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

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.

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.

Near-Earth objects are asteroids and comets with orbits that get close to Earth's orbit. That doesn't mean they are going to hit the Earth, of course. It's sort of like driving on a busy street; just because there are a lot of cars zipping by on either side of you, it doesn't necessarily mean your car is going to hit one. The cars would have to be at the same place at the same time for that to happen. So even though the paths each car has traveled might get close, there is no collision.

WISE finds asteroids by using a sophisticated piece of software called the WISE Moving Object Processing System, or WMOPS. When we first get a set of images from WISE, we have software that automatically searches the images for all the sources in them, be they stars, galaxies or asteroids. The software records their positions and how bright they are. WMOPS goes into that source list and figures out which sources are moving compared to the fixed stars and galaxies in each frame. Then, it figures out which sources are actually the same object -- just observed at different times. So it's a pretty smart piece of code. The whole system has to be highly automated, since when the WISE survey is done, the source catalog will contain several hundred million sources! You can imagine that trying to sort through all of these to find individual objects would be very challenging without a nifty program like WMOPS.

Our newest addition to the approximately 6,600 near-Earth Asteroids that are currently known is shown in this new image above.

2010 AB78 shows up like a glowing red ember at the center of the image, because it's glowing brightly in infrared light with a wavelength of 12 microns, which is about 20 times redder than your eye can see. The stars appear blue, because they're much hotter, and they emit proportionally less of their energy at these long wavelengths. The color that the asteroids appear to WISE is an important feature we use to distinguish them from other stars and galaxies, in addition to their motion.

With this first asteroid discovery, we are flexing our muscles in preparation for the heavy lifting we're about to start.

TAGS: WISE, ASTEROIDS & COMETS, SPACECRAFT, MISSION, UNIVERSE

• 

  ABOUT THE AUTHOR

  Amy Mainzer, Scientist

  Amy Mainzer is an astronomer specializing in astrophysical instrumentation and infrared astronomy. She is currently the principal investigator for the NEOWISE comet-hunting mission.

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.

These last few weeks and days before launch require a lot of flexibility of the team, since the schedule can change on a dime. There are about a million things having nothing to do with the launch vehicle or the spacecraft that can delay a launch -- winds, too much fog, too many clouds, lightning and even something as mundane as a fishing boat or aircraft straying into the "keepout" zone that's established around the launch site. You would think that the prospect of running into a giant, 330,000-pound rocket loaded with fuel would be enough to make people move out of the way, but sometimes they don't seem to get the message! Any of these items is enough to scrub a launch attempt.

But that's why we've built in the ability to make two consecutive launch attempts with WISE, separated by 24 hours. We get two tries. After that, our tank full of frozen hydrogen starts to warm up too much, and it takes two days for us to cool it back down. To keep the tank of frozen hydrogen a frosty 7 degrees above absolute zero (minus 447 Fahrenheit), we circulate an even colder refrigerant, liquid helium, around the outside of the tank. But the process of re-cooling takes two days; we have to hook all the hoses back up, cool everything down, then disconnect the hoses again before the next launch attempt.

So we have to be flexible. We've all put our lives on hold for the duration, since we have to be ready for anything that happens. Meanwhile, I've frantically tried to take care of stuff like cleaning the house and laying in supplies, because once WISE launches, things will go into overdrive. Needless to say, our families have all been very patient with us!

TAGS: WISE, ASTEROIDS & COMETS, SPACECRAFT, MISSION, UNIVERSE

• 

  ABOUT THE AUTHOR

  Amy Mainzer, Scientist

  Amy Mainzer is an astronomer specializing in astrophysical instrumentation and infrared astronomy. She is currently the principal investigator for the NEOWISE comet-hunting mission.

It turns out that the main culprit that drives us into space and into an orbit more than 500 kilometers (about 360 miles) above the Earth's surface is our atmosphere. As wonderful as our atmosphere is for life on Earth, it wreaks havoc on astronomical images in many ways. For one, shifting pockets of warm and cool air drifting above a telescope -- or a human observer-- cause stars to twinkle. While pretty, this twinkling makes it difficult to get a good measurement of a star's true brightness (or, in astronomical terms, its "photometry"). The twinkling also reduces the telescope's sensitivity and resolution by enlarging the images it produces, making them blurrier and less sharp. This is true for all kinds of telescopes not just infrared ones.

Secondly, the atmosphere acts like a sponge at many wavelengths, soaking up light from the stars so that it never reaches the ground at all. Everybody's seen a rainbow at one time or another, and that range of colors -- from violet to red -- spans the maximum range of wavelengths that our eyes can see. But that is only a small fraction of the entire spectrum of light that's really out there in the universe. Our sun puts out most of its radiation in visible light, and most of that visible light makes it through our atmosphere to the ground. However, our atmosphere is only partially transparent to infrared wavelengths. Filled with water vapor, carbon dioxide, and methane, our atmosphere absorbs almost all infrared light, so most of the infrared light emitted by distant stars, asteroids, and planets doesn't make it to observers on the ground. These molecules grab infrared light and trap it, preventing it from passing through the atmosphere (which is why they are called greenhouse gases). To see anything at all in most infrared colors, we have to get entirely above the Earth's atmosphere.

The final problem posed by our atmosphere for infrared astronomers is that it -- and the Earth itself -- is warm. Infrared light is characteristically emitted by room-temperature objects. Objects like you and I glow brightly in infrared light, and so does the Earth and its atmosphere. If you could see in infrared light, the night sky would look as bright as daylight! So when we're trying to detect the faint heat signatures of distant astronomical objects, a glowing, warm atmosphere is almost impossible to see through. This is why we must cool the WISE telescope to a mere 12 degrees above absolute zero (minus 438 Fahrenheit). Being in space with a cold telescope makes such a huge difference that the relatively modest-size WISE telescope, which is 40 centimeters (16 inches) in diameter, is equivalent in sensitivity to literally thousands of 8-meter (26-foot) telescopes on the ground. That small WISE telescope packs a punch.

So with that cleared up, we're just about ready to put WISE into the nose cone and crane it up onto the Delta II rocket that's waiting for us on the launch pad. Let's go see some stars!

TAGS: WISE, ASTEROIDS & COMETS, SPACECRAFT, MISSION, UNIVERSE

• 

  ABOUT THE AUTHOR

  Amy Mainzer, Scientist

  Amy Mainzer is an astronomer specializing in astrophysical instrumentation and infrared astronomy. She is currently the principal investigator for the NEOWISE comet-hunting mission.

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.