Dear Asdawnished Readers,
Dawn is scrutinizing Vesta from its low-altitude mapping orbit (LAMO), circling the rocky world five and a half times a day. The spacecraft is healthy and continuing its intensive campaign to reveal the astonishing nature of this body in the mysterious depths of the main asteroid belt.
Since the last log, the robotic explorer has devoted most of its time to its two primary scientific objectives in this phase of the mission. With its gamma ray and neutron detector (GRaND), it has been patiently measuring Vesta's very faint nuclear emanations. These signals reveal the atomic constituents of the material near the surface. Dawn also broadcasts a radio beacon with which navigators on distant Earth can track its orbital motion with exquisite accuracy. That allows them to measure Vesta's gravity field and thereby infer the interior structure of this complex world. In addition to these top priorities, the spacecraft is using its camera and its visible and infrared mapping spectrometer (VIR) to obtain more detailed views than they could in the higher orbits.
As we have delved into these activities in detail in past logs, let's consider here some more aspects of controlling this extremely remote probe as it peers down at the exotic colossus 210 kilometers (130 miles) beneath it.
Well, the first aspect that is worth noting is that it is incredibly cool! Continuing to bring this fascinating extraterrestrial orb into sharper focus is thrilling, and everyone who is moved by humankind's bold efforts to reach into the cosmos shares in the experience. As a reminder, you can see the extraordinary sights Dawn has by going here for a new image every weekday, each revealing another intriguing aspect of the diverse landscape.
The data sent back are providing exciting and important new insights into Vesta, and those findings will continue to be announced in press releases. Therefore, we will turn our attention to a second aspect of operating in LAMO. Last month, we saw that various forces contribute to Dawn moving slightly off its planned orbital path. (That material may be worth reviewing, either to enhance appreciation of what follows or as an efficacious soporific, should the need for one ever arise.) Now let's investigate some of the consequences. This will involve a few more technical points than most logs, but each will be explained, and together they will help illustrate one of the multitudinous complexities that must be overcome to make such a grand adventure successful.
Far away, traveling through the vast expanse of (mostly) empty space, Dawn only knows where it is because of information the mission control team installs in it. This is typical for interplanetary spacecraft. Earth-orbiting satellites may be able to use the Global Positioning System (GPS) constellation or other means to find their own location, but only a few spacecraft that have gone far from Earth have the means to independently establish their own location. This should not be confused with a spacecraft's ability to determine its own orientation, which Dawn does with its star trackers, gyros, and sun sensors. In the same way, if you were in a dark and unidentified place on your planet, you could determine the direction you were looking by recognizing patterns of stars, but that would not help you ascertain your position.
Throughout the mission, controllers regularly transmit to the spacecraft a mathematical description of its location in the solar system at any instant over a given period of time. They also provide it with the information needed to calculate where Earth is. That's how it is able to point its main antenna in the correct direction when it needs to do so. During the Vesta phase of the mission, the probe is given the additional means it needs to determine its location relative to Vesta. All the information sent to the spacecraft is based on navigators' best prediction of where the spacecraft will be in the future. Dawn remains unaware of any deviations from its expected course, so it always behaves as if it were exactly where it would be if its motion matched the team's projections perfectly, without the discrepancies that are sure to occur. For the majority of the mission, both in interplanetary cruise and at higher altitude orbits at Vesta, the effects of being slightly off the predicted trajectory are insignificant. In LAMO, they are not.
For Dawn to aim its scientific sensors at Vesta, controllers instruct it to point straight "down." Again, it knows how to compute where "down" is because of the information it was given by navigators. Any disparity between where the craft was predicted to be and where it really is along its orbit causes it to point in a slightly different direction, not quite truly straight down. This does not compromise the observations; it could tolerate larger pointing errors and still capture the desired targets in the field of view of the instruments.
Dawn is a very large spacecraft. Indeed, the wingspan from one solar array tip to the other is 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest span of any probe NASA had ever dispatched on an interplanetary journey. The large area of solar cells is needed to capture faint sunlight in the asteroid belt to meet all of the electrical power needs. Each solar array wing is the width of a singles tennis court, and the whole spacecraft would reach from a pitcher's mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.
Now consider that when Dawn is looking precisely down, directly toward the center of Vesta, its wings are level. If it is pointed off even a little, then one of those long extensions is slightly closer to the massive body it is circling and one is slightly farther away. Because gravity diminishes with increasing distance, the one that is closer is subject to a very slightly stronger pull than the farther other. If unchecked, that lower side would gently be pulled down even more, thus increasing the difference in gravitational attraction between the two wings still more. Eventually, this would cause Dawn to be oriented so that one wing points straight down toward the ancient surface below and the other points straight up, back into the depths of space. Because this phenomenon depends on the change in gravity from the lower point to the higher one, it is known as "gravity gradient." Some satellites that orbit Earth are designed to take advantage of the gravity gradient to align their long axis with the planet below, but Dawn (and most other spacecraft) need greater flexibility in where they point.
Rather than accepting the passive method of orienting provided by the gravity gradient, Dawn uses its reaction wheels to train its science instruments on Vesta. By electrically changing the rate at which these devices spin, the ship can control its orientation in the zero-gravity, frictionless conditions of spaceflight. When a small deviation from the perfect orbit causes it to tip its wings a little when pointing to where it calculates "down" to be, the spacecraft's reaction wheels work to prevent it from succumbing to the gravity gradient, countering the tendency of the wings to deviate still more from being level. As a consequence, the ship remains stable and the wheels gradually spin faster and faster as it conducts its observations.
