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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The visible and infrared mapping spectrometer (VIR) and the primary science camera also were turned on for the first time in more than half a year. As these sensors yield complementary data, controllers want to refine earlier measurements of exactly how their views overlap. This will allow scientists to correlate observations from the instruments in order to glean as much as possible about the nature of the protoplanets the craft will orbit. Dawn rotated to point at a star and then observed it simultaneously with VIR and the camera. By measuring precisely where the star registers in each device, their relative alignments can be pinned down. Upon completing the sequence of commands to acquire the desired data, the spacecraft turned to point its main antenna to Earth again and began transmitting the results during the next scheduled session with the Deep Space Network a few hours later.

The VIR team quickly discovered that a subtle incompatibility between certain instructions in the program for recording the signals from the star caused its shutter to remain closed. (VIR also has a reusable protective cover, but that operated as intended.) The unit continued to function and stayed healthy, but it did not perform the planned observations. The science camera imaged the target, but the purpose was to compare where the star appeared in the 2 instruments. The VIR commands are easily corrected, and the calibration will be executed again early next year.

Earlier this year, engineers developed new software for the science camera to improve its efficiency in mapping the distant worlds Vesta and Ceres. The software was updated once before in space, and the process followed this week was the same. As last year, loading software into the primary and the backup cameras was performed as entirely separate activities; each camera was off while the other was being upgraded. This was the only major work this week that was not accomplished with commands that had previously been stored on the spacecraft. After the new software was installed, each camera was directed to carry out a set of tests, and the results confirmed that both were operating correctly.

Among the other tasks this week was an annual evaluation of the backup star tracker, a device that recognizes star patterns so the spacecraft can calculate its orientation. To verify that the tracker remained healthy, the unit was powered on and operated. It correctly took pictures, identified the stars, and then determined the direction it was pointed. The tests verified that the unit remains in good condition and ready to be called into service in the unlikely event a problem with the primary tracker occurs.

On December 4, after completing all of its scheduled activities for the week, Dawn turned once again to point ion thruster #1 in the direction needed for propelling itself to Vesta, and resumed emitting high-speed xenon ions. It has continued since then with its familiar schedule of quiet cruise.

As the effect of the thrust continues to build up, tomorrow Dawn will pass another milestone. The thrusting since the beginning of the mission will have achieved the equivalent of accelerating the spacecraft by 2.00 miles per second (3.22 kilometers per second, or 7200 miles per hour). This is well in excess of what most spacecraft accomplish with their propulsion systems but is less than 1/3 of the planned maneuvering for the mission. To achieve this extraordinary velocity, Dawn has expended less than 126 kg (278 pounds) of xenon propellant during 474 days of powered flight. While the day-to-day change is small (as we will discuss in greater detail in February), with 24 hours of thrusting yielding just 7.2 meters per second (16 miles per hour), the benefit of its acclaimed patience is becoming evident.

As we have discussed several times (see, for example, this previous log), Dawn’s actual speed has not changed by the values just presented. In the complex orbital dance it performs, partnered principally by the Sun but with others joining in as well (Mars being the most significant this year), the more it thrusts and climbs away from the Sun, the slower it travels. Nevertheless, the equivalent change in speed (that is, the change that would be achieved in the absence of the complications from being in orbit) is a handy measure of the effect of any spacecraft’s maneuvering.

While Dawn continues pushing away from the Sun and deeper into the asteroid belt, the distance to Earth is still declining, as it has been since November 2008. The separation between the planet and the probe varies just as the distance between the tips of the hour hand and minute hand increases and decreases every hour. That suggests that it’s time once again to refer to one of the clocks available in the Dawn gift shop on your planet. (If you didn't get around to preparing for the recent festivities marking the universe's reaching its present age, don't despair. Although there are only 5 trillion shopping days until the next such gala celebration, Dawn gift shops in most galaxies are offering attractive discounts right now.)

