 degrees. Seasons are caused by the annual motion of the sun between 23 degrees north latitude and 23 degrees south. A large area around each pole remains in the dark during winter. Vesta's axis is tipped 27 degrees, and when Dawn arrived, the high northern latitudes were not illuminated by the sun. The probe took advantage of its extraordinary maneuverability to fly to a special mapping orbit late in its residence there, after the sun had shifted north. That will not be necessary at Ceres. That world's axis is tipped at a much smaller angle, so throughout a Cerean year (lasting 4.6 Earth years), the sun stays between 4 degrees north latitude and 4 degrees south. Seasons are much less dramatic. Among Dawn's many objectives is to photograph Ceres. Because the sun is always near the equator, the illumination near the poles will change little. It is near the beginning of southern hemisphere winter on Ceres now, but the region around the south pole hidden in hibernal darkness is tiny. Except for possible shadowing by local variations in topography (as in deep craters), well over 99 percent of the dwarf planet's terrain will be exposed to sunlight each day.
Guiding Dawn from afar, the operations team incorporates the new information about Ceres into occasional updates to the flight plan, providing the spacecraft with new instructions on the exact direction and throttle level to use for the ion engine. As they do so, subtle aspects of the trajectory change. Last month we described the details of the plan for observing Ceres throughout the four-month approach phase and predicted that some of the numbers could change slightly. So, careful readers, for your convenience, here is the table from January, now with minor updates.
|Beginning of activity in Pacific Time zone||Distance from Dawn to Ceres in miles (kilometers)||Ceres diameter in pixels||Resolution in miles (kilometers) per pixel||Resolution compared to Hubble||Illuminated portion of disk||Activity|
|Dec 1, 2014||740,000
|Jan 13, 2015||238,000
In addition to changes based on discoveries about the nature of Ceres, some changes are dictated by more mundane considerations (to the extent that there is anything mundane about flying a spacecraft in the vicinity of an alien world more than a thousand times farther from Earth than the moon). For example, to accommodate changes in the schedule for the use of the Deep Space Network, some of the imaging sessions shifted by a few hours, which can make small changes in the corresponding views of Ceres.
The only important difference between the table as presented in January and this month, however, is not to be found in the numbers. It is that OpNav 3, RC1 and RC2 are now in the past, each having been completed perfectly.
As always, if you prefer to save yourself the time and effort of the multi-billion-mile (multi-billion-kilometer) interplanetary journey to Ceres, you can simply go here to see the latest views from Dawn. (The Dawn project is eager to share pictures promptly with the public. The science team has the responsibility of analyzing and interpreting the images for scientific publication. The need for accuracy and scientific review of the data slows the interpretation and release of the pictures. But just as with all of the marvelous findings from Vesta, everything from Ceres will be available as soon as practicable.)
In November we delved into some of the details of Dawn's graceful approach to Ceres, and last month we considered how the trajectory affected the scene presented to Dawn's camera. Now that we have updated the table, we can enhance a figure from both months that showed the craft's path as it banks into orbit and maneuvers to its first observational orbit. (As a reminder, the diagram illustrates only two of the three dimensions of the ship's complicated route. Another diagram in November showed another perspective, and we will include a different view next month.)
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We can zoom out to see where the earlier OpNavs were.
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As the table and figures indicate, in OpNav 6, when Ceres and the sun are in the same general direction from Dawn's vantage point, only a small portion of the illuminated terrain will be visible. The left side of Ceres will be in daylight, and most of the hemisphere facing the spacecraft will be in the darkness of night. To get an idea of what the shape of the crescent will be, terrestrial readers can use the moon on March 16. It will be up much of the day, setting in the middle of the afternoon, and it will be comparable to the crescent Dawn will observe on April 10. (Of course, the exact shape will depend on your observing location and what time you look, but this serves as a rough preview.) Fortunately, our spacecraft does not have to contend with bad weather, but you might, so we have generously scheduled a backup opportunity for you. The moon will be new on March 20, and the crescent on March 23 will be similar to what it was on March 16. It will rise in the mid morning and be up until well after the sun sets.
Photographing Ceres as it arcs into orbit atop a blue-green beam of xenon ions, setting the stage for more than a year of detailed investigations with its suite of sophisticated sensors, Dawn is sailing into the history books. No spacecraft has reached a dwarf planet before. No spacecraft has orbited two extraterrestrial destinations before. This amazing mission is powered by the insatiable curiosity and extraordinary ingenuity of creatures on a planet far, far away. And it carries all of them along with it on an ambitious journey that grows only more exciting as it continues. Humankind is about to witness scenes never before seen and perhaps never even imagined. Dawn is taking all of us on a daring adventure to a remote and unknown part of the cosmos. Prepare to be awed.
Dawn is 24,600 miles (39,600 kilometers) from Ceres, or 10 percent of the average distance between Earth and the moon. It is also 3.42 AU (318 million miles, or 512 million kilometers) from Earth, or 1,330 times as far as the moon and 3.46 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
7:00 a.m. PST February 25, 2015
Pardawn Me, Dear Readers,
Far away from Earthlings who look forward to a new year, Dawn looks forward to a new world. On the far side of the sun, the interplanetary explorer is closing in on Ceres, using its advanced ion propulsion system to match solar orbits with the dwarf planet.
Since breaking out of orbit around the giant protoplanet Vesta in September 2012, the spaceship has patiently flown in interplanetary cruise. That long mission phase is over, and now Dawn is starting the Ceres chapter of its extraordinary extraterrestrial expedition. Configured for its approach phase, the craft is following a new and carefully designed course described in detail last month. In March it will slip ever so gracefully into orbit for an ambitious and exciting exploration of the alien world ahead.
Over the past year, we have provided previews of the major activities during all the phases of Dawn’s mission at Ceres. This month, let’s take a look at Ceres itself, an intriguing and mysterious orb that has beckoned for more than two centuries. Now, finally, after so long, Earth is answering the cosmic invitation, and an ambassador from our planet is about to take up permanent residence there. Over the course of Dawn’s grand adventure, our knowledge will rocket far, far beyond all that has been learned before.