To reduce the wheels' speeds, mission planners schedule a period almost every day in LAMO during which the spacecraft fires its reaction control system thrusters, a function known as "desaturating the wheels." Indeed, the principal reason Dawn is outfitted with these small thrusters and a modest supply of conventional rocket propellant known as hydrazine is to manage the speed of the wheels.
The thruster firings not only provide the torque needed to reduce the rotation rate of the wheels, but they also have the incidental effect of propelling the spacecraft slightly. The push is small, changing the orbital speed by no more than about one centimeter per second (around one fiftieth of a mph, or about 120 feet per hour). But that causes Dawn to deviate from its planned orbit, and the accumulated force from all the firings is the largest source of trajectory discrepancies in LAMO.
To summarize so far, once Dawn has any variance at all between the predicted orbital motion that mission controllers have radioed to it and its actual path, its long wings will be tipped a little while it observes Vesta. In opposing the resultant gravity gradient effect, the reaction wheels will accelerate. When the reaction control system thrusters fire to decelerate the wheels, they will nudge Dawn still more off course, and the cycle will continue.
Of course, engineers have devised strategies to accommodate this contribution (and others) to deviations from the plan. In LAMO, they frequently measure the ship's trajectory and revise their estimates of the future course. They transmit to the spacecraft a new prediction for the orbit twice a week, so the main computer usually has a very good estimate of where it is relative to Vesta and hence how to orient itself so that its long solar arrays remain level as it acquires its fabulous pictures and other scientific information. With the updated knowledge of its position, Dawn can aim its sensors accurately and keep the thruster firings from being excessive, even when it is not following its orbit perfectly. This solution works well, but let's continue delving into the consequences of the orbital perturbations.
While the operations team has the capability to provide the ship regularly with a good description of where it will be, it is much more difficult to make such frequent adjustments to its detailed itinerary. The schedule of its myriad activities has to be planned longer in advance. The sequences of commands, which are timed to the second, are very complicated to develop and verify, and the operations team does not have the resources to refine the timing as often as they can send updates on the craft's predicted location.
Engineers took many factors into account in selecting the orbits Dawn uses for its science observations. We saw in November that the orbits are characterized not only by the altitude but also by the orientation of the orbital plane. A subsequent log will explain the choices for the planes more fully, but for now, what matters is that, among other considerations, the orbits were designed to ensure Dawn remains in constant sunlight. It always has the sun in sight, never entering Vesta's shadow. Keeping Earth in view at all times was not part of the design, and on every one of the more than 600 revolutions around the gigantic rocky body since August 28 (the seventh circuit in survey orbit), the spacecraft has been temporarily behind Vesta from the geocentric point of view. In its present orbit, these occultations last for about half an hour in every 4.3-hour loop.
When Dawn is observing Vesta, that doesn't matter. When it is using its ion propulsion system to transfer from one orbit to another, it also doesn't matter. It does matter, however, when it is in contact with Earth, because Vesta blocks the radio signal. Controllers give the spacecraft a detailed schedule of which data to transmit and when, making the best possible use of the precious communications link that stretches across the solar system. The timed plan tells it not to send high priority data during the radio blackout, but the timing of the occultations can shift a little as the orbit departs from the plan.
The strategy to deal with the slight deviations in the timing of the interruption in the radio link principally involves including some padding in the plan. The schedule for the transmission of the highest priority data places it well away from the expected gap, so no important losses occur if Dawn is a little ahead in its orbit or a little behind (causing the gap to occur a little earlier or a little later).
But what is there to do during and near the time the craft is predicted to be blocked by Vesta while conducting a communications session? Dawn rotates too slowly to make it worth turning to point its sensors at the surface just for these periods. Of course, it could simply transmit nothing at all. Instead, the team has it transmit data that otherwise would be lost. There is never enough time to send to Earth all the information the probe generates and collects. So most of the time it is behind Vesta, it broadcasts many of the measurements of its own subsystems that cannot be stored and sent later. And during the periods immediately before and after the expected occultation, when there is a chance that the signal will reach Earth, it sends bonus pictures and VIR spectra. If the deviations from the planned orbit are small, then the antenna will have an unobstructed view of Earth, and these data will make it home. And if the spacecraft enters the blackout period late (or early), then it will exit late (or early) as well, so the bonus results sent before (or after) the occultation will be received. But in the rest of the cases, well, Dawn will transmit those bits right back where they came from, sending the photos and spectra into the vast rocky surface between the spacecraft and Earth.
Last month we described one of the limitations in how much bonus information could be obtained from LAMO. Now we have another. In summary, because the probe can acquire more images and other data than it is possible to return, it radios some of them during times that it is possible they will make it to Earth. Because of realistic causes of variation from its predicted orbital path, however, some of these measurements will be transmitted when, from Dawn's perspective, Vesta blocks Earth, thus preventing the broadcast signals from getting through. The GRaND observations (as well as essential telemetry on the health of the ship) are scheduled to be sent during times that, even with the reasonable range of orbit discrepancies, the communications link will not be obstructed. In this way, mission planners return as much data as possible, taking maximum advantage of the time Dawn points its main antenna to Earth. Having a sophisticated robot in orbit around the second most massive resident of the asteroid belt presents truly unique opportunities for the exploration of the solar system, and the team has devised every strategy they could to use the time as productively as possible.
The spacecraft aims GRaND at Vesta most of the time in order to develop a good picture of the weak nuclear glow. Controllers schedule three periods per week, each about eight hours, in which it directs its antenna to Earth. The orbit predictions have been extremely good, matching the actual motion quite well. Moreover, some time is allocated to return the camera and VIR data apart from the times that Vesta might be in the way. As a result, the team has been rewarded with more than 3200 photos from LAMO so far. Every one is bonus, and every one is neat!