To picture the changing alignment, let’s recall the clock described 365 days ago, with the Sun at the center. Dawn is at the tip of the minute hand and Earth is at the tip of the shorter hour hand. One year ago today, the celestial alignment corresponded to the position of the hands at about 6:01:45. At that time, Dawn was 2.49 astronomical units (AU) from Earth. In the intervening year, Earth has completed 1 orbit around the Sun, returning to where it was. Having traveled more slowly, Dawn is in a different position now that happens to be much closer to Earth. Today the alignment is similar to that at 6:30:00. Even though Dawn is farther from the Sun today than it was 1 year ago (as if the length of the minute hand had increased), in its current location around the clock face, it is 0.84 AU from Earth, only 1/3 of what it was at the end of last year. The cosmic hands will continue to move into still-closer alignment until late next month, when the Sun, Earth, and Dawn will lie nearly along a straight line.

Picturing Dawn’s position relative to Earth and the Sun may help some readers gain perspective on the explorer’s interplanetary journey, and we will continue to present such illustrations (at least as long as the increased revenue for the gift shop makes it profitable to do so). Nevertheless, it is worth bearing in mind that from Dawn’s perspective, the location of Earth is of little importance (except when it needs to point its antenna there). The ship travels on its own course around the Sun, independent of the motions of the distant celestial port from which it set sail more than 2 years ago. Dawn’s sights remain firmly fixed on the destinations ahead, where it seeks to unlock secrets about the dawn of the solar system.

Dawn is 0.84 AU (125 million kilometers or 78 million miles) from Earth, or 345 times as far as the moon and 0.85 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
6:30:00 pm PST December 30, 2009

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

After crossing the threshold of the belt earlier this month, Dawn will travel 7.7 astronomical units (AU), or nearly 1.2 billion kilometers (almost 720 million miles), to its July 2011 rendezvous with Vesta. Yet in all that time, and across all that distance, the closest the probe will come to a catalogued asteroid is 1.0 million kilometers (greater than 600 thousand miles), or more than 2.5 times the distance between Earth and the moon. Certainly travelers on Earth would not consider something that far away to be a hazard (especially compared to what many Dawn team members regularly experience on the freeways in Los Angeles), and neither would our intrepid explorer.

To bring this down to a more tractable scale, we can imagine Dawn’s journey through the asteroid belt to Vesta as a trip from New York City to Los Angeles, with rocks littered along the way. In this case, along the entire route to a bizarre and forbidding land, the nearest we would come to one of these rocks would be 3.4 kilometers (2.1 miles) -- hardly a close call. At that distance, it would be difficult even to detect the rock, as it would be a mere 1.5 centimeters (less than 5/8 of an inch) in diameter; this corresponds to an asteroid less than 5 kilometers (under 3 miles) across. Even looking out to 20 kilometers (12 miles) during our trek, the largest object we would pass would be just 3.4 centimeters (1.3 inches), representing a 10-kilometer (6-mile) asteroid Dawn will miss by 15 times the distance between Earth and the moon.

Dawn is bound for the giants of the asteroid belt. Vesta’s equatorial diameter is about 580 kilometers (360 miles), and Ceres is 975 km (605 miles) across. (Remember that when thinking about three-dimensional worlds such as these, the diameter may fail to illustrate how large they really are.) Together these two behemoths contain more than a third of all the mass in the main asteroid belt. On the scale of our cross-country drive, Vesta would be 2.0 meters (6.5 feet) wide and Ceres would be 3.3 meters (11 feet). Rather than missing them by great distances, we would move to within 0.6 meters (2 feet) of the first target and 2.4 meters (8 feet) of the second.

Dawn’s science instruments are optimized for studying these immense bodies in detail from orbit around them, just as many Earth-observation spacecraft peer down constantly on our planet. Diverting the probe to zip past a chunk of rock for a very brief view would be possible, but doing so would take precious time away from the far richer and more valuable investigations planned for Vesta. That is where Dawn will find the rewards of the next 20 months of travel.

While astronomers observe members of the asteroid belt as small as about a kilometer (a mile), what about still smaller rocks that are large enough to damage the spacecraft? Because available telescopes generally are not powerful enough to detect such objects from Earth, mathematical models are used to predict their prevalence and thus Dawn’s likelihood of encountering them. Although far more abundant than the larger asteroids, there still are too few pebbles distributed over the enormous volume of space through which the ship sails to pose a serious threat.

