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.
A new version of the Dawn spacecraft is continuing the ambitious journey through the asteroid belt to uncharted distant worlds. Now holding a new solar system record, the probe is thrusting with its ion propulsion system, patiently and gently changing its orbit around the sun to match that of the immense protoplanet Vesta (and subsequently dwarf planet Ceres).
Even as Dawn continues pushing deeper into space, another spacecraft that used ion propulsion to conduct an exciting mission at a near-Earth asteroid has concluded. After traveling to and studying the diminutive Itokawa, Japan’s Hayabusa spacecraft returned to Earth on June 13. This was long one of your correspondent’s favorite missions, and he has joined many, many other enthusiasts in congratulating the team responsible for this impressive achievement.
When Dawn reaches each of its destinations, it will have a very full program of activities to acquire pictures and other scientific information. Brief overviews of some of its plans for Vesta were described in recent logs, and more will be presented later. To accomplish its mission of exploration, the spacecraft needs some enhancements to the capabilities it has been using for its travel through deep space to reach its targets. Those new capabilities are now onboard.
For the third time since it left Earth in September 2007, the spacecraft has received an upgrade of the software that runs in its primary computer. With a sense of grandeur and drama befitting this unique adventure, ever-poetical engineers fulfilled their dream of more than a year by denominating it OBC flight software version 9.0. Revealing their surprisingly cute and playful nature, however, most Dawn team members prefer the hypocorism “9.0” (or “nine oh”).
Engineers at JPL and Orbital Sciences Corporation began work on 9.0 almost immediately after 8.0 was installed on the spacecraft in April 2009. They continued with the careful and deliberate process of modifying the software until January, when the extensive test program commenced. It was crucial to verify not only that the new functions would work correctly but also that no unintended differences were introduced and that the existing capabilities were not compromised.
The latest software has 23 sets of changes from the previous version. Some of these are new methods of controlling the way the spacecraft will point its sensors at Vesta and Ceres in order to optimize the acquisition of data. Other modifications, based on experience gained in the ongoing operation of the spacecraft, improve its ability to handle certain potential anomalies on its own. In addition, just as 7.0 and 8.0 did, 9.0 corrects some bugs.
While it may seem quite elementary to load software into a computer that is in control of a spacecraft more than twice as far from Earth as the sun, it actually turns out to be somewhat complex and delicate. Even in “quiet cruise,” the computer is responsible for a great deal of activity onboard. The ion propulsion system was inactive, which is typical when the main antenna is pointed to Earth, but otherwise the computer was busy keeping all systems operating.
To install 9.0, controllers used exactly the same processes they followed for 8.0 in April 2009. It went quite smoothly again this time, right down to the on-time delivery of pizza to mission control during the first day of returning the spacecraft to its normal configuration after rebooting the computer. We know almost all readers accepted the advice offered last year to retain a copy of the log that presented the details of the 8.0 installation, but we happily include a link here for the convenience of the sole reader who did not and wishes to recall what is involved. (For all other readers, congratulations on the handsome profit you have realized on your investment in that previous log.)
As last year, controllers had run a few tests to verify the integrity of some critical components during the normal weekly communications sessions in the weeks leading up to the loading of the new software. On June 15, the spacecraft stopped thrusting on schedule, turned to point its main antenna to Earth, and kept it there rather than returning to the thrust direction a few hours later. That allowed operators to perform the rest of these detailed checks. After confirming that both the primary and backup computers were fully healthy, they transmitted the files containing the new software.
On June 16, with all stations in mission control at JPL reporting all subsystems were healthy and stable, and all systems at the Deep Space Network performing equally well, the command to reset the computer was radioed to the distant ship. The computer dutifully rebooted for the first time since the installation of 8.0 and began running with version 9.0. Whenever the computer reboots, it puts the craft into safe mode. The team verified that the new software was running smoothly and then initiated the process of guiding the spacecraft out of safe mode and back to its normal interplanetary cruise configuration. The schedule had allowed until June 24, but by June 18, the robotic explorer was fully prepared to resume its normal duties.