After well over four years of travel in deep space and already half a year in orbit around Vesta, engineers recently encountered a bug lurking in the spacecraft's software. As with most bugs, this one had waited silently until just the right circumstances occurred to provoke it. The combination of conditions was achieved late in the day on January 13, and the bug caused the main computer to reboot. Dawn correctly responded by going into safe mode. The mission control team observed this the next day, and promptly began investigating the reason. They soon determined the nature of the bug (as well as ways to ensure it would never be activated again) and restored the spacecraft to its usual operating configuration for LAMO. Even with the slow communications in safe mode, the long time for radio signals to travel between Earth and Dawn, and the frequent interruptions by the regular occultations by Vesta, they had fully restored all systems by January 19. It took a few more days to configure GRaND, but it, along with the other instruments, is now back to its intensive inspection of Vesta.
We saw last month that the mission has been progressing so well that the time originally allocated to deal with anomalies had not been needed, so it is being applied to extend the duration of LAMO. This allows even more scientific observations to be conducted in this lowest altitude. Far from the planet it left in 2007, in a region of the solar system in which no other spacecraft has ever taken up residence, Dawn will continue its exploration of Vesta, alternating between examining the alien world below and transmitting its discoveries to Earth. Meanwhile, everyone who ponders what undiscovered lands lie beyond our sight, everyone who hungers for exciting challenges and noble adventures, and everyone who values turning the unknown into the known profits from the great treasures this stalwart cosmic ambassador sends to its erstwhile home, a faraway place it will never visit again.
Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.08 AU (461 million kilometers or 286 million miles) from Earth, or 1155 times as far as the moon and 3.13 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.
In November 1969 Apollo 12 astronauts Alan Bean and Pete Conrad landed on the moon less than 600 feet from NASA's Surveyor 3 spacecraft, which had been there since April 1967. They removed the camera, some cable and tubing, and the trenching scoop from the lander and brought them back to Earth so that the effects of prolonged lunar exposure could be studied by Hughes Aircraft Company (the spacecraft prime contractor) and NASA's Jet Propulsion Laboratory. The Surveyor 3 camera was kept under quarantine and studied for several weeks at the Lunar Receiving Laboratory in Houston. Then it was shipped to the Hughes facility in Culver City, Calif. This photo was taken in January 1970, probably at the Hughes facility, where Hughes and JPL employees photographed, disassembled and studied the camera in detail.
Dawn concludes 2011 more than 40 thousand times nearer to Vesta than it began the year. Now at its lowest altitude of the mission, the bold adventurer is conducting its most detailed exploration of this alien world and continuing to make thrilling new discoveries.
Circling the protoplanet 210 kilometers (130 miles) beneath it every 4 hours, 21 minutes on average, Dawn is closer to the surface than the vast majority of Earth-orbiting satellites are to that planet. There are two primary scientific objectives of this low altitude mapping orbit (LAMO). With its gamma ray and neutron detector (GRaND), the probe is measuring the faint emanations of these subatomic particles from Vesta. Some are the by-products of the bombardment by cosmic rays, radiation that pervades space, and others are emitted through the decay of radioactive elements. Vesta does not glow brightly when observed in nuclear particles, so GRaND needs to measure the radiation for weeks at this low altitude. This is analogous to using a long exposure with a camera to photograph a dimly lit subject. If GRaND only detected the radiation, it would be as if it took a black and white picture, but this sophisticated instrument does more. It measures the energy of each particle, just as a camera can measure the color of light. The energies reveal the identities of the elements that constitute the uppermost meter (yard) of the surface. Dawn devotes most of its time now flying over Vesta to collecting the glimmer of radiation. It requires a long time, but this spacecraft has demonstrated tremendous patience in its use of the gentle but efficient ion propulsion system that made the mission possible, so it can be patient in making these measurements.
The second motivation for diving down so low is to be close enough that Vesta's interior variations in density affect the spacecraft's orbit discernibly. We have seen before that the distribution of mass inside the protoplanet reveals itself through the changing strength of its gravitational tug on Dawn. Exquisitely sensitive measurements of the ship's course can be translated into a three-dimensional map of the mass. In the plans discussed for LAMO one year ago, the delicate tracking of the spacecraft required pointing the main antenna to Earth. That provides a radio signal strong enough to achieve the required accuracy. Since then, navigators have determined that the radio signal received from one of the craft's auxiliary antennas, although far weaker, is sufficient. The main antenna broadcasts a tight beam, whereas the others emit over a much larger angle, exchanging signal strength for flexibility in pointing.
This allows an extremely valuable improvement. The spacecraft cannot aim GRaND at the surface and the main antenna at Earth concurrently, because both are mounted rigidly, just as you cannot simultaneously point the front of your car north and the back east. Therefore, in the original plan, gravity measurements and GRaND measurements were mutually exclusive. Now, as Dawn turns throughout its orbit to keep Vesta in GRaND's sights, it can transmit a weak radio signal that is just perceptible at Earth. This enables an even greater science return for the time in LAMO. Unlike the science camera and the visible and infrared mapping spectrometer (VIR), GRaND and gravity observations do not depend on the sun's illumination of the surface. Even as it orbits over a dark, cold, silent landscape, Dawn is fully capable of continuing to build its maps of elements and the interior structure.
The signal from the auxiliary antenna is just sufficient for the measurement of the spacecraft's motion, but it is not strong enough to carry data as well. So the spacecraft is still programmed to point its main antenna to Earth three times each week, allowing the precious GRaND observations that have been stored in computer memory to be transmitted. As always, the myriad measurements of temperatures, voltages, currents, pressures, and other parameters that engineers use to ensure the health of the ship are returned during these communications sessions as well.