The spacecraft was designed so that the tiniest particles, which are sufficiently plentiful that some likely will strike it, cannot inflict significant damage. Dawn’s largest area is in its solar arrays, and asteroidal dust cracking a few of the 11,480 cells is inconsequential. More sensitive components are covered with protective materials that will cause the high-speed grains to break up and slow down before they reach the vulnerable elements. There is good reason to believe Dawn’s travels in the asteroid belt will be safe.

Even as Dawn recedes from the Sun, Earth (moving faster in its tighter solar orbit) is approaching the spacecraft; indeed, the distance has been decreasing for more than a year (and will continue to do so for another 2 months). On December 5, the craft and the star will be equidistant from the planet. We saw instances of these 3 members of the solar system family forming a triangle with 2 equal sides, known as an isosceles triangle, on May 28, 2008 and again on September 18 of this year. In those cases however, the equal sides were those between Dawn and Earth and between Dawn and the Sun. Next month, it will be Earth at the apex of the astronomical triangle, with both the spacecraft and the Sun at a distance of 0.99 AU. The third leg of the triangle, from Dawn to the Sun, will be 1.70 AU.

To illustrate the geometry, let’s use one of the new clocks that have just reached the shelves of the Dawn gift shop on your planet. (And note that for any purchase through the end of 2009, we will donate a used xenon ion to the charity of your choice.) With Earth at the center of the clock face, if the Sun were at the 10, Dawn would be the same distance but at the 2. (The clock hands are not important here; the objective is to illustrate the relative lengths and the angles of the isosceles triangle. Ignoring the hands also lets us offer the clock at a very low price!)

Any readers who happen to reside on or be visiting Earth on December 5 may find this arrangement a convenient opportunity to contemplate something of the nature of an interplanetary voyage. Dawn is quite invisible even to the most powerful telescopes, but it will be at the same distance as the most easily detectable extraterrestrial body, the Sun. The spacecraft has been more remote (as have other probes) and will be again later in the mission, but on that day it will be just as far from Earth as the star that rules from the center of the solar system. While the Sun has seemed -- indeed, has been -- unreachably distant for the overwhelming majority of human history, farther even than any horizon travelers could set their sights on, a craft that we set sail upon the cosmic ocean will be exactly that far away.

To add more dimensions to our mental imagery of Dawn’s location, we can take advantage of another celestial reference on December 6, before the triangular alignment of the previous day has changed noticeably. At about 8:30 am PST, the spacecraft will appear just over 2 degrees (or a little more than 4 times the moon’s diameter) north of the moon. As the moon’s orbit carries it around Earth, it will be less than twice that far from the apparent position of the spacecraft for the 6 hours before and after that time, so anyone who can see the moon during that interval can get a rough fix on Dawn’s location. For readers in North America, the alignment occurs when the moon is the western sky after dawn (yes!). From the vantage point of the center of the clock, observers may be able to see both the Sun and the approximate location of the spacecraft at the same distance, letting their imaginations take over where their eyes leave off. Out there, in that direction, as far as the Sun, will be Dawn, patiently, reliably, silently continuing its bold voyage of exploration.

Dawn is 1.03 AU (154 million kilometers or 96 million miles) from Earth, or 395 times as far as the moon and 1.05 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 17 minutes to make the round trip.

Dr. Marc D. Rayman
4:30 pm PST November 27, 2009

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The probe has been here before. On June 30, 2008 it passed the outermost part of Mars’s orbit. But its elliptical path reached its greatest distance from the Sun of more than 1.68 AU on August 8, 2008, and 40 days after that, it crossed the orbit of Mars again. On April 17, 2009, then at 1.37 AU from the Sun, its momentum began carrying it outwards once again. By then it was in a larger orbit, and thanks to the extensive additional orbital energy imparted to the spacecraft by its persistent ion thrusting, it will sail smoothly through 1.68 AU next month and continue deeper into the asteroid belt.

As Dawn continuously enlarges its solar orbit still more, mission controllers work diligently to ensure the distant craft remains healthy. They are also preparing to give it some additional tasks before the year is out, and inside sources reveal that these may be described in an upcoming log. In the meantime, emitting its eerie bluish glow, the probe silently streaks toward unexplored worlds, seeking to reveal new secrets and likely new questions as well.