There can be two accounts of Ceres: its own history, which dates back to near the dawn of the solar system almost 4.6 billion years ago, and its history in the scope of human knowledge, which is somewhat shorter. Both are rich topics, with much more than we can cover here (or in the first log for this entire mission), but let’s touch on a few tidbits. We begin with the latter history.
In 1800, the known solar system contained seven planets: Mercury, Venus, Earth (home to some of our readers), Mars, Jupiter, Saturn and Uranus. This reflected a new and sophisticated scientific understanding, because Uranus had first been noticed in telescopes not long before, in 1781. (The other planets had been known to ancient sky watchers.) Even before William Herschel’s fortuitous sighting of a planet beyond Saturn, astronomers had wondered about the gap between Mars and Jupiter and speculated about the possibility of a planet there. Although some astronomers had searched, their efforts had not yielded a new planet.
Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, but he spotted an unfamiliar dot of light that moved slowly among the stars. He named it for Ceres, the Roman goddess of agriculture, and if you had cereal this morning, you have already had an etymological connection with the goddess.
The Dawn project worked with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon and broadens Piazzi’s theme. Craters will be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.
Because Ceres was fainter than the other known planets, it was evident that it was smaller. Nevertheless, many astronomers considered it to be a planet too.
It is worth noting the significance of this. Modern astronomy had chanced upon only one other planet, so Piazzi’s discovery was A Big Deal. When a new chemical element was found a couple of years later, it was named cerium in tribute to the new planet Ceres. (Uranus had been similarly honored with the 1789 naming of uranium. That element’s peculiar property of emitting radiation would not be known for another century.)
In the six years following the discovery of Ceres, three more bodies were detected orbiting between Mars and Jupiter. (One of them is Vesta, now known in spectacular detail thanks to Dawn’s extensive exploration in 2011-2012.) There then ensued a gap of more than 38 years before another was found, so for well over a generation, the sun’s family of planets was unchanged.
So if you had been reading about all this 200 years ago, there would have been at least two important differences from now. One is that your Internet connection would have been considerably slower. The other is that you might have learned in school or elsewhere that Ceres was a planet.
In 1846, a planet was discovered beyond Uranus, and we call it Neptune. Nothing else of comparable size has subsequently been seen in our solar system.
With scientific knowledge and technology progressing in the middle of the nineteenth century, new objects were glimpsed between Mars and Jupiter. As more and more were seen over the years, what we now know as the main asteroid belt was gradually recognized. Terminology changed too. One of the great strengths of science is that it advances, and sometimes we have to modify our vocabulary to reflect the improved, refined view of the universe.
By the time Pluto was sighted in 1930, Ceres had long been known as a “minor planet” and an “asteroid.” For a while thereafter, Pluto enjoyed planetary status similar to what Ceres had had. In fact, in 1940, scientists named two more additions to the periodic table of the elements neptunium and plutonium. While the histories are not identical, there is a certain parallel, with more and more objects in Pluto’s part of the solar system later being found. Terminology changed again: Pluto was subsumed into the new category of “dwarf planets” defined by the IAU in 2006. Ceres was the first body to be discovered that met the criteria established by the IAU, and Pluto was the second. (Spacecraft are now on their way to both dwarf planets: Dawn to orbit Ceres 214 years after its discovery and the wonderful New Horizons mission to fly past Pluto 85 years after it was found.)
We discussed this new nomenclature in some detail shortly after it was adopted. We understand that the designation then, as now, is controversial among some scientists and the public, and there are strong emotions on this topic. We will not delve into it here (nor in the blog comments below), preferring instead to focus on the extraordinary successes of science, the great power of the scientific method and the thrill of bold adventures far from home. The Dawn team remains both unperturbed and confident in what to call this intriguing and alluring world: we call it “Ceres.” And our goal is to develop that faint smudge of light amidst the stars into a richly detailed portrait.
One of the advances of science was the recognition that Ceres really is entirely different from typical residents of the main asteroid belt. It is a colossus! There are millions upon millions of asteroids, and yet Ceres itself contains roughly 30 percent of the mass in that entire vast region of space. By the way, Vesta, the second most massive body there, constitutes about eight percent of the asteroid belt’s mass. It is remarkable that Dawn will single-handedly explore around 40 percent of the asteroid belt’s mass.
With an equatorial diameter of about 605 miles (975 kilometers), a value that Dawn will refine very soon, Ceres is the largest body between the sun and Pluto that a spacecraft has not yet visited. It is occasionally described as being comparable in size to Texas, which is like comparing a basketball to a flat sheet of paper. Ceres has a surface area 38 percent of that of the continental United States, or more than four times the area of Texas. (Nevertheless, until Dawn shows evidence to the contrary, we will assume Texas has more rodeos.) It is nearly a third of the area of Europe and larger than the combined lands of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. Such a large place offers the promise of tremendous diversity and many marvelous and exciting sights to behold. Earth is about to be introduced to a fascinating new world.
How did Ceres come to be? And why is that being phrased as a question instead of a more declarative introduction to the history and nature of this dwarf planet? For that matter, why is this paragraph composed exclusively of questions? At least this sentence isn’t a question, right? OK, really, shouldn’t we stay more on topic?
At the dawn of the solar system almost 4.6 billion years ago, the young sun was surrounded by a swirling cloud of dust and gas. Sometimes some particles would happen to hit and stick together. Then more and more and more particles would stick to them, and eventually these agglomerations would grow so large that their gravity would pull in even more material. It was through mechanisms like this that the planets formed.
But when massive Jupiter developed, its powerful gravity terminated the growth of objects nearby, leaving bits and pieces as asteroids. Ceres and Vesta, already sizable by then, might have grown to become even larger, each incorporating still more of the nearby material, had Jupiter not deprived them of such an opportunity. Not having made it to full planetary proportions, Ceres and Vesta are known as protoplanets, and studying them provides scientists with insight into the largest building blocks of planets and into worlds that are intriguing in their own rights.