Although the pictures of Vesta from survey orbit and the high altitude mapping orbit (HAMO) have exceeded scientists' expectations, not only in quality and quantity but also in the truly fascinating content, as enthusiastic explorers, the Dawn team could not pass up the opportunity for more. When GRaND is pointed at the surface, the camera is too, and already well over one thousand images have been returned, revealing detail three times finer than the spectacular images from HAMO. For readers who cannot go to Vesta on their own, go here for a selection of the best views, each showing surprising and captivating alien landscapes.
In addition to the bonus photography, beginning in January VIR will take observations. Although the instrument has already acquired nearly seven million spectra in the higher orbits, this new vantage point will allow sharper resolution, just as it does for the camera.
The ultra-long-distance communication between Dawn and Earth requires extraordinary technology on both ends. Even with all the sophistication, the amount of information that can be transmitted in a given time remains very limited. The remote spacecraft sends data at speeds significantly lower than a typical home Internet connection. Engineers use that precious communications link very carefully, judiciously selecting what information to instruct the probe to return. Because of the high priority given to GRaND, which needs to be pointed at the surface as long as possible, much of the limited time spent with the main antenna aimed at Earth is devoted to transmitting that instrument's findings (and the measurements of spacecraft subsystems). This restricts how much data from the camera and VIR can be communicated.
In the next log, we will see another limitation on the number of camera images and VIR spectra in LAMO. It is a consequence of another aspect of the complex operations in this low orbit around a massive body, and that is the small but real differences between the predicted orbit and the actual orbit. We will cover the first part of the explanation here.
Navigators use their best knowledge of the many forces acting on Dawn to chart an orbital course for it. The forces can be traced to three principal sources: gravity, light, and Dawn itself. We have discussed all of these before in detail (see, for example, this explication of the last two), but let's review them here. This is an involved story, so readers are advised to be in a comfortable orbit while following it. You can safely skip the next four paragraphs and no one ever need know.
Vesta has a complicated gravity field, and that leads to a complicated orbit. The spacecraft does not follow a perfectly circular, repetitive path because the gravitational pull on it changes according to where it is as the colossus beneath it rotates and it loops around. The map of the gravity field has been improving throughout Dawn's residence there, but its completion awaits the LAMO gravity measurements. In the meantime, unknown details of the variation of mass lead to small divergences in the orbit. All the other bodies in the solar system exert gravitational pulls on the spacecraft as well (just as they do on you), but those are more easily accounted for. The distances from Dawn are so great that the variations in their gravity fields don't matter. So although the effects of the faraway objects need to be accounted for, they do not contribute much to the discrepancies.
Dawn depends on sunlight for its power, using its large solar arrays to make electricity to run all systems. The sun also propels the spacecraft, because in the frictionless conditions of spaceflight, the ship recoils slightly in response to the miniscule but persistent pressure of the light. The force depends on whether the light is absorbed (whereupon it is converted to electrical power by the arrays or to heat by whatever component it illuminates) or reflected. If it is reflected, the angle makes a difference, so smooth shiny surfaces that act like mirrors cause different effects from the materials that present a matte finish or are curved or angled. As the spacecraft rotates to keep GRaND pointed at the ground below, different parts of the ship are presented to the sun, so the force from the light changes, and the orbit is constantly subjected to a variable disturbance.
Dawn itself adds to the complexity of its orbital path. The spacecraft carries reaction wheels, which are spun to help it control its orientation. These devices gradually spin faster, so every few days they need to be slowed down. That is accomplished by firing the small reaction control system thrusters during windows specified by mission controllers. In addition to the thrusters providing the needed torque on the craft to reduce the wheels' speeds, they impart a force that changes the orbit slightly.
The physical principles underlying all these phenomena that perturb Dawn's orbit are understood with exceptional clarity. Although the values of the myriad parameters involved are ascertained quite accurately, they are not known perfectly. As a result, navigators' prediction of the ship's course includes some degree of uncertainty. Even their ability to determine the present orbit is subject to a variety of small errors typical in sensitive physical measurements.
For all of these reasons, the craft's actual orbit departs slightly from the plan, and the deviations tend to grow, albeit gradually. As designers expected, in survey orbit and HAMO, the differences were small enough that they did not affect the complex operations plans. Analysis well before Dawn arrived at Vesta predicted that the discrepancies in LAMO would be large enough that occasional adjustments of the orbit would be necessary. Therefore, mission controllers scheduled a window every week (on Saturdays, as it turned out) to use the ion propulsion system to fine-tune the spacecraft's trajectory, bringing it back to the intended orbit. These are known as "orbit maintenance maneuvers," and succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)
The LAMO phase began on December 12, and OMMs were performed on December 17 and 24. In contrast to the long periods of thrusting required with ion propulsion for other parts of the mission, the corrections needed were so small that each OMM needed less than 15 minutes. The whisper-like thrust changed the spacecraft's speed by less than five centimeters per second (one-tenth of a mph). But that was enough to nudge Dawn back to the planned orbit.
The ship was so close to the designated course that the OMMs for December 31 and even January 7 have already been canceled. Not executing the OMMs allows the probe to spend more time collecting neutrons and gamma rays from Vesta. The operations team productively uses the time saved in designing, checking, and transmitting the OMM commands to do other work to ensure LAMO proceeds smoothly and productively.