Dawn is 1.25 AU (187 million kilometers or 116 million miles) from Earth, or 485 times as far as the moon and 1.26 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
11:30 pm PDT October 31, 2009

P.S. Although Dawn works tirelessly in interplanetary space, the team on Earth is taking a break for Halloween. Observant readers have already noticed that this correspondent has dawned his costume, and it is a delightful and impressive disguise indeed. In an act of astonishing creativity, he is pretending to be someone who can write a (relatively) short log.

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Since launch, our readers who have remained on or near Earth have completed 2 revolutions around the Sun, covering about 1.88 billion kilometers (1.17 billion miles). Orbiting farther from the Sun, and moving at a more leisurely pace, Dawn has traveled 1.57 billion kilometers (980 million miles). As it climbs away from the Sun to match its orbit to that of Vesta, it will continue to slow down to Vesta’s speed. Since Dawn’s launch, Vesta has traveled only 1.18 billion kilometers (730 million miles).

Readers with nothing better to do have already discovered that much of the text in the 3 preceding paragraphs is taken verbatim from the log that commemorated Dawn’s first anniversary of being in space, with the principal changes being that the numbers are updated here. (In addition, most of the humor was removed to comply with a request from the Glum Legion of Ardent Dawnniversaries). This is not a result of any more otiosity than normally displayed by your correspondent; rather, comparing the beginning of this log with last year’s may be helpful for measuring the progress in the intervening time. Of course, most of the last 12 months was devoted to coasting, and the gravitational boost from Mars is not reflected in the effect of the ion thrusting, but the comparison may be illuminating for some readers. This also provides a handy preview of the beginning of the September 27, 2010 log. [Note to self: Perhaps there really is an option here for greater lassitude. Think about that after taking a nap.]

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the Sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the Sun, they sometimes move closer and sometimes move farther from it. For thinking about these distances, we may remind ourselves once again of the convenient unit of measure in the solar system, the astronomical unit (AU).

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the Sun is a good reference. Other planets and interplanetary spacecraft travel in orbits that are tipped at some angle to that. Vesta and Ceres do not orbit the Sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the plane of Earth’s orbit, and no spacecraft has had to venture as far out of that plane to orbit another body as Dawn will.)

Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the Sun) and the angle of its orbit on its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy is to link to the experts’ sites when their readership extends to 1 more elliptical galaxy than ours does.)

The table below shows what the orbit would be if the spacecraft terminated thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on September 27, 2007, its orbit around the Sun was exactly Earth’s orbit. After launch, it had its own orbit.

Readers may disregard the table or gaze into it for insight or inspiration for as long as they like. The point of it, however, is to illustrate both that Dawn has come a long way since the launch pad, and it has a long journey ahead before it begins its exploration of Vesta.

But the trek will be a little shorter than mission planners had anticipated until quite recently. As we have seen in a previous log, the plan for thrusting depends on how much electrical power will be available to the ion propulsion system, which converts electrical power into thrusting power. Greater electrical power translates into higher (but still exceptionally gentle) thrust.

Last year Dawn’s engineers, who remain on distant Earth, devised a method to calibrate the solar arrays, and the spacecraft dutifully carried it out. The resulting data, combined with an extensive refinement of the mathematical model that predicts solar array power, allowed the team to be confident in increasing the prediction of the future availability of power by up to 10%. Equipped with this crucial information, they could update the plan for thrusting.

Many other factors affect the design of the thrust profile as well. As one example, how effective the thrust is depends on how massive the spacecraft is. Although weightless, Dawn still has mass (the resistance to a change in its velocity), and the greater the mass, the lower the acceleration provided by the ion thruster. This phenomenon is no different from what readers experience frequently even in the gravity of their home planet. The heavier the load you carry, the more gradually you will accelerate, whether the effort is exerted by the muscles in your legs (or wings or tentacles, depending on your species) or the engine in your car (or spaceship). Dawn’s mass decreases as the mission progresses because the ion propulsion system expends xenon and the reaction control system expends hydrazine. By refining predictions for how much of these propellants will be onboard at all times for the rest of the mission, engineers could predict how long it will take Dawn to propel itself into the same orbit around the Sun as Vesta and then later into the same orbit as Ceres.