Because the software upgrade went so well, the Dawn project has decided to present this exciting offer: we will install a functional copy of 9.0 on your computer or smartphone at no charge. Simply place your device in the asteroid belt, send us the coordinates, and we’ll do the rest.
On June 17, after the majority of reconfigurations had been completed and while all members of the team but the insomniacs and the spacecraft itself were slumbering, protective software that is always running onboard detected an increase in the internal friction in reaction wheel no. 4. Reaction wheels are devices used to control a ship’s orientation in the zero-gravity of spaceflight. By electrically controlling the speed of these spinning units, the spacecraft can hold steady or rotate as needed. Dawn is outfitted with four reaction wheels, although it only uses three during normal operations. As we have seen before, operators let each wheel have its turn at being off for a part of the mission. The software that detected the friction in no. 4 responded correctly by powering that unit off. If only three wheels had been in use, it would have activated the unused wheel; but it was unnecessary to do so this time because, by coincidence, all wheels were operating, as is normal when the spacecraft enters safe mode. The team had been planning to turn reaction wheel no. 1 off later on June 17 as part of the reconfiguration. Instead, after taking some time to reassess the spacecraft’s condition, they simply left wheel no. 4 off and continued with their plans, now using wheels 1, 2 and 3 instead of 2, 3 and 4.
Dawn resumed ion thrusting on schedule on June 24. As it continues propelling itself to Vesta, it does so with the recognition that it has accomplished a greater propulsive change in velocity than any other craft ever to leave Earth. Some spacecraft have experienced larger velocity changes through gravitational interactions with planets, but thanks to the extensive use of its extremely efficient ion propulsion system, Dawn surpassed the record for the greatest change in velocity under a ship’s own power on June 5.
The previous record holder, Deep Space 1, was the first interplanetary mission to use ion propulsion. In its 11-month primary mission of testing advanced technologies (including ion propulsion), its two-year extended mission devoted to the exploration of a comet, and its final three-month hyperextended mission of additional technology testing, DS1 accumulated so much thrust time that it achieved an effective change in speed of 4.3 kilometers per second (9,600 mph). (As we have seen in several earlier discussions, such as here, this “effective change in speed” is not the speed at which the craft travels. It is a very commonly used way to express the effectiveness of a spacecraft’s propulsion system that avoids the confounding effects of orbital mechanics.)
Having thrust now for 635 days, or 63 percent of its time in space, Dawn has attained a change of more than 4.4 kilometers per second (9,800 mph), and it has much, much more powered flight ahead.
The record itself and even the total velocity change, while perhaps fun, really are not important, however. They are convenient measures of the progress this ship is making on its ambitious expedition, one that would not have been possible without ion propulsion and other innovations. The exploration of the cosmos is not a competition; it is a shared undertaking of all humankind. Each mission, each record, each accomplishment, each discovery builds on the successes (and even the failures) of those that preceded it and helps pave the way for those that will follow. Together they all contribute to the advancement of our understanding of the universe and our humble place within it.
Dawn is 0.32 AU (48 million kilometers or 30 million miles) from Vesta, its next destination. It is also 2.29 AU (342 million kilometers or 213 million miles) from Earth, or 855 times as far as the moon and 2.25 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.
Dr. Marc D. Rayman
10:30 p.m. PDT June 27, 2010
After more than 2.5 years of spaceflight, and more than 6 months in the asteroid belt, Dawn's interplanetary journey continues smoothly. The mission remains on course and schedule for this expedition to the dawn of the solar system.