In the last log we discussed the complicated and dynamic spiral descent from HAMO to LAMO, which was still in progress. The flight required not only reducing the altitude from 680 kilometers (420 miles) to 210 kilometers (130 miles) but also twisting the plane of Dawn's orbit around Vesta. As with all orbiting bodies, whether around Vesta, Earth, or the sun, the lower the orbital altitude, the shorter the orbital period. Vesta's gravitational grip strengthened as Dawn closed in, forcing the spacecraft to make faster loops around it. This meant that as the probe performed the intricate choreography to align its ion thruster with the changing direction needed to alter its orbit, it had to pirouette faster.
When engineers command Dawn to rotate, they usually instruct it to use the same stately speed as the minute hand on a clock. The spacecraft may have to move a little faster however, as it pivots to keep its solar arrays pointed at the sun while accomplishing the required turn. Sometimes it knows that at the end of a turn, it will have to initiate another turn. For example, it may rotate to the orientation required to begin a session of ion thrusting. But while it is thrusting and curving around its orbit, it generally needs to steer the thruster to execute the maneuver. As a result, the robot may choose to turn at a slightly different rate from what its human team members command in order to make a smooth transition from the first turn to the second.
On Dec. 3, when preparing for one of the final thrust segments required to reach LAMO, the combination of all these factors caused the spacecraft to rotate faster than usual. That led to a temporary discrepancy between where it was pointed and where it expected to be pointed during the turn. When protective software detected the inconsistency, it interrupted the ongoing activities and put the spacecraft into safe mode.
When the safe mode signal was received by the Deep Space Network, the operations team responded with its usual calm and skill. They quickly determined that Dawn was fully healthy, diagnosed the cause of the safing, and began guiding the spacecraft back to its normal operational configuration. In addition, they devised a new flight profile that would compensate for the thrusting that was not completed. The team also determined how to prevent the same problem from recurring for subsequent maneuvers. While doing all this work, they were putting the finishing touches on the first LAMO science observation sequences. Controllers managed to complete everything flawlessly and even kept the mission on schedule, allowing LAMO to commence on Dec. 12.
The general plan for Dawn's three-month approach plus one year in orbit around Vesta was described in logs in 2010. The time was apportioned among the different science phases and the transfers between science orbits to ensure a comprehensive and balanced exploration of this mysterious and fascinating world. Fully appreciating that in such an exceedingly ambitious undertaking, some unexpected problems are inevitable, mission planners worked hard to devise an itinerary that left 40 days uncommitted. Their strategy was that as they recovered from anomalies, they would draw from that time and still not have to compromise any of their carefully designed activities. They also planned that any unspent margin would be used to extend LAMO.
To the great delight (and, to be honest, surprise) of all, not one day of the 40-day reserve has been needed. Although there have indeed been unanticipated difficulties, from the beginning of approach on May 3 to this point, the team has been able to resolve all of them without having to withdraw from that account. This is remarkable considering that Dawn is the first visitor from Earth to Vesta, with its many unknown physical properties. This expedition is the first ever in which humankind has sent a spacecraft to orbit such a massive body without first conducting a reconnaissance with a flyby spacecraft. Dawn has maintained a rapid pace of scrutinizing its enigmatic destination. Performing all of this so successfully without needing to use even a little of the spare time they provided for themselves was considered quite unlikely. And yet the entire 40 days remain available.
More ambitious operations lie ahead, with the rest of LAMO, the spiral ascent to HAMO2, HAMO2 itself, and the escape in July to begin the long interplanetary cruise to reach Ceres on schedule in February 2015. We will see in 2012 that each of these phases includes new challenges, and it is certain new problems will arise. Nevertheless, all 40 days are being used to extend LAMO. Therefore, the indomitable explorer will remain at this low altitude through the end of March, continuing to tease out secrets about the dawn of the solar system and revealing more startling and thrilling discoveries on behalf of everyone on distant Earth who yearns to reach out into the vastness of space.
Dawn is 210 kilometers (130 miles) from Vesta. It is also 2.79 AU (418 million kilometers or 260 million miles) from Earth, or 1045 times as far as the moon and 2.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 p.m. PST December 30, 2011
Recently, one of our fans on the NASAJPL Facebook page asked a good question about the efficiency of solar arrays on the Dawn and Rosetta spacecraft.
"A question about Dr. Marc D. Rayman's comment in his Dawn journal, saying that 'its tremendous solar arrays [are] the most powerful ever used on an interplanetary mission.' Is that really true? According to JPL's Dawn website, the solar arrays have a total span of 19.7 meters. By comparison, each of Rosetta's two arrays is 14 meters in length (28m total). Are Dawn's arrays so much more efficient? Thanks."
Here's an answer from Dawn Chief Engineer Marc Rayman:
Yes, this is really true. Dawn's solar arrays, although smaller than Rosetta's, could indeed produce more power because they are more efficient. Fortunately, it is not a competition! Both missions seek fascinating new insights into the complex history and character of the solar system as they take all of us on adventures to exotic destinations. To overcome some of the daunting challenges of traveling moderately far from the sun, engineers on each mission have turned to powerful solar arrays.
Rosetta is a fabulous mission, promising exciting results from comet Churyumov-Gerasimenko. It is with the greatest enthusiasm that I look forward to the astonishing discoveries that await its rendezvous with this solar system relict.
The spacecraft carries the largest solar arrays ever flown on an interplanetary mission. The two 14-meter (47-foot) arrays project in opposite directions from the main spacecraft itself, creating a structure about 32 meters (105 feet) tip-to-tip, and the total area of solar cells is 53 square meters (573 square feet). Composed of silicon, these cells could have produced somewhat in excess of seven kilowatts when at Earth's distance from the sun. Of course, Rosetta did not need that much power, but as it travels into the depths of space, every watt will be precious. When the spacecraft is more than five times Earth's distance from the sun and the light from our star is much weaker, the giant arrays still will generate 400 watts, just enough to keep the probe operating. (Rosetta will arc out to that distance on its way to the comet, but it will be closer to the sun, and hence able to produce more power, when it arrives and conducts its investigations of this mysterious body).