After an extended set of analyses late in 2008 and the first half of 2009, all the elements needed to update the thrust plan were in place. The seemingly modest improvement in solar array power is by far the dominant one. When all were combined, the result revealed that Dawn’s remarkable maneuvering capability over the course of the mission will be even better than engineers had been counting on. The probe will be able to reach Vesta about 6 weeks earlier than had previously been planned. Moreover, the newfound capability will enable the craft to travel from Vesta to Ceres more quickly, so the deadline for leaving the first world to reach the second on schedule in 2015 is about 6 weeks later.

Together, these changes allow the explorer to increase its planned 9-month stay at Vesta to 12 months. This is of extraordinary benefit to the project. Vesta promises to be a fascinating place to visit, and we know quite well from other solar system adventures that no matter how much data we collect, there is always still more to learn. Mission planners had been working hard to squeeze as much as possible into the precious time they expected Dawn could spend at Vesta, so being able to increase the duration of its residency there by a third makes a tremendous difference. As details continue to be formulated for all the activities necessary to operate at and study this alien world, the additional time will prove extremely valuable in allowing the team to accommodate the glitches that are inevitable in such a complex expedition and to uncover as much of Vesta’s intriguing story as possible.

Dawn is already following the new flight plan, targeting where Vesta will be in July 2011. It is not enough, though, just for them to be in (nearly) the same place at the same time. That would result in a flyby, but our probe will enter orbit around the protoplanet, accompanying it on its orbit around the Sun, just as satellites of Earth remain close by throughout the planet’s solar orbit. The craft will remain with Vesta until July 2012, when it will begin thrusting to Ceres. We have discussed before why flying by (providing only a glimpse of each body) is significantly less challenging than matching orbits (enabling more extensive explorations), a capability that would be essentially impossible without the ion propulsion system. A subsequent log will delve further into this issue, as it is a fundamental feature of this ambitious mission.

As Dawn begins its third year in space, now on its new and better course, much work remains before it can return the scientific bounty it seeks. We hope readers will continue to follow the progress of this bold adventure in the exciting years to come.

Dawn is 1.50 AU (225 million kilometers or 140 million miles) from Earth, or 555 times as far as the moon and 1.50 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 25 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 am PDT September 27, 2009

P.S. The astronomical unit has been mentioned in these logs frequently enough that we will include that convenient unit of measurement from now on in the famously unimaginative concluding paragraph. It might appear redundant to present the distance from Earth both in astronomical units and in terms of how many times as far as the Sun it is. Isn’t that simply 2 different ways to describe exactly the same quantity? Well, no it is not; they are different, although they are close. An astronomical unit is the average distance between Earth and the Sun and hence does not change. The actual distance varies slightly throughout the year, so Earth’s distance from the Sun at any given time may not be precisely the average value of 1.00000000 AU (149,597,871 kilometers or 92,955,629 miles). This would be more apparent if your correspondent did not round off the numbers as dramatically. The details on that closing text are that Dawn is 1.50456971 AU (225,080,425 kilometers or 139,858,224 miles) from Earth. At the same time, Earth is 1.00222102 AU (149,928,510 kilometers or 93,161,078 miles) from the Sun, very close to the average, but not exactly equal to it. So Dawn is 1.50123544 times as far from Earth as the Sun is, given the distance to the Sun now. When rounded off, the distance in astronomical units and the distance in terms of how far the Sun is both come out to 1.50, but we see they are not really equal. Other times of the year, when the actual distance to the Sun is farther from the average, the difference will be apparent. As long as these secrets of the final paragraph are being revealed, here are the rest: the distance relative to the moon is rounded to the nearest multiple of 5, and the travel time for radio signals to the nearest minute. But just for this special occasion: Dawn is 556.865373 times as far as the moon right now, and radio signals take 25 minutes 1.574966 seconds. Approximately. Best regards to the Numerivores.