Our Dawn is not the first spacecraft to use this name, although it is traveling farther from home than any other Dawn. This month 2 more craft traveled into space carrying that appellation, at least when translated into English. The Japanese Aerospace Exploration Agency sent Akatsuki to Earth's neighbor Venus, and Russia's Rassvet module was attached to the International Space Station in Earth orbit. The solar system is vast, however, and there is plenty of room for all such spacecraft. We send our best wishes for success to these other Dawns as they embark on their missions.
While our Dawn patiently and reliably thrusts with its ion propulsion system, gradually reshaping its path around the Sun to match orbits with the protoplanet Vesta, the human members of the team are very busy on distant Earth. Among their many activities is developing the sequences the robotic explorer will use when it begins studying that mysterious, alien world next year. We have seen recently what will occur during the “approach phase” and how Dawn will slip into orbit around Vesta. Now let's have a preview of what the ship will do once it has reached the first science orbit, known as “survey orbit.” Engineers are developing those sequences now, for execution in August 2011.
In survey orbit, the probe will be about 2700 kilometers (1700 miles) above the surface. During the approach phase, navigators will measure the strength of Vesta's gravitational tug on the spacecraft so they can compute the giant asteroid's mass with much greater accuracy than astronomers have yet been able to determine it. (The mass is calculated now using observations of how Vesta perturbs the orbits of other asteroids and even of Mars.) That knowledge will allow them to refine the survey orbit altitude, and they may target it to be somewhat higher or lower, depending on whether Vesta is more massive or less massive than the current calculations show. The sequences for acquiring science data are being designed to accommodate a reasonable range of masses.
Dawn will be in a near-polar orbit. Its trajectory will take it over the north pole (which will be in darkness, because it will be northern hemisphere winter at that time), then over the terminator (the boundary between the illuminated and unilluminated sides), down over the equator, over the south pole, and then across the terminator again to pass over Vesta's night side. Such an orbit allows the spacecraft to have a view of virtually every part of the lit surface at some time. Each revolution in survey orbit will take 2.5 to 3 days to complete. While this may seem like a leisurely pace, the spacecraft will be busy the entire time.
When on the day side of Vesta, Dawn will conduct an intensive campaign of observations. Vesta rotates on its axis in about 5 hours, 20 minutes (one Vestian “day”), which is faster than Dawn will be advancing in its orbit. So from the spacecraft's perspective, as it progresses slowly from north to south, the globe beneath it will complete several turns on its axis. That affords excellent opportunities for mapping the body.
During most of approach, Vesta will be so far away that it will fit comfortably in the fields of view of the science camera and the visible and infrared mapping spectrometer. Before Dawn reaches survey orbit, however, it will be too close to capture all of the expansive surface with its sensors in one glance. On each revolution, the sequences will command the spacecraft to point the instruments through profiles that will allow them to observe as much of the surface as possible.
The primary objective of survey orbit is to get a broad overview of Vesta with color pictures and with ultraviolet, visible, and infrared spectra. The camera will obtain views with 250 meters (820 feet) per pixel, about 150 times sharper than the best images from the Hubble Space Telescope. The mapping spectrometer will reveal much of the surface at better than 700 meters (2300 feet) per pixel. While subsequent science orbits will yield more detail, these first, new perspectives of this ancient world will represent an exciting step in the exploration of the solar system.
Throughout the year at Vesta, gamma-ray spectra and neutron spectra will be recorded with GRaND, and ultrasensitive measurements of the spacecraft's motion using the radio signal will reveal ever greater details of the protoplanet's gravity field and hence its internal structure. Although such information will be acquired in survey orbit, these investigations will benefit most from the lower altitude orbits.
Survey orbit is planned to last for 6 revolutions, or about 17 days. For most of the time it is on the day side, Dawn will fill its memory buffers with images and spectra. For most of the other half of each orbit, as it travels over the night side, the spacecraft will transmit those precious data through its main antenna to eager scientists and all others curious about the cosmos who reside on Earth. (Even when the surface below the spacecraft is in darkness, Dawn itself will be high enough that it will remain in sunlight, so its solar arrays will continue to provide electrical power.) There is so much to see at Vesta, and the instruments generate so much data, that a simple strategy of filling the memory on the day side and emptying it on the night side would be too limiting. Therefore, in the middle of its second, fourth, and fifth passes over the sunlit side, Dawn will halt its acquisition of data to spend a few hours radioing some of its findings to Earth, making more room for subsequent measurements.