Dawn's solar arrays, while the largest used on a NASA interplanetary mission, are smaller than Rosetta's. This bird's wingspan is about 20 meters (65 feet), and the solar arrays, each more than eight meters (27 feet) in length, have a total of about 32 square meters (341 square feet) to capture sunlight. The panels are populated with advanced cells composed of three different materials that work together to convert a larger percentage of the incident light into electrical power. The combination of indium gallium phosphide, indium gallium arsenide, and germanium makes these cells so much more efficient that despite the smaller collecting area, together they produce higher power under the same conditions. These arrays could have generated more than 10 kilowatts at Earth's distance from the sun. Dawn not only did not need such tremendous power, but like Rosetta, it was not even capable of using it all. But it too ventures far from home to remote locations where sunlight is less abundant.
Dawn's ambitious mission to orbit the two most massive residents of the asteroid belt, Vesta and Ceres, would be quite impossible without its use of ion propulsion. The key to ion propulsion's extraordinary capability is its conversion of electrical power into thrust, so Dawn carries such powerful arrays to ensure that even when exploring dwarf planet Ceres at three times Earth's distance from the sun, it can produce sufficient power to thrust and operate all other systems. I describe more about the importance of power to the mission in my Dawn Journal of July 27, 2008.
I appreciate your interest in Dawn, and I hope you will continue to join us as we travel to two of the last unexplored worlds in the inner solar system. In only 10 months, Dawn will become the first spacecraft ever to orbit a resident of the main asteroid belt as it begins its exploration of protoplanet Vesta, and to put it quite simply, this is going to be really cool!
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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.
Dr. Marc D. Rayman
9:00 pm PDT May 27, 2010
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
Pushing ever farther into space, deeper into the asteroid belt, Dawn is continuing to progress smoothly on its solar system journey.
The spacecraft spends most days climbing away from the Sun atop its pillar of blue-green xenon ions. A day’s thrusting is enough to change the spacecraft’s speed by a very modest 7.3 meters/second (16.3 miles/hour). While such an effect would be entirely inadequate for an interplanetary mission as ambitious as Dawn’s, the extraordinary efficiency of ion propulsion allows the probe to thrust for much more than a day. Although almost all spacecraft coast most of the time, as do planets, moons, and asteroids, this explorer usually maintains a gentle pressure on its orbit, constantly changing it so that it can rendezvous with Vesta next year, leave in 2012, and then rendezvous with Ceres in 2015. Dawn has spent 60% of its time since launch patiently accelerating with the ion propulsion system. It has already managed to change its speed by more than 3.6 kilometers/second (8100 miles/hour), far exceeding the capability of most spacecraft, yet it has a great deal more thrusting ahead. (For a comparison with probes that enter orbit around Mars, visit a previous log.)
In contrast to conventional chemical propulsion systems, ion propulsion achieves its astonishingly high performance by using electrical power to create the thrust. Outfitted with the most powerful solar arrays ever carried on an interplanetary probe, Dawn converts sunlight into the electricity consumed by the ion thrusters. And yet even as its travels take it ever more distant from the luminous source of its electrical power, the effect of the ion thrust becomes greater each day, not less. At the beginning of the mission, a day of thrust yielded only about 6.5 meters/second (14.5 miles/hour). Now, more than 1.8 times farther from the Sun, the acceleration is greater, and by this summer, when Dawn is still farther from the Sun, it will climb to 7.6 meters/second (17.0 miles/hour) every 24 hours. The reason for the paradoxical increase is deceptive, yet simple.
Dawn’s solar arrays are so large that they can produce enough power to operate an ion thruster at the maximum throttle level (as well as all other spacecraft systems) even when twice as far from the Sun as Earth is. Therefore, the propulsion system will not have to be throttled to lower power until the probe is more than 2 astronomical units (AU) from the Sun, a distance it will reach this summer. In the meantime, because the arrays produce excess power, the thrust is independent of the distance to the Sun.
The acceleration depends on more than the thrust, however; the thruster pushes against the spacecraft, so the change in speed depends on the spacecraft’s total mass. A rocket engine (whether powerful and inefficient or soft and efficient) imparts a lower velocity to a more massive craft than to a less massive one. (Gravity, or the absence of it, is not relevant.) This is quite familiar from terrestrial experience. If you throw a baseball with the same force as you throw a shot put, the baseball will depart with a higher speed.
When Dawn began its mission in September 2007, it was about 1218 kilograms (2685 pounds). Since then, it has expended 140 kilograms (309 pounds) of xenon plus about 5 kilograms (11 pounds) of hydrazine from the reaction control system (the system that uses small conventional thrusters to aid in orienting the spacecraft in the zero-gravity of spaceflight). The ion thruster now is pushing against less mass, so the effect of the thrust is greater. As Dawn continues to expel its propellants, it will become still less resistant to the thruster’s efforts to change its speed. In summary, with the thrust staying constant and the mass decreasing, the acceleration is increasing.
The mass will always go down when the ion propulsion or reaction control systems are operated. Once the spacecraft is far enough from the Sun that it needs to reduce the throttle level, the availability of power, and hence the thrust, generally will decline faster than the mass, so the effect of the thrust will diminish. By the end of the mission, a day of thrusting will provide less than half of the change in speed that it does now.