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Another event that is now considered routine occurred on August 15. For the second time this year, a particle of space radiation struck a particularly sensitive electrical component on the spacecraft, depositing enough energy to interfere with the operation of a circuit. When this happened in January 2008, it caused Dawn to enter safe mode, interrupting its other activities. Thanks to software the team transmitted to the ship later that year, now the interplanetary explorer is immune to strikes in that formerly vulnerable location.

As Dawn continues its long (in space and in time) solar system journey to match orbits with Vesta and later with Ceres, both of which reside farther from the Sun than the probe has yet traveled, some readers may note a surprising trend in the statistics for the mission. The famously unimaginative ending of each of these logs reveals that Dawn’s distance from Earth has been diminishing since November 2008. Indeed, the probe’s maximum separation from its planet of origin occurred on November 10. Today, it is as far from Earth as it was on June 2, 2008. By January 2010, it will be as close as it was in March 2008. Is this progress?

Earth and Dawn, each following its own path, are both in orbit around the Sun. As grateful residents of the planet know, their world’s orbit doesn’t change very much. The planet keeps following the same nearly circular path around the Sun year after year after year. Today Earth is about 1.01 astronomical units (AU) from the Sun, and it never strays very far from its average distance of 1.00 AU. As Dawn has traveled independently of Earth, thanks to the push from its Delta rocket, its orbit has been farther from the Sun than Earth’s. The gradual effect of ion thrusting and the much more abrupt boost from Mars have caused that orbit to change considerably since then. To enter orbit around Vesta, Dawn will have to match the giant asteroid’s orbit around the Sun, ranging from 2.15 AU to 2.57 AU. Today, Dawn is 1.53 AU from the Sun and headed outward.

As we saw a year ago (that is, one Earth-orbit-around-the-Sun ago), objects at different orbital distances travel at different speeds. The probe, orbiting the Sun at a greater range than Earth, travels more slowly, because the Sun’s gravitational attraction diminishes with distance. So as Dawn heads slowly for Vesta, gradually spiraling away from the Sun, and Earth speeds around more quickly in its orbit, sometimes our planet moves closer to the spacecraft and sometimes it moves farther away.

In a continuing effort to offset the extraordinary cost of these logs with the handsome revenue from subtle product placements, we can refer to still another in the apparently endless line of Dawn clocks (many of which have been described in recent logs and all of which are available in the Dawn gift shop on your planet). On this clock, the minute hand is shorter than the hour hand. The motion of the former represents Earth, traveling closer to the Sun (at the clock’s center) and more quickly. Dawn is at the tip of the hour hand, moving more slowly in its larger orbit. (We’ll ignore for now that the hour hand should be growing in length, as the spacecraft recedes from the Sun.) Some of the time (such as between noon and about 12:30), the distance between the ends of the hands increases, but then the situation reverses; the faster minute hand begins moving closer and closer to the hour hand as the time approaches about 1:05.

Earth and Dawn are exhibiting the same repetitive behavior, albeit more complicated because of Dawn’s ever-changing orbital speed and distance from the Sun. They will continue to draw closer until January, when Earth, coming from behind, passes Dawn and moves on ahead. The explorer will not need to take note however, as its sights are set on the asteroid belt.

So for readers tracking the distances reported in each log, don’t despair. The continuously declining separation between Earth and its celestial envoy is a reflection of the elegant mechanics of the cosmos and not the result of inattentive engineers setting the spacecraft on the wrong path.

Next month, as Earth and the spacecraft continue their separate solar system dances, together with the Sun they will briefly make an attractive arrangement. On September 18, Dawn will be just as far from the Sun as it is from Earth, at 1.56 AU from each. Earth and the Sun will be 1.00 AU apart. The trio formed a very similar pleasing pattern last year. A triangle such as this, with two sides of equal length, is usually called “isosceles.”

Although there is nothing inherently significant for the mission about this alignment, we can use one more clock example to illustrate this isosceles triangle. In this case, we put Dawn at the center and Earth at the 12. (This clock may not be as useful for telling time as some of the others that are available, but it would still make a great gift.) The Sun would be next to the sixth little tick mark, where the minute hand would point at about 6 minutes and 15 seconds after the hour. (Note that this depiction of the geometry illustrates the angles and the relative separations of Dawn, Earth, and the Sun; hence, the clock may be any size. Several sizes are available in the gift shop, and we helpfully recommend the most expensive one.)