Ceres apparently formed far enough from the sun under conditions cool enough for it to hang on to water molecules. Indeed, scientists have good reason to believe that water (mostly in the form of ice) may make up an astonishing 30 percent of its mass. Ceres may contain more water than Mars or any other body in the inner solar system except Earth. (Comets, of course, have high proportions of water too, but they are so minuscule compared to this behemoth that each one harbors a quite negligible amount of water when measured against Ceres’ huge inventory.)
Although some of the moons of the outer planets also are ice and rock, and they display very interesting characteristics to the impressive and capable spacecraft that have flown past (in some cases repeatedly, as the craft orbited the host planet), no probe has had the capability to linger in orbit around any of them. Dawn’s in-depth exploration of Ceres will yield more detailed and complete views than we have obtained of any icy moon.
Radioactive elements incorporated into Ceres when it was forming would supply it with some heat, and its great bulk would provide thermal insulation, so it would take a very long time for the heat to escape into space. The sun, faraway though it is, adds still more heat. As a result, there may be some water warm enough to be liquid. (The concentration of any chemical impurities in the water that affect its freezing point, as salt does, may make an important difference in how much is liquid.) This distant, alien world may have lakes or even oceans of liquid water deep underground. What a fantastic possibility!
There will be no liquid on the frigid surface. Even ice on the surface, exposed to the cold vacuum of space, would sublimate before long. But ice could be just beneath the surface, perhaps well less than a yard (a meter) deep.
Ceres then may have a thin, dusty crust over a mantle rich in ice that might be more than 60 miles (100 kilometers) thick. Its warmer core is likely composed mostly of rock.
As heat dissipated from Ceres’ interior over the eons, it may have undergone convection, with the warmer material rising and cooler material sinking very slowly. This is reminiscent of what occurs in pot of heated water and in Earth’s interior. Even if it did occur at some time in Ceres’ history, it probably is not happening any longer, as too much heat would have been lost by now, so there would not be enough left to power the upward movement of warm material. But the convective process might have written its signature in structures or minerals left behind when ice sublimated after being pushed to the surface. Dawn’s photos of geological features and measurements of the composition may provide a window to forces in the interior of the protoplanet sometime in its past.
Even if convection is no longer occurring, Ceres is not entirely static. We have very tantalizing information from a marvelously productive far-infrared space telescope named for the only known astronomer who found a planet before Piazzi made his discovery. The Herschel Space Observatory recently detected a tiny amount of water vapor emanating from the distant dwarf planet. Scientists do not know how the water vapor makes it into space. It might be from ice sublimating (possibly following a powerful impact that exposed subsurface ice) or perhaps from geysers or even erupting cryovolcanoes (“cold volcanoes”) powered by heat that Ceres has retained since its formation. In any case, Herschel saw water, albeit in very, very small quantity.
It is not certain whether water vapor is there all the time. It is unknown whether, for example, it depends on solar heating and hence where Ceres is in its somewhat elliptical orbit around the sun (not as circular as Earth’s orbit but more circular than Mars’), which requires 4.6 years to complete.
Even if the water vapor is present during Dawn’s 1.3-year primary mission in orbit, it would be extremely difficult to detect. Herschel made its findings when our ship was already far, far from Earth, well along its interplanetary itinerary. The probe’s sensors were designed for studying the solid surfaces of airless bodies, not an exceedingly tenuous veil of water molecules. For context, the water vapor Herschel measured is significantly less dense than Earth’s atmosphere is even far above the International Space Station, which orbits in what most people consider to be the vacuum of space. Dawn will not need windshield wipers! Nevertheless, as we saw in February, the Dawn team, ever creative and dedicated to squeezing as much out of the mission as possible, investigated techniques this year that might be effective in searching for an exceptionally thin vapor. They have augmented the plan with many hours of observations of the space above Ceres when the spacecraft is over the night side during its first science orbit in April and May at an altitude of 8,400 miles (13,500 kilometers). It is possible that if there is some water vapor, the instruments may pick up a faint signature in the sunlight that passes through it.
Regardless of the possibility of detecting traces of water from Ceres, Dawn will focus its measurements on the uncharted surface and the interior, as it did at Vesta. Vesta displayed landscapes battered by craters from impacts during more than 4.5 billion years in the rough and tumble asteroid belt. Ceres has spent most or all of its history also in the asteroid belt, but it is possible it will not show its age so clearly. Ice, although very hard at such low temperatures, is not as hard as rock. So it may be that the surface gradually “relaxes” after an impact, just as your skin restores its shape after pressure has been removed. Craters older than a few tens of millions of years may have slowly disappeared. (That may sound old, but it is a small fraction of Ceres’ lifetime.) Near the poles, where it is colder so ice is harder, the scars of impact craters may be preserved for longer.
Ceres has more than water-ice and rock. It probably contains organic materials, some produced by chemical processes with the minerals already there and some delivered by asteroids that fell to its surface. This is noteworthy, because water and organic chemicals are ingredients for life. The combination of Ceres’ internal heat and the weak but persistent heating from the sun provides energy, which also is essential for life. Even if the possibility of life itself there is extremely remote (and it is beyond Dawn’s capability to detect), the conditions for “prebiotic” chemistry would be tremendously interesting. That is why, as we explained in August, we want to protect the special environment on the ground from contamination by the terrestrial chemicals in our orbiting spacecraft.
While there is more known about Ceres, there is much, much more that is unknown. Dawn seeks to discover many of the secrets of this unfamiliar, fascinating member of the solar system family. One of the measures of its success would be if, upon answering many of our questions about Ceres, we are left with even more questions. Now on the threshold of an old world which will be new to us, we do not have long to wait for the great rewards of new knowledge, new insight, new thrills and new mysteries to solve.
Dawn is 382,000 miles (614,000 kilometers) from Ceres, or 1.6 times the average distance between Earth and the moon. It is also 3.77 AU (351 million miles, or 564 million kilometers) from Earth, or 1,500 times as far as the moon and 3.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.
Dr. Marc D. Rayman
8:00 a.m. PST December 29, 2014
A new version of the Dawn spacecraft is continuing the ambitious journey through the asteroid belt to uncharted distant worlds. Now holding a new solar system record, the probe is thrusting with its ion propulsion system, patiently and gently changing its orbit around the sun to match that of the immense protoplanet Vesta (and subsequently dwarf planet Ceres).