Of course, engineers have been accounting for this since they began designing the project. The entire flight plan from Earth to Vesta (via Mars) and from Vesta to Ceres is based on how much the craft can accelerate throughout its mission.
While the spacecraft will not reach Vesta until July 2011, the Dawn team has been hard at work developing the detailed plans for what it will do there. This month, they initiated the long process of formulating the specific instructions that will be radioed to the probe to carry out those plans, guiding it through all the steps it must follow to get into orbit, to perform the myriad scientific measurements that are planned, to transmit the results to Earth, and to remain healthy and productive in that distant and forbidding environment. The team is beginning with the “approach phase,” which commences in May 2011 and concludes when Dawn has completed thrusting its way to the first orbit from which it will conduct intensive observations in August 2011. (Of course, it will stop occasionally to peer at Vesta as it closes in on the enormous asteroid during the approach phase.) As work on each of the Vesta phases is completed, the team will turn its focus to the next, so by the time Dawn begins its approach, most of the instructions for its year at Vesta will have been prepared.
The commands will be checked and double checked just as they are for carrying out the interplanetary flight. For operating at Vesta so long from now, however, they also are being designed so that shortly before it is time to transmit them to the spacecraft, controllers can update them to account for the exact trajectory the spacecraft is on and other details that may change slightly.
In upcoming logs, we will describe some of the highlights of the plans that are being readied for when the ship reaches its first celestial destination.
To help refine preparations for flying near Vesta and studying it from Dawn’s vantage point, scientists are taking advantage of the convenient alignment between Earth and the protoplanet to observe it with the Hubble Space Telescope on February 25 and 28. The venerable 200-inch (5.1-meter) Hale Telescope on Palomar Mountain in California will be used for other Vesta measurements in April.
While engineers and scientists focus their attentions on Dawn, the ship continues to grow more remote. As we saw in the previous log, after closing in on each other for 14 months, Earth and the spacecraft are now separating again, their independent orbits around the Sun carrying them farther apart. On February 28, Earth will be equidistant from Dawn and the Sun. Readers on that planet will be at the apex of a broad cosmic isosceles triangle, 0.99 AU from both the tremendous star that has governed the solar system for 4.6 billion years and the tiny probe that is quietly and patiently making its way to investigate unexplored alien worlds to help us understand the dawn of the solar system. The third leg, between Dawn and the Sun, will be 1.84 AU long.
Although quite undetectable with all but the most sensitive radio receivers of the Deep Space Network, those who are share in the profundity and the passion for the exploration of the cosmos may wish to gaze upon the spacecraft with their minds’ eyes. It has been farther from Earth before, and other spacecraft have been much farther still, but while it is at the same distance as the Sun, it presents an occasion to reflect upon humankind’s achievements. Dawn’s milestone represents much more than the opportunity to gain fascinating new insights into the solar system and an exciting adventure to reveal vistas previously unseen. At the same distance as the Sun, it symbolizes the extraordinary success of science and engineering. At the same distance as the Sun, it compels us to mediate upon what humankind can accomplish when we are inspired to translate our grand ambitions into action. At the same distance as the Sun, it reminds us that some feats that once were even beyond imagination may be achieved. While physically we are confined to the vicinity of our home planet, the power of uncounted millennia of thoughts, hopes, and dreams combined with persistence, discipline, and tremendous cognitive effort allows us to have an extension of ourselves as far as the Sun. A spacecraft as far as the Sun is a triumph of humble creatures bold enough to reach out into the universe.
To aid in contemplating the nature of such grand undertakings, some readers may wish to peer in the direction of the invisible, distant craft. It is 5.5 degrees northwest of Mars, an easily identified ruby among the gems of the evening sky. (For reference, 5.5 degrees is 11 times the diameter of the Moon or about the width of 3 fingers held together at arm’s length.) It is roughly 2/3 of the way from Mars to the bright star Pollux. As the month ends, your correspondent (reporting on location from Earth) plans to contemplate his view of the Sun during the day and the sky near Dawn in the evening.
This will be the last chance to peer toward the spacecraft while it is this close. For the rest of its mission, and effectively forever, Dawn will be farther from Earth than the Sun. Yet it will remain eternally tied to the planet by virtue of the unique human thirst for knowledge, spirit of adventure, and insatiable yearning to know the cosmos, all of which propel it beyond distant horizons.
Dawn is 0.96 AU (144 million kilometers or 90 million miles) from Earth, or 395 times as far as the Moon and 0.97 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 16 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 pm PST February 24, 2010
Dear Plausible Dawniabilities,
Patiently and reliably continuing with its interplanetary voyage, Dawn is now flying in a new configuration and, from the perspective of those readers who may be on Earth, in a new direction.
The spacecraft still spends most of its time gradually changing its orbit around the Sun by thrusting with its ion propulsion system. The probe is outfitted with 3 ion thrusters, assigned the heartwarming names thruster #1, thruster #2, and thruster #3. (The nomenclature and locations of the units were divulged in a log shortly after launch, before such information could be distorted and used unethically by others.) The ship only uses 1 thruster at a time. All 3 were tested during the 80-day initial checkout phase of the mission, and when the interplanetary cruise phase commenced in December 2007, it was thruster #3 that was responsible for pushing the spacecraft away from the Sun. It performed flawlessly, but engineers plan to share the workload among the thrusters over the course of the 8-year mission, so thruster #1 was called into action in June 2008. By that time, stalwart #3 had been operated in space for 158 days. (For those readers who have just returned from an enjoyable excursion back to that log, the apparent discrepancy between the 158 days of operating time given here and the 149 days presented there is not an error. The smaller value is the operating time in the interplanetary cruise phase. Thruster #3 had accumulated about 9 days of operation during the initial checkout phase.)