Not only is Dawn on course for the asteroid belt, on course for returning new and exciting discoveries from its enigmatic destinations, it is on a new and better course than it had been. According to inside sources, the vast team of writers specifically assigned to create the next log is already planning to explain what has changed and why. Just as all loyal readers, your correspondent is hoping for an interesting description of this improvement in the mission.

Dawn is 250 million kilometers (156 million miles) from Earth, or 620 times as far as the moon and 1.66 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 28 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 pm PDT August 30, 2009

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The task is far more difficult than simply enlarging its orbit to have precisely the same shape as Vesta’s. In addition, Dawn has to change the orientation of its orbit so it overlaps Vesta’s. Even that is not sufficient, however, because the spacecraft also has to be in the same place in its orbit that Vesta is in its. It wouldn’t be helpful to be on exactly the same path but be very far apart.

Still, let’s consider just the problem of changing the size of the orbit. If Dawn thrust all day today but stopped tomorrow, the one day’s worth of work would change the orbit by only about 0.0004 AU. For another perspective on the effect of the thrusting, compare the orbit size 2 paragraphs above with the orbit Dawn was in after the boost from Mars. The difference is the result of 7 weeks of powered flight, yet on the scale of these vast distances, it is so small it barely registers. Yes, there is indeed a great deal of work ahead.

Dawn will reach Vesta in about 2 years. The key to getting there is the combination of the extraordinary efficiency of the ion propulsion system with the patience and reliability of all systems. The probe’s persistence in quiet cruise will pay off with the excitement of its discoveries in the asteroid belt.

Now why is this deemed “quiet cruise”? It’s true that in space no one can hear you thrust, but that’s not the reason. Rather, it is considered quiet because the spacecraft is not engaging in any special routines or functions (assuming you don’t consider traveling for years in deep space to be special). Thrusting is Dawn’s most familiar activity. Even with the long coast from October 31, 2008 to June 8, Dawn has spent about half of its time since launch tirelessly adjusting its orbit. In contrast, the great preponderance of spacecraft coast all of the time (or nearly so), just as planets and asteroids do, simply going where their orbits take them.

Dawn has already had plenty of time in space that was not quiet (readers of these logs have shared in some such times), and much more lies ahead, particularly when it is in orbit around its two very distant targets. For now though, the spacecraft’s focus is on its trek to Vesta. While its engineering colleagues will continue to be diligent in maintaining Dawn’s health and providing regular updates to its flight plan, the quiet cruise does afford the team more time to develop and refine plans for operations at Vesta. A great deal of work remains to prepare for that not-so-quiet time.

But really, how quiet is quiet cruise? In fact, Dawn is a hive of activity. For the probe to reach its targets in the asteroid belt, all engineering systems labor together. For the following discussion, reminders of the essentials of each of these systems are available by clicking here (note that this link works only for loyal readers), and another detail is available here.

For the ion propulsion system to keep emitting a steady stream of high-speed xenon ions, it must regulate the delicate flow of xenon from the main tank at more than 1000 pounds per square inch (for you readers on Earth, that’s about 70 times atmospheric pressure, and for you readers on Venus, that’s low pressure) to the thruster at roughly one millionth of a pound per square inch (around one ten-millionth of atmospheric pressure). It constantly ionizes the xenon and electrically accelerates it, requiring careful control of the high currents and voltages. The electrical power system delivers to the ion propulsion system the needed power; it supplies all other systems as well. It draws its energy from the solar array wings and converts it to the voltage its onboard customers need.

To keep the huge arrays pointed at the Sun and the ion thruster aimed in the direction needed to travel to Vesta, the attitude control system is always at work. Five times every second it takes a fix with its star tracker and computes the orientation, or “attitude,” of the craft in the void of space. If needed, it rotates the solar arrays, and it uses several different means to adjust the entire spacecraft’s attitude to achieve the required thruster pointing.