Even as Dawn continues pushing deeper into space, another spacecraft that used ion propulsion to conduct an exciting mission at a near-Earth asteroid has concluded. After traveling to and studying the diminutive Itokawa, Japan’s Hayabusa spacecraft returned to Earth on June 13. This was long one of your correspondent’s favorite missions, and he has joined many, many other enthusiasts in congratulating the team responsible for this impressive achievement.
When Dawn reaches each of its destinations, it will have a very full program of activities to acquire pictures and other scientific information. Brief overviews of some of its plans for Vesta were described in recent logs, and more will be presented later. To accomplish its mission of exploration, the spacecraft needs some enhancements to the capabilities it has been using for its travel through deep space to reach its targets. Those new capabilities are now onboard.
For the third time since it left Earth in September 2007, the spacecraft has received an upgrade of the software that runs in its primary computer. With a sense of grandeur and drama befitting this unique adventure, ever-poetical engineers fulfilled their dream of more than a year by denominating it OBC flight software version 9.0. Revealing their surprisingly cute and playful nature, however, most Dawn team members prefer the hypocorism “9.0” (or “nine oh”).
Engineers at JPL and Orbital Sciences Corporation began work on 9.0 almost immediately after 8.0 was installed on the spacecraft in April 2009. They continued with the careful and deliberate process of modifying the software until January, when the extensive test program commenced. It was crucial to verify not only that the new functions would work correctly but also that no unintended differences were introduced and that the existing capabilities were not compromised.
The latest software has 23 sets of changes from the previous version. Some of these are new methods of controlling the way the spacecraft will point its sensors at Vesta and Ceres in order to optimize the acquisition of data. Other modifications, based on experience gained in the ongoing operation of the spacecraft, improve its ability to handle certain potential anomalies on its own. In addition, just as 7.0 and 8.0 did, 9.0 corrects some bugs.
While it may seem quite elementary to load software into a computer that is in control of a spacecraft more than twice as far from Earth as the sun, it actually turns out to be somewhat complex and delicate. Even in “quiet cruise,” the computer is responsible for a great deal of activity onboard. The ion propulsion system was inactive, which is typical when the main antenna is pointed to Earth, but otherwise the computer was busy keeping all systems operating.
To install 9.0, controllers used exactly the same processes they followed for 8.0 in April 2009. It went quite smoothly again this time, right down to the on-time delivery of pizza to mission control during the first day of returning the spacecraft to its normal configuration after rebooting the computer. We know almost all readers accepted the advice offered last year to retain a copy of the log that presented the details of the 8.0 installation, but we happily include a link here for the convenience of the sole reader who did not and wishes to recall what is involved. (For all other readers, congratulations on the handsome profit you have realized on your investment in that previous log.)
As last year, controllers had run a few tests to verify the integrity of some critical components during the normal weekly communications sessions in the weeks leading up to the loading of the new software. On June 15, the spacecraft stopped thrusting on schedule, turned to point its main antenna to Earth, and kept it there rather than returning to the thrust direction a few hours later. That allowed operators to perform the rest of these detailed checks. After confirming that both the primary and backup computers were fully healthy, they transmitted the files containing the new software.
On June 16, with all stations in mission control at JPL reporting all subsystems were healthy and stable, and all systems at the Deep Space Network performing equally well, the command to reset the computer was radioed to the distant ship. The computer dutifully rebooted for the first time since the installation of 8.0 and began running with version 9.0. Whenever the computer reboots, it puts the craft into safe mode. The team verified that the new software was running smoothly and then initiated the process of guiding the spacecraft out of safe mode and back to its normal interplanetary cruise configuration. The schedule had allowed until June 24, but by June 18, the robotic explorer was fully prepared to resume its normal duties.
Because the software upgrade went so well, the Dawn project has decided to present this exciting offer: we will install a functional copy of 9.0 on your computer or smartphone at no charge. Simply place your device in the asteroid belt, send us the coordinates, and we’ll do the rest.
On June 17, after the majority of reconfigurations had been completed and while all members of the team but the insomniacs and the spacecraft itself were slumbering, protective software that is always running onboard detected an increase in the internal friction in reaction wheel no. 4. Reaction wheels are devices used to control a ship’s orientation in the zero-gravity of spaceflight. By electrically controlling the speed of these spinning units, the spacecraft can hold steady or rotate as needed. Dawn is outfitted with four reaction wheels, although it only uses three during normal operations. As we have seen before, operators let each wheel have its turn at being off for a part of the mission. The software that detected the friction in no. 4 responded correctly by powering that unit off. If only three wheels had been in use, it would have activated the unused wheel; but it was unnecessary to do so this time because, by coincidence, all wheels were operating, as is normal when the spacecraft enters safe mode. The team had been planning to turn reaction wheel no. 1 off later on June 17 as part of the reconfiguration. Instead, after taking some time to reassess the spacecraft’s condition, they simply left wheel no. 4 off and continued with their plans, now using wheels 1, 2 and 3 instead of 2, 3 and 4.
Dawn resumed ion thrusting on schedule on June 24. As it continues propelling itself to Vesta, it does so with the recognition that it has accomplished a greater propulsive change in velocity than any other craft ever to leave Earth. Some spacecraft have experienced larger velocity changes through gravitational interactions with planets, but thanks to the extensive use of its extremely efficient ion propulsion system, Dawn surpassed the record for the greatest change in velocity under a ship’s own power on June 5.
The previous record holder, Deep Space 1, was the first interplanetary mission to use ion propulsion. In its 11-month primary mission of testing advanced technologies (including ion propulsion), its two-year extended mission devoted to the exploration of a comet, and its final three-month hyperextended mission of additional technology testing, DS1 accumulated so much thrust time that it achieved an effective change in speed of 4.3 kilometers per second (9,600 mph). (As we have seen in several earlier discussions, such as here, this “effective change in speed” is not the speed at which the craft travels. It is a very commonly used way to express the effectiveness of a spacecraft’s propulsion system that avoids the confounding effects of orbital mechanics.)