Thruster #1 was in service until this month. Although it remains in excellent condition, engineers transmitted instructions in December for the spacecraft to reconfigure for use of a different thruster after its weekly communications session on January 4. By that time, #1 had thrust for almost 318 days. With its famously efficient use of xenon propellant, all that maneuvering consumed only 84.6 kilograms (187 pounds), yet it imparted 2.2 kilometers/second (4900 miles/hour) to the spacecraft.
Now it is #2’s turn. It had barely more than 1 day of total running time in space prior to this month, having been used only for some tests in November 2007 and April and May 2009 . Now 2010 will be its year to shine (with a lovely blue-green glow). In addition, as we will see in the next log, for the entirety of the mission, thruster #2 will have the distinction of providing the greatest acceleration to the spacecraft of any of the thrusters.
There is much more to the ion propulsion system than the thrusters. As explained in more detail in an earlier log, the system also includes 2 computer controllers and 2 units that draw as much as 2500 watts from Dawn’s solar arrays and converts the power to the currents and voltages the thrusters need. Controller #1 and power unit #1 are used for both thruster #1 and #3, so those electrical devices have already worked extensively during the mission, although most of their operation still lies ahead. For now, though, controller #2 and power unit #2 are in charge.
Although thruster #2 and its associated components have spent most of their time in space unpowered, they all are now performing just as smoothly as the other ion propulsion system elements did when they were in use.
Most of the artistic depictions of the spacecraft in flight happen to show it using thruster #2, the one nearest the main antenna. So the next time you see such an image, probably even at the top of this very page, you might consider that it is very much the way the spacecraft would look right now if you could see it.
Of course, Dawn is much too far from Earth to be seen by human eyes, even aided by the most powerful telescopes. But it has recently come nearer to the planet than it had been for nearly 2 years. As we have discussed in many logs (see, for example, November 2008), Earth and Dawn move independently through the solar system. Just as the hands of a clock sometimes move closer together and sometimes farther apart, Dawn and Earth sometimes approach each other and sometimes separate.
Some readers may not be at all surprised that even as the probe is receding from the Sun well over 2 years after launch, blazing a trail through the asteroid belt, constantly changing its own orbit (unlike most spacecraft, which coast most of the time, just as planets do), it is no farther from Earth than it was just 5 months after launch. They are excused from reading the material below. Others, however, may find this discussion helpful in thinking more about why this occurs. It is not important for the mission, but it may be satisfying for those who wish to direct a metaphorical gaze to the distant craft.
Unlike clock hands, Dawn does not travel in a circular path. Following the initial push away from Earth by the Delta rocket that carried it from Cape Canaveral into space, its orbit around the Sun was elliptical (see the second row of the table here). Its path has changed a great deal since then, principally because of the extensive thrusting (but also because of the gravitational boost from Mars).
Although elliptical orbits distort the picture a little, the essentials of the clock analogy are valid, so let’s imagine this alignment by considering the same clock we have used twice before, most recently last month (For readers who now have more clocks than room to display them, we promise that this will be that last reference to a clock from the Dawn gift shop, at least until your clocks’ warranties have expired.) With the Sun at the center, Earth is at the tip of the shorter hand and Dawn at the tip of the longer one. On January 25, the star, planet, and spacecraft were aligned as closely as the hands of the clock would be at 6:32:16.
When positioned that way, the Sun and Dawn were nearly in opposite directions from Earth’s vantage point. Suppose you were on Earth on that date and wanted to look in the direction of the spacecraft. You would have put the Sun at your back and Dawn would have been less than 6 degrees from your line of sight, equivalent to being in the center of a (different) clock, having the 12 at your back, and instead of looking at the 6, shifting your gaze almost to the next tic mark. (The positions constantly change, and by the middle of February, you would need to readjust your gaze to the 7, still keeping the Sun at the 12.)
Although the alignment is the result of the motion of both Earth and the spacecraft, from the terrestrial perspective, with our deceptive sense of cosmic immobility, it seemed that Dawn had been moving closer to us. Now it seems to be moving away.
Dawn reached its greatest distance from Earth so far in the mission on November 10, 2008. [Note: We had decided that it was unnecessary to include a link to that paragraph, thanks to our encouragement therein for readers to memorize it. According to our new consultants, Prescient Telepaths ‘R Us, you are the sole reader who did not commit it to memory. Therefore, in our goal to make every customer happy, we are pleased to include the link specifically for you. Enjoy!] At that time, it was 2.57 astronomical units (AU) from Earth. Since then, while its orbit has carried it closer to the Sun and then farther again, the distance to Earth has been declining the entire time. The spacecraft and its planet of origin finally moved to their closest point on January 18, when their travels brought them to 0.80 AU from each other. (It occurred at about 2:00 am PST, so if you sleep deeply, you may have missed it.) The minimum distance did not occur at exactly the same time as the nearly linear arrangement because the orbits are not as simple as the circular motion of the clock hands.
The last time they were this close was on March 11, 2008. They will never be so near each other again. Earth follows the same orbit around the Sun year after year, but with Dawn constantly changing its trajectory, pushing deeper into the solar system, the next time it and Earth are aligned on the same side of the Sun (in August 2011), the explorer will be much farther away. Indeed, if all goes according to plan, it will be in orbit around Vesta by then, beginning to reap the rewards for its long expedition through the cold depths of space, as it explores a distant and alien world that waits silently for its first visitor.
Dawn is 0.82 AU (123 million kilometers or 76 million miles) from Earth, or 345 times as far as the moon and 0.83 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 14 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 pm PST January 30, 2010