The telecommunications system continually transmits a radio signal through one of the small antennas, broadcasting a very wide beam that encompasses Earth so that mission controllers can listen in whenever they choose. The system is also unceasingly alert, listening for an almost imperceptible radio whisper from Earth in case human team members need to contact the probe between the weekly sessions that use the main antenna.

Every second the thermal control system reads more than 100 temperature sensors and decides which heaters to turn on or off to keep each component from becoming too cold or too hot.

The command and data handling system, including the main computers, is orchestrating most of this activity. It switches its attention 200 times per second as it communicates with other components. About 35 times every second it assesses all the data available from around the ship and selects some for storage and subsequent transmission to Earth through the main antenna. More than 200 parameters are checked each second so that if there are any problems, the software can take action, promptly issuing instructions to protect the craft and the mission, ensuring unexpected situations do not get out of control. And while performing these and many other functions on its own, every second the software checks to find out whether there is a stored command from mission control. Engineers formulate these directives many weeks in advance and place them onboard to be carried out at the precise second planned.

So quiet cruise really is not so quiet. And yet, this is what Dawn was designed to do. In the forbidding depths of space, all the systems work together in a near frenzy of activity to hold the stalwart ship steady, keeping it healthy and on course, as it maintains its sights on the distant horizon, where the asteroid belt beckons.

Dawn is 275 million kilometers (171 million miles) from Earth, or 710 times as far as the moon and 1.84 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc D. Rayman
12:30 am PDT July 28, 2009

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Dawn thrusts all but about 6 to 8 hours per week, providing only a brief opportunity to turn away from the direction it needs to aim its ion thruster in order to point its main antenna at Earth. That weekly radio communications session affords the robotic explorer its sole contact with mission controllers. While it is thrusting, Dawn is programmed to broadcast signals from one of its small, auxiliary antennas, spreading its radio signal in a wide swath that encompasses distant Earth. Usually one of the exquisitely sensitive receivers of NASA’s Deep Space Network will listen in on the spacecraft for a few hours halfway through the week, capturing the extraordinarily faint transmission showing the spacecraft is sailing smoothly.

Spending so much time thrusting is possible thanks to the extremely frugal use of xenon propellant. The ion thruster expels only about 0.26 kilograms (10 ounces) in a day. So while Dawn would need nearly 4 days to accelerate from 0 to 60 miles/hour, it would consume little more than 1 kilogram (about 2.3 pounds) of its supply of xenon during that time.

As the probe climbs away from the Sun to reach the cold depths of the asteroid belt, the multiyear thrust profile is designed to make its solar orbit match that of Vesta. The current flight plan has it arriving at the massive protoplanet in September 2011, requiring it to thrust for more than 700 days along the way, the significant majority of the time.

Prior to resuming thrust, the spacecraft carried out a routine check of one its scientific instruments. All of the instruments designed to uncloak the secrets Vesta and Ceres hold about the dawn of the solar system spend most of the time during the interplanetary cruise switched off, waiting for their opportunities to go to work in orbit. Each instrument is powered on occasionally to verify its health. On May 27, the visible and infrared mapping spectrometer (VIR) was activated. On this occasion, VIR repeated the routines it first executed in space in October 2007. All of its mechanisms were exercised, and they operated smoothly. Instead of aiming at distant celestial targets, its visible and infrared detectors measured emissions from built-in lamps. VIR passed the 4-hour test with flying colors (some of which are outside the range of human vision).

The VIR operation was one of many assignments for the coast period, most of which have been described in logs since November 2008. With all activities completed successfully, the spacecraft set about thrusting right on schedule. On June 8, executing instructions already stored in its main computer, Dawn rotated to point thruster #1 in the required direction. It powered on the ion beam shortly after 11:59 am PDT. Any readers who happened to be in the vicinity during their own deep-space excursions would immediately have recognized the familiar scene: Dawn majestically perched once again atop a blue-green pillar of xenon ions, as its ambitious journey of exploration continues.

Dawn is 291 million kilometers (181 million miles) from Earth, or 775 times as far as the moon and 1.91 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

Dr. Marc D. Rayman
5:00 am PDT June 28, 2009

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.