Having thrust now for 635 days, or 63 percent of its time in space, Dawn has attained a change of more than 4.4 kilometers per second (9,800 mph), and it has much, much more powered flight ahead.
The record itself and even the total velocity change, while perhaps fun, really are not important, however. They are convenient measures of the progress this ship is making on its ambitious expedition, one that would not have been possible without ion propulsion and other innovations. The exploration of the cosmos is not a competition; it is a shared undertaking of all humankind. Each mission, each record, each accomplishment, each discovery builds on the successes (and even the failures) of those that preceded it and helps pave the way for those that will follow. Together they all contribute to the advancement of our understanding of the universe and our humble place within it.
Dawn is 0.32 AU (48 million kilometers or 30 million miles) from Vesta, its next destination. It is also 2.29 AU (342 million kilometers or 213 million miles) from Earth, or 855 times as far as the moon and 2.25 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.
Dr. Marc D. Rayman
10:30 p.m. PDT June 27, 2010
After more than 2.5 years of spaceflight, and more than 6 months in the asteroid belt, Dawn's interplanetary journey continues smoothly. The mission remains on course and schedule for this expedition to the dawn of the solar system.
Our Dawn is not the first spacecraft to use this name, although it is traveling farther from home than any other Dawn. This month 2 more craft traveled into space carrying that appellation, at least when translated into English. The Japanese Aerospace Exploration Agency sent Akatsuki to Earth's neighbor Venus, and Russia's Rassvet module was attached to the International Space Station in Earth orbit. The solar system is vast, however, and there is plenty of room for all such spacecraft. We send our best wishes for success to these other Dawns as they embark on their missions.
While our Dawn patiently and reliably thrusts with its ion propulsion system, gradually reshaping its path around the Sun to match orbits with the protoplanet Vesta, the human members of the team are very busy on distant Earth. Among their many activities is developing the sequences the robotic explorer will use when it begins studying that mysterious, alien world next year. We have seen recently what will occur during the “approach phase” and how Dawn will slip into orbit around Vesta. Now let's have a preview of what the ship will do once it has reached the first science orbit, known as “survey orbit.” Engineers are developing those sequences now, for execution in August 2011.
In survey orbit, the probe will be about 2700 kilometers (1700 miles) above the surface. During the approach phase, navigators will measure the strength of Vesta's gravitational tug on the spacecraft so they can compute the giant asteroid's mass with much greater accuracy than astronomers have yet been able to determine it. (The mass is calculated now using observations of how Vesta perturbs the orbits of other asteroids and even of Mars.) That knowledge will allow them to refine the survey orbit altitude, and they may target it to be somewhat higher or lower, depending on whether Vesta is more massive or less massive than the current calculations show. The sequences for acquiring science data are being designed to accommodate a reasonable range of masses.
Dawn will be in a near-polar orbit. Its trajectory will take it over the north pole (which will be in darkness, because it will be northern hemisphere winter at that time), then over the terminator (the boundary between the illuminated and unilluminated sides), down over the equator, over the south pole, and then across the terminator again to pass over Vesta's night side. Such an orbit allows the spacecraft to have a view of virtually every part of the lit surface at some time. Each revolution in survey orbit will take 2.5 to 3 days to complete. While this may seem like a leisurely pace, the spacecraft will be busy the entire time.
When on the day side of Vesta, Dawn will conduct an intensive campaign of observations. Vesta rotates on its axis in about 5 hours, 20 minutes (one Vestian “day”), which is faster than Dawn will be advancing in its orbit. So from the spacecraft's perspective, as it progresses slowly from north to south, the globe beneath it will complete several turns on its axis. That affords excellent opportunities for mapping the body.
During most of approach, Vesta will be so far away that it will fit comfortably in the fields of view of the science camera and the visible and infrared mapping spectrometer. Before Dawn reaches survey orbit, however, it will be too close to capture all of the expansive surface with its sensors in one glance. On each revolution, the sequences will command the spacecraft to point the instruments through profiles that will allow them to observe as much of the surface as possible.
The primary objective of survey orbit is to get a broad overview of Vesta with color pictures and with ultraviolet, visible, and infrared spectra. The camera will obtain views with 250 meters (820 feet) per pixel, about 150 times sharper than the best images from the Hubble Space Telescope. The mapping spectrometer will reveal much of the surface at better than 700 meters (2300 feet) per pixel. While subsequent science orbits will yield more detail, these first, new perspectives of this ancient world will represent an exciting step in the exploration of the solar system.
Throughout the year at Vesta, gamma-ray spectra and neutron spectra will be recorded with GRaND, and ultrasensitive measurements of the spacecraft's motion using the radio signal will reveal ever greater details of the protoplanet's gravity field and hence its internal structure. Although such information will be acquired in survey orbit, these investigations will benefit most from the lower altitude orbits.
Survey orbit is planned to last for 6 revolutions, or about 17 days. For most of the time it is on the day side, Dawn will fill its memory buffers with images and spectra. For most of the other half of each orbit, as it travels over the night side, the spacecraft will transmit those precious data through its main antenna to eager scientists and all others curious about the cosmos who reside on Earth. (Even when the surface below the spacecraft is in darkness, Dawn itself will be high enough that it will remain in sunlight, so its solar arrays will continue to provide electrical power.) There is so much to see at Vesta, and the instruments generate so much data, that a simple strategy of filling the memory on the day side and emptying it on the night side would be too limiting. Therefore, in the middle of its second, fourth, and fifth passes over the sunlit side, Dawn will halt its acquisition of data to spend a few hours radioing some of its findings to Earth, making more room for subsequent measurements.
Because the program of activities during the residence at Vesta is so full, and it all has to be planned in detail long before Dawn arrives, the project needs plans that are resilient to the inevitable problems, both large and small, that arise in such complex and challenging endeavors. While every observation in survey orbit is of interest, many more are scheduled than are necessary to fulfill the scientific objectives. Therefore, even if some are missed because of glitches in systems on the spacecraft or on Earth, as long as others are acquired, the mission will proceed. With the extremely rich set of measurements planned, there is no intention of repeating some that are lost.