Because the program of activities during the residence at Vesta is so full, and it all has to be planned in detail long before Dawn arrives, the project needs plans that are resilient to the inevitable problems, both large and small, that arise in such complex and challenging endeavors. While every observation in survey orbit is of interest, many more are scheduled than are necessary to fulfill the scientific objectives. Therefore, even if some are missed because of glitches in systems on the spacecraft or on Earth, as long as others are acquired, the mission will proceed. With the extremely rich set of measurements planned, there is no intention of repeating some that are lost.
After it has completed its survey of Vesta, Dawn will resume thrusting, spiraling down to its next science orbit for an even closer view. We will learn more about that in a subsequent log.
Meanwhile, as the craft continues to propel itself toward its destination, traveling farther and longer than ever, it will pass 3 milestones on its journey next month. Look for a NASA news release soon on a record it will set as it keeps thrusting with its ion propulsion system. We will describe that in the next log.
On June 23, Dawn will have been in flight for 1000 days. No doubt readers will enjoy taking a minute (at least, for those who read 61,000 words per minute) to reread all the logs since launch to recall some of what has occurred so far during the mission. While much has already been accomplished, the great rewards lie ahead, as Dawn pushes deeper into the asteroid belt, where it will explore faraway new worlds.
On June 3, Dawn will be exactly twice as far from Earth as Earth is from the Sun. Of course, the distance between the planet and the star does not matter for the spacecraft; it is on its own independent journey through the solar system. Nevertheless, such an occasion may provide some terrestrial readers with another opportunity to reflect upon the nature of such a journey. Dawn's trek is not simply that of a robot in space. Although in a narrow sense the ship is sailing the cosmic seas on its own, there is much more to the voyage than that. Such a mission represents a journey by a remarkable species that does not allow its physical confinement to the vicinity of its home planet to keep it from reaching ever farther in its pursuit of knowledge and its quest for grand and noble adventures.
Dawn is 1.96 AU (293 million kilometers or 182 million miles) from Earth, or 760 times as far as the Moon and 1.93 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.
Dr. Marc D. Rayman
9:00 pm PDT May 27, 2010
Dawn remains on course and on schedule for its appointments with Vesta and Ceres, colossal protoplanets in the main asteroid belt. Under the gentle and continuous thrust of its ion propulsion system, its journey through the solar system brings it ever closer to its first target.
Last month’s log included an overview of many of the spacecraft’s activities during the final 3 months before its August 2011 arrival in the first science orbit at Vesta. In this “approach phase,” the probe will observe Vesta with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the alien world prior to initiating its in-depth studies. The discussion did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time will be devoted to continuing its ion-powered flight. Let’s take a more careful look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Vesta.
Thrusting is not necessary for a spacecraft to remain in orbit, just as the Moon remains in orbit around Earth and Earth and other planets remain in orbit around the Sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. With its extraordinarily efficient ion propulsion system, Dawn’s near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the Sun. The flight profile was carefully designed to send the craft by Mars in February 2009, so our explorer could appropriate some of the planet’s orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn’s orbit, Dawn lowered Mars’s orbit, ensuring the solar system’s energy account remained balanced.
While spacecraft have flown past a few asteroids in the main belt (although none as large as the behemoth Vesta nor the still more massive dwarf planet Ceres), no probe has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two solar system targets. Dawn’s unique assignment would be quite impossible without ion propulsion. But with its light touch on the accelerator, taking nearly 4 years to travel from Earth past Mars to Vesta and more than 2.5 years from Vesta to Ceres, how will it enter orbit around Vesta, how will it break back out of orbit, and how will it enter orbit around Ceres?
Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its orbit around the Sun. This intriguing challenge was addressed in part in February 2007, as all readers with perfect memory recall. In August 2008, we considered another aspect of what is involved in climbing the solar system’s hill, with the Sun at the bottom, Earth partway up, and the asteroid belt even higher. (Readers at that time in the past thoughtfully sent greetings through time to us, which we are now delighted to receive! On behalf of all present readers, we return the kind gesture with our own greetings.) We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the Sun, but then need to turn in order to move along with the body they will orbit.
Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet’s gravity can hold it in orbit and carry it around the Sun. These exciting events can range from around 0.6 to 1.5 kilometers per second (1300 to 3400 miles per hour). With 10 thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn’t have to.
Dawn’s method of getting into orbit is quite different, and the key is expressed in an attribute of the ion propulsion system that has been referred to 26 times (trust or verify; it’s your choice) before in these logs: it is gentle. Dawn’s entire thrust profile for its long interplanetary flight has been devoted largely to the gradual reshaping of its orbit around the Sun so that by the time it is in the vicinity of Vesta, its orbit will be very much like Vesta’s. Only a small change will be needed to let the giant asteroid’s gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit.
To get into orbit, a spacecraft has to match speed, direction, and location with its target. A mission with conventional propulsion first gets to the location and then, with the planet’s gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling out to the orbit of Vesta (and later Ceres), Dawn works on its speed, direction, and location all at the same time, so they all gradually reach the needed values just at the right time.
To think about this facet of the difference between achieving this goal with the different technologies, imagine you want to drive your car along next to another traveling west at 100 kilometers per hour (60 miles per hour). The analogy with the conventional technology would be similar to heading north toward an intersection where you know the other car will be. You arrive there at the same time and execute a whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator, and probably some adrenaline. When you drive an ion propelled car, operating with 10 times the fuel efficiency, you take a different path from the start, one more like a long, curving entrance ramp to a highway. When you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is entirely different from the first method, but the sophisticated techniques of orbital navigation are up to the task.
In late July 2011, as the probe follows its approach trajectory to Vesta, their paths will be so similar they will be moving at nearly the same direction and speed around the Sun (about 20.5 kilometers per second or almost 46 thousand miles per hour). When at a range of about 16 thousand kilometers (9900 miles), the spacecraft will be traveling at less than 50 meters per second (110 miles per hour) relative to its destination. That combination of distance and velocity will allow Vesta to take gentle hold of Dawn. The spacecraft will not even notice the difference, but it will be in orbit around its first celestial target, even as it continues ion thrusting to reach the planned orbit more than 2 weeks later.
With the gradual trajectory changes inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. Dawn is propelling itself along a spiral path around the Sun as it journeys from Earth out to Vesta, the first loop having been completed in June 2009. It will arrive at Vesta before it completes the second revolution. Then its flight profile will be designed to spiral around Vesta as the probe and protoplanet together orbit the Sun. Dawn’s first loop around Vesta will be about 10 days, and its second will take 4. It will stop thrusting when it is in “survey orbit,” where one revolution takes just under 3 days. After collecting a rich bounty of pictures and other important scientific data from this altitude of about 2700 kilometers (1700 miles), it will resume thrusting, spiraling down to lower and lower orbits, requiring hundreds of revolutions. Dawn’s speed will increase as its orbital altitude decreases, so the loops will progressively become shorter.
In 2012, after completing months of close-range scientific observations, it will reverse the spirals, gradually climbing away from the world it has been studying just as it gradually climbed away from the Sun. Vesta’s gravitational hold will weaken as Dawn moves farther and faster, its graceful motion ultimately exceeding the strength of the invisible tether that bound it. As gently as it arrived, it will depart. In July of that year, it will once again be on its own in orbit around the Sun, and navigators will instruct it to point its ion thruster to spiral outward more in order to undertake its pursuit of Ceres.