After it has completed its survey of Vesta, Dawn will resume thrusting, spiraling down to its next science orbit for an even closer view. We will learn more about that in a subsequent log.
Meanwhile, as the craft continues to propel itself toward its destination, traveling farther and longer than ever, it will pass 3 milestones on its journey next month. Look for a NASA news release soon on a record it will set as it keeps thrusting with its ion propulsion system. We will describe that in the next log.
On June 23, Dawn will have been in flight for 1000 days. No doubt readers will enjoy taking a minute (at least, for those who read 61,000 words per minute) to reread all the logs since launch to recall some of what has occurred so far during the mission. While much has already been accomplished, the great rewards lie ahead, as Dawn pushes deeper into the asteroid belt, where it will explore faraway new worlds.
On June 3, Dawn will be exactly twice as far from Earth as Earth is from the Sun. Of course, the distance between the planet and the star does not matter for the spacecraft; it is on its own independent journey through the solar system. Nevertheless, such an occasion may provide some terrestrial readers with another opportunity to reflect upon the nature of such a journey. Dawn's trek is not simply that of a robot in space. Although in a narrow sense the ship is sailing the cosmic seas on its own, there is much more to the voyage than that. Such a mission represents a journey by a remarkable species that does not allow its physical confinement to the vicinity of its home planet to keep it from reaching ever farther in its pursuit of knowledge and its quest for grand and noble adventures.
Dawn is 1.96 AU (293 million kilometers or 182 million miles) from Earth, or 760 times as far as the Moon and 1.93 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.
Dawn remains on course and on schedule for its appointments with Vesta and Ceres, colossal protoplanets in the main asteroid belt. Under the gentle and continuous thrust of its ion propulsion system, its journey through the solar system brings it ever closer to its first target.
Last month’s log included an overview of many of the spacecraft’s activities during the final 3 months before its August 2011 arrival in the first science orbit at Vesta. In this “approach phase,” the probe will observe Vesta with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the alien world prior to initiating its in-depth studies. The discussion did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time will be devoted to continuing its ion-powered flight. Let’s take a more careful look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Vesta.
Thrusting is not necessary for a spacecraft to remain in orbit, just as the Moon remains in orbit around Earth and Earth and other planets remain in orbit around the Sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. With its extraordinarily efficient ion propulsion system, Dawn’s near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the Sun. The flight profile was carefully designed to send the craft by Mars in February 2009, so our explorer could appropriate some of the planet’s orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn’s orbit, Dawn lowered Mars’s orbit, ensuring the solar system’s energy account remained balanced.
While spacecraft have flown past a few asteroids in the main belt (although none as large as the behemoth Vesta nor the still more massive dwarf planet Ceres), no probe has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two solar system targets. Dawn’s unique assignment would be quite impossible without ion propulsion. But with its light touch on the accelerator, taking nearly 4 years to travel from Earth past Mars to Vesta and more than 2.5 years from Vesta to Ceres, how will it enter orbit around Vesta, how will it break back out of orbit, and how will it enter orbit around Ceres?
Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its orbit around the Sun. This intriguing challenge was addressed in part in February 2007, as all readers with perfect memory recall. In August 2008, we considered another aspect of what is involved in climbing the solar system’s hill, with the Sun at the bottom, Earth partway up, and the asteroid belt even higher. (Readers at that time in the past thoughtfully sent greetings through time to us, which we are now delighted to receive! On behalf of all present readers, we return the kind gesture with our own greetings.) We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the Sun, but then need to turn in order to move along with the body they will orbit.
Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet’s gravity can hold it in orbit and carry it around the Sun. These exciting events can range from around 0.6 to 1.5 kilometers per second (1300 to 3400 miles per hour). With 10 thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn’t have to.
Dawn’s method of getting into orbit is quite different, and the key is expressed in an attribute of the ion propulsion system that has been referred to 26 times (trust or verify; it’s your choice) before in these logs: it is gentle. Dawn’s entire thrust profile for its long interplanetary flight has been devoted largely to the gradual reshaping of its orbit around the Sun so that by the time it is in the vicinity of Vesta, its orbit will be very much like Vesta’s. Only a small change will be needed to let the giant asteroid’s gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit.
To get into orbit, a spacecraft has to match speed, direction, and location with its target. A mission with conventional propulsion first gets to the location and then, with the planet’s gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling out to the orbit of Vesta (and later Ceres), Dawn works on its speed, direction, and location all at the same time, so they all gradually reach the needed values just at the right time.
To think about this facet of the difference between achieving this goal with the different technologies, imagine you want to drive your car along next to another traveling west at 100 kilometers per hour (60 miles per hour). The analogy with the conventional technology would be similar to heading north toward an intersection where you know the other car will be. You arrive there at the same time and execute a whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator, and probably some adrenaline. When you drive an ion propelled car, operating with 10 times the fuel efficiency, you take a different path from the start, one more like a long, curving entrance ramp to a highway. When you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is entirely different from the first method, but the sophisticated techniques of orbital navigation are up to the task.
In late July 2011, as the probe follows its approach trajectory to Vesta, their paths will be so similar they will be moving at nearly the same direction and speed around the Sun (about 20.5 kilometers per second or almost 46 thousand miles per hour). When at a range of about 16 thousand kilometers (9900 miles), the spacecraft will be traveling at less than 50 meters per second (110 miles per hour) relative to its destination. That combination of distance and velocity will allow Vesta to take gentle hold of Dawn. The spacecraft will not even notice the difference, but it will be in orbit around its first celestial target, even as it continues ion thrusting to reach the planned orbit more than 2 weeks later.
With the gradual trajectory changes inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. Dawn is propelling itself along a spiral path around the Sun as it journeys from Earth out to Vesta, the first loop having been completed in June 2009. It will arrive at Vesta before it completes the second revolution. Then its flight profile will be designed to spiral around Vesta as the probe and protoplanet together orbit the Sun. Dawn’s first loop around Vesta will be about 10 days, and its second will take 4. It will stop thrusting when it is in “survey orbit,” where one revolution takes just under 3 days. After collecting a rich bounty of pictures and other important scientific data from this altitude of about 2700 kilometers (1700 miles), it will resume thrusting, spiraling down to lower and lower orbits, requiring hundreds of revolutions. Dawn’s speed will increase as its orbital altitude decreases, so the loops will progressively become shorter.