These spiral paths do not occur naturally. Under the predictable and calculable effects of the gravity of the Sun and other bodies (including Vesta or Ceres), Dawn is programmed to orient its thruster in just the right direction at the right time to propel itself on the desired trajectory. A great deal of work was required before launch to devise such a plan. Changes since then have been determined by knowledge gained during the mission, such as an update to the prediction of how much power the solar array will yield.
Engineers have completed work on the approach phase for now. They have reviewed the sequences (the timed instructions the spacecraft will follow) in detail and have tested portions of them in the spacecraft simulator at JPL. The sequences are mature enough that they will be ready for controllers to update and refine as necessary next year before being radioed to the spacecraft. Now the operations team is turning its attention to the subsequent phase of the Vesta mission, survey orbit, where the intensive observations will begin. We will learn more about that in the next log.
Dawn’s controllers certainly do not focus all their efforts on preparing for Vesta. (Your correspondent devotes some of his to dancing, but perhaps that’s a topic for a future log.) Of course, keeping the spacecraft healthy and on course is essential as well. In addition to commanding it to sustain the needed thrusting, with a weekly hiatus for telecommunications, they perform routine maintenance to ensure the ship remains in top shape. For example, engineers recently adjusted the spacecraft’s master clock. Always in the glow of the distant Sun, and never needing to rest or take a break from its duties, the robot has no need to switch to daylight saving time. Nevertheless, a time change was called for because the onboard time had gradually drifted from the correct value. It had last been set on February 27, 2008, and has remained sufficiently accurate for all Dawn’s needs. With the gradual nature of this mission, precise timing is generally not necessary, so although they have closely monitored the clock, controllers have allowed it to run without correction. When they commanded the transition from ion thruster #1 to thruster #2 in January in January they expected the clock to change slightly, and indeed it did. Thruster #2 uses a different power control unit from thrusters #1 and #3. The #2 controller is mounted closer to the electronics assembly that includes Dawn’s clock, and now that that device is powered, the heat it dissipates warms the clock a little, so the clock rate is slightly altered. Although much larger values could be accommodated, when the time offset had crept up to 1.37 seconds, operators set it back to the correct time, and they included a change to account for the warmer environment. (Readers may wish to pause for 1.37 seconds to contemplate the difficulties of synchronizing clocks that are farther apart than the Sun.)
An improved version of the test to measure the overlap of the views of the visible and infrared mapping spectrometer (VIR) and the prime science camera was executed successfully. When the measurement was carried out in December, a conflict between commands in the VIR sequence prevented the intended data from being acquired.
As if maintaining the spacecraft’s health and powered flight and developing detailed plans for Vesta weren’t enough to keep Dawn’s engineers happy, they also are continuing work on a new version of the software for the primary computer, scheduled to be transmitted to the spacecraft in June. The mission also will mark 3 milestones that month, and it may not be a surprise if your correspondent marcs them in the next log.
Dawn is 1.62 AU (243 million kilometers or 151 million miles) from Earth, or 650 times as far as the Moon and 1.61 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 am PDT April 28, 2010
Dawn continues patiently forging through the asteroid belt, its permanent residence, as it climbs away from Earth and the Sun. Having thrust with its ion propulsion system for more than 1.5 years, the spacecraft remains healthy and on target for its rendezvous with alien worlds.
Our interplanetary adventurer still has a great deal of ion thrusting to complete before it can begin its orbital exploration of Vesta next year. Although it will suspend thrusting for a few weeks this summer to conduct some special activities (to follow along, be sure to renew your subscription to these logs the first time our helpfully persistent telemarketers call), it will devote most of the time until early August 2011 in powered flight, continuously reshaping its orbit around the Sun.
In addition to keeping the ship sailing smoothly and on course, Dawn’s engineers (who reside and work on distant Earth) are developing the detailed instructions that will guide it into orbit around Vesta and throughout its year of operations there. This process began last month and will continue even as the probe begins executing the first of the commands in May 2011.
Mission controllers compile Dawn’s instructions by assigning a time to each individual command. Groups of these timed commands are known as a “sequence.” During the current interplanetary cruise phase of the mission, sequences generally extend for 5 weeks, but some special activities may use sequences as short as a few hours. Usually more than one sequence is executing at a time, but like all the instruments in an orchestra, they are carefully synchronized and coordinated so the overall score accomplishes the composer’s artistic intent.
Readers may recall that the mission is separated into phases. Following the “launch phase” was the 80-day “checkout phase”. The current “interplanetary cruise phase,” which began on December 17, 2007, is the longest. It ends when the “Vesta phase” begins. (Other phases may occur simultaneously with those phases, such as the “oh man, this is so cool phase,” the “what clever name are we going to give this phase phase,” and the “lunch phase.”) Because the mission at Vesta is so complex, it is further divided into sub-phases. The Vesta sequences that are being developed now are for the “approach phase.” Approach begins in early May 2011 and concludes 3 months later when Dawn will have maneuvered to the first orbit from which it will conduct intensive science observations, known as survey orbit.
Most of the approach phase is dedicated to the final ion thrusting required to slip into orbit around Vesta. All of Dawn’s thrusting contributes to rendezvousing with Vesta, but the terminal thrusting will be controlled slightly differently. We will describe the process of using ion propulsion to enter orbit around another solar system body in an upcoming log. For now, however, let’s take a look at some of the other activities during the approach phase. While these are being timed in the sequences down to the second, part of the strategy for developing these sequences is to allow the team a means to update the times as the probe closes in on its target. The ion propulsion system provides flexibility in the timing that is different from most missions, and to take advantage of the benefits, the sequences must be correspondingly flexible. All the relative timing within a sequence will be fixed, but the time each sequence is activated can change. So, for example, even though we may change the date the first Vesta approach sequence begins executing by as much as a few days, once that adjustment is made, all the events within the sequence will shift by exactly the same interval. Some small changes other than timing, such as details of the probe’s orientation, may be made as well to reflect the latest information available before it is time to transmit the sequences to the spacecraft more than a year from now.
The principal activity other than thrusting during approach is the acquisition of images of Vesta with Dawn’s main science camera, primarily for navigation. From the distant vantage point of Earth, astronomers can determine Vesta’s location with astonishing accuracy, and the Dawn navigation team achieves extraordinary accuracy in establishing the probe’s position, but for the craft to enter orbit, still greater accuracy is required. Therefore, Dawn will observe Vesta’s location against the background of stars, and the photographs will be analyzed by celestial navigators to pin down the relative location of the ship and the port of call it is approaching. To distinguish this method from the one by which Dawn is usually navigated, making use of its radio signal, this supplementary technique with pictures is generally known as “optical navigation.” There are 24 optical navigation sessions during the 3-month approach phase. Many of these will be combined with observations of Vesta designed to help prepare for subsequent scientific measurements.
The positions of the spacecraft and protoplanet will be determined well enough with the current navigation method that engineers will know which stars will appear to be near Vesta from Dawn’s perspective. It is the analysis of precisely where Vesta appears relative to those stars that will yield the necessary navigational refinement. When Dawn is closer to Vesta, the giant asteroid will occupy most or all of the camera’s view, and stars won’t be visible. Then the optical navigation will be based on determining the location of the spacecraft with respect to specific surface features that have been charted in previous images.
For the optical navigation observations, Dawn will halt thrusting and align itself so that Vesta and, when possible, the stars are in view of the camera. It will spend half an hour or more taking images and storing them for transmission at the next scheduled communications session. The information extracted from the images will be used to calculate where the probe is relative to its destination. Engineers then will update the design of the trajectory for the spacecraft to follow to reach its intended orbit and fine-tune the ensuing thrust profile to ensure that Dawn accomplishes the revised flight plan.