In 2012, after completing months of close-range scientific observations, it will reverse the spirals, gradually climbing away from the world it has been studying just as it gradually climbed away from the Sun. Vesta’s gravitational hold will weaken as Dawn moves farther and faster, its graceful motion ultimately exceeding the strength of the invisible tether that bound it. As gently as it arrived, it will depart. In July of that year, it will once again be on its own in orbit around the Sun, and navigators will instruct it to point its ion thruster to spiral outward more in order to undertake its pursuit of Ceres.
These spiral paths do not occur naturally. Under the predictable and calculable effects of the gravity of the Sun and other bodies (including Vesta or Ceres), Dawn is programmed to orient its thruster in just the right direction at the right time to propel itself on the desired trajectory. A great deal of work was required before launch to devise such a plan. Changes since then have been determined by knowledge gained during the mission, such as an update to the prediction of how much power the solar array will yield.
Engineers have completed work on the approach phase for now. They have reviewed the sequences (the timed instructions the spacecraft will follow) in detail and have tested portions of them in the spacecraft simulator at JPL. The sequences are mature enough that they will be ready for controllers to update and refine as necessary next year before being radioed to the spacecraft. Now the operations team is turning its attention to the subsequent phase of the Vesta mission, survey orbit, where the intensive observations will begin. We will learn more about that in the next log.
Dawn’s controllers certainly do not focus all their efforts on preparing for Vesta. (Your correspondent devotes some of his to dancing, but perhaps that’s a topic for a future log.) Of course, keeping the spacecraft healthy and on course is essential as well. In addition to commanding it to sustain the needed thrusting, with a weekly hiatus for telecommunications, they perform routine maintenance to ensure the ship remains in top shape. For example, engineers recently adjusted the spacecraft’s master clock. Always in the glow of the distant Sun, and never needing to rest or take a break from its duties, the robot has no need to switch to daylight saving time. Nevertheless, a time change was called for because the onboard time had gradually drifted from the correct value. It had last been set on February 27, 2008, and has remained sufficiently accurate for all Dawn’s needs. With the gradual nature of this mission, precise timing is generally not necessary, so although they have closely monitored the clock, controllers have allowed it to run without correction. When they commanded the transition from ion thruster #1 to thruster #2 in January in January they expected the clock to change slightly, and indeed it did. Thruster #2 uses a different power control unit from thrusters #1 and #3. The #2 controller is mounted closer to the electronics assembly that includes Dawn’s clock, and now that that device is powered, the heat it dissipates warms the clock a little, so the clock rate is slightly altered. Although much larger values could be accommodated, when the time offset had crept up to 1.37 seconds, operators set it back to the correct time, and they included a change to account for the warmer environment. (Readers may wish to pause for 1.37 seconds to contemplate the difficulties of synchronizing clocks that are farther apart than the Sun.)
An improved version of the test to measure the overlap of the views of the visible and infrared mapping spectrometer (VIR) and the prime science camera was executed successfully. When the measurement was carried out in December, a conflict between commands in the VIR sequence prevented the intended data from being acquired.
As if maintaining the spacecraft’s health and powered flight and developing detailed plans for Vesta weren’t enough to keep Dawn’s engineers happy, they also are continuing work on a new version of the software for the primary computer, scheduled to be transmitted to the spacecraft in June. The mission also will mark 3 milestones that month, and it may not be a surprise if your correspondent marcs them in the next log.
Dawn is 1.62 AU (243 million kilometers or 151 million miles) from Earth, or 650 times as far as the Moon and 1.61 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 am PDT April 28, 2010
Dawn continues patiently forging through the asteroid belt, its permanent residence, as it climbs away from Earth and the Sun. Having thrust with its ion propulsion system for more than 1.5 years, the spacecraft remains healthy and on target for its rendezvous with alien worlds.
Our interplanetary adventurer still has a great deal of ion thrusting to complete before it can begin its orbital exploration of Vesta next year. Although it will suspend thrusting for a few weeks this summer to conduct some special activities (to follow along, be sure to renew your subscription to these logs the first time our helpfully persistent telemarketers call), it will devote most of the time until early August 2011 in powered flight, continuously reshaping its orbit around the Sun.
In addition to keeping the ship sailing smoothly and on course, Dawn’s engineers (who reside and work on distant Earth) are developing the detailed instructions that will guide it into orbit around Vesta and throughout its year of operations there. This process began last month and will continue even as the probe begins executing the first of the commands in May 2011.
Mission controllers compile Dawn’s instructions by assigning a time to each individual command. Groups of these timed commands are known as a “sequence.” During the current interplanetary cruise phase of the mission, sequences generally extend for 5 weeks, but some special activities may use sequences as short as a few hours. Usually more than one sequence is executing at a time, but like all the instruments in an orchestra, they are carefully synchronized and coordinated so the overall score accomplishes the composer’s artistic intent.
Readers may recall that the mission is separated into phases. Following the “launch phase” was the 80-day “checkout phase”. The current “interplanetary cruise phase,” which began on December 17, 2007, is the longest. It ends when the “Vesta phase” begins. (Other phases may occur simultaneously with those phases, such as the “oh man, this is so cool phase,” the “what clever name are we going to give this phase phase,” and the “lunch phase.”) Because the mission at Vesta is so complex, it is further divided into sub-phases. The Vesta sequences that are being developed now are for the “approach phase.” Approach begins in early May 2011 and concludes 3 months later when Dawn will have maneuvered to the first orbit from which it will conduct intensive science observations, known as survey orbit.
Most of the approach phase is dedicated to the final ion thrusting required to slip into orbit around Vesta. All of Dawn’s thrusting contributes to rendezvousing with Vesta, but the terminal thrusting will be controlled slightly differently. We will describe the process of using ion propulsion to enter orbit around another solar system body in an upcoming log. For now, however, let’s take a look at some of the other activities during the approach phase. While these are being timed in the sequences down to the second, part of the strategy for developing these sequences is to allow the team a means to update the times as the probe closes in on its target. The ion propulsion system provides flexibility in the timing that is different from most missions, and to take advantage of the benefits, the sequences must be correspondingly flexible. All the relative timing within a sequence will be fixed, but the time each sequence is activated can change. So, for example, even though we may change the date the first Vesta approach sequence begins executing by as much as a few days, once that adjustment is made, all the events within the sequence will shift by exactly the same interval. Some small changes other than timing, such as details of the probe’s orientation, may be made as well to reflect the latest information available before it is time to transmit the sequences to the spacecraft more than a year from now.
The principal activity other than thrusting during approach is the acquisition of images of Vesta with Dawn’s main science camera, primarily for navigation. From the distant vantage point of Earth, astronomers can determine Vesta’s location with astonishing accuracy, and the Dawn navigation team achieves extraordinary accuracy in establishing the probe’s position, but for the craft to enter orbit, still greater accuracy is required. Therefore, Dawn will observe Vesta’s location against the background of stars, and the photographs will be analyzed by celestial navigators to pin down the relative location of the ship and the port of call it is approaching. To distinguish this method from the one by which Dawn is usually navigated, making use of its radio signal, this supplementary technique with pictures is generally known as “optical navigation.” There are 24 optical navigation sessions during the 3-month approach phase. Many of these will be combined with observations of Vesta designed to help prepare for subsequent scientific measurements.
The positions of the spacecraft and protoplanet will be determined well enough with the current navigation method that engineers will know which stars will appear to be near Vesta from Dawn’s perspective. It is the analysis of precisely where Vesta appears relative to those stars that will yield the necessary navigational refinement. When Dawn is closer to Vesta, the giant asteroid will occupy most or all of the camera’s view, and stars won’t be visible. Then the optical navigation will be based on determining the location of the spacecraft with respect to specific surface features that have been charted in previous images.
For the optical navigation observations, Dawn will halt thrusting and align itself so that Vesta and, when possible, the stars are in view of the camera. It will spend half an hour or more taking images and storing them for transmission at the next scheduled communications session. The information extracted from the images will be used to calculate where the probe is relative to its destination. Engineers then will update the design of the trajectory for the spacecraft to follow to reach its intended orbit and fine-tune the ensuing thrust profile to ensure that Dawn accomplishes the revised flight plan.
The first optical navigation images will be acquired when Dawn is about 1.2 million kilometers (750 thousand miles) from Vesta, or more than 3 times the separation between Earth and the Moon. Dawn’s camera is designed for mapping Vesta from orbit. Therefore, instead of a high-power telescope with a narrow field of view, the camera has a relatively low magnification but covers a broad area. The camera achieves the equivalent of a magnification of about 3 compared to unaided human eyes. When these first optical navigation images are taken, distant Vesta will appear to be only about 5 pixels across. But at that stage, navigators will need to know its location, not its appearance, so the images will be of great value.
For 8 of the approach observation periods, in addition to the camera, the visible and infrared mapping spectrometer (VIR) will be trained on Vesta. By taking some early measurements with the camera and VIR, scientists will have the opportunity to make fine adjustments to the instrument parameters in the sequences for later observations.
In one of the optical navigation sessions in July, the camera will acquire many images of the space around Vesta in a search for moons. Astronomers have looked for moons of Vesta before, and will do so again before the explorer reaches its vicinity. Although none has been discovered, Dawn’s unique vantage point will provide more data. The existence of moons would be of interest both for science and for mission safety.
When Dawn suspends thrusting to check for moons, it also will collect a series of images as Vesta rotates. Like Earth and all other solar system bodies, Vesta spins. It completes one turn on its axis (one Vestian “day”) in about 5 hours, 20 minutes. These measurements will help characterize the alien world still more to aid in navigation and to prepare for subsequent observations with the science instruments. The moon search will be during the second of 3 observations of a full rotation.
Over the course of the 3-month approach, it will be exciting to watch Vesta grow from little more than a tiny smudge in the first optical navigation images until it is too large to fit in the camera’s view at the end of the phase. By early June 2011, the images will surpass the best that can be obtained with the Hubble Space Telescope. All succeeding observations will yield better and better views, both rewarding us and tantalizing us as Dawn prepares for its more intensive studies in later Vesta phases.
The spacecraft will glide into a very high orbit in late July and continue thrusting, gently as always, until early August, when it will arrive in its survey orbit at an orbit at an altitude of about 2700 kilometers (1700 miles). The activities to be conducted in the survey phase will be described when mission planners are working on those sequences.
In the meantime, the team is running some of the approach sequences through the Dawn spacecraft simulator at JPL down the hall from mission control. The simulator includes some hardware that is virtually identical to what is on the spacecraft and some software to take the place of other hardware components. The simulator is one of several methods used to check complex sequences before they are approved for transmission to the spacecraft.
It is both unnecessary and impossible to test all sequences. The simulator operates in real-time, so it would take 3 months to run all the approach sequences, and the Dawn team has too many other tests to perform with the simulator to allow that. Because much of the approach phase consists of ion thrusting, an activity which is quite familiar not only to the spacecraft but also to mission controllers (as well as regular readers of these logs), there is no need to test the thrusting periods. Engineers review each sequence to determine which portions would benefit from testing.
While the spacecraft simulator is hard at work at JPL, the actual spacecraft continues its work elsewhere. On February 28, Dawn and the Sun were equidistant from Earth. Now, as the distant explorer continues to propel itself toward its rendezvous with Vesta, it is farther from Earth than the Sun ever is. Moreover, even as the probe and the planet follow their separate paths around the Sun, Dawn will remain farther from Earth than the Sun. The orbits of Mercury, Venus, Mars, and many other members of the solar system family occasionally bring them closer to our planet than the Sun, but Dawn has enlarged its orbit so much that it never will return to the region of the solar system in which it began its ambitious journey of discovery.
Dawn is 1.27 AU (191 million kilometers or 118 million miles) from Earth, or 525 times as far as the Moon and 1.28 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 21 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 pm PDT March 28, 2010