The first optical navigation images will be acquired when Dawn is about 1.2 million kilometers (750 thousand miles) from Vesta, or more than 3 times the separation between Earth and the Moon. Dawn’s camera is designed for mapping Vesta from orbit. Therefore, instead of a high-power telescope with a narrow field of view, the camera has a relatively low magnification but covers a broad area. The camera achieves the equivalent of a magnification of about 3 compared to unaided human eyes. When these first optical navigation images are taken, distant Vesta will appear to be only about 5 pixels across. But at that stage, navigators will need to know its location, not its appearance, so the images will be of great value.
For 8 of the approach observation periods, in addition to the camera, the visible and infrared mapping spectrometer (VIR) will be trained on Vesta. By taking some early measurements with the camera and VIR, scientists will have the opportunity to make fine adjustments to the instrument parameters in the sequences for later observations.
In one of the optical navigation sessions in July, the camera will acquire many images of the space around Vesta in a search for moons. Astronomers have looked for moons of Vesta before, and will do so again before the explorer reaches its vicinity. Although none has been discovered, Dawn’s unique vantage point will provide more data. The existence of moons would be of interest both for science and for mission safety.
When Dawn suspends thrusting to check for moons, it also will collect a series of images as Vesta rotates. Like Earth and all other solar system bodies, Vesta spins. It completes one turn on its axis (one Vestian “day”) in about 5 hours, 20 minutes. These measurements will help characterize the alien world still more to aid in navigation and to prepare for subsequent observations with the science instruments. The moon search will be during the second of 3 observations of a full rotation.
Over the course of the 3-month approach, it will be exciting to watch Vesta grow from little more than a tiny smudge in the first optical navigation images until it is too large to fit in the camera’s view at the end of the phase. By early June 2011, the images will surpass the best that can be obtained with the Hubble Space Telescope. All succeeding observations will yield better and better views, both rewarding us and tantalizing us as Dawn prepares for its more intensive studies in later Vesta phases.
The spacecraft will glide into a very high orbit in late July and continue thrusting, gently as always, until early August, when it will arrive in its survey orbit at an orbit at an altitude of about 2700 kilometers (1700 miles). The activities to be conducted in the survey phase will be described when mission planners are working on those sequences.
In the meantime, the team is running some of the approach sequences through the Dawn spacecraft simulator at JPL down the hall from mission control. The simulator includes some hardware that is virtually identical to what is on the spacecraft and some software to take the place of other hardware components. The simulator is one of several methods used to check complex sequences before they are approved for transmission to the spacecraft.
It is both unnecessary and impossible to test all sequences. The simulator operates in real-time, so it would take 3 months to run all the approach sequences, and the Dawn team has too many other tests to perform with the simulator to allow that. Because much of the approach phase consists of ion thrusting, an activity which is quite familiar not only to the spacecraft but also to mission controllers (as well as regular readers of these logs), there is no need to test the thrusting periods. Engineers review each sequence to determine which portions would benefit from testing.
While the spacecraft simulator is hard at work at JPL, the actual spacecraft continues its work elsewhere. On February 28, Dawn and the Sun were equidistant from Earth. Now, as the distant explorer continues to propel itself toward its rendezvous with Vesta, it is farther from Earth than the Sun ever is. Moreover, even as the probe and the planet follow their separate paths around the Sun, Dawn will remain farther from Earth than the Sun. The orbits of Mercury, Venus, Mars, and many other members of the solar system family occasionally bring them closer to our planet than the Sun, but Dawn has enlarged its orbit so much that it never will return to the region of the solar system in which it began its ambitious journey of discovery.
Dawn is 1.27 AU (191 million kilometers or 118 million miles) from Earth, or 525 times as far as the Moon and 1.28 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 21 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 pm PDT March 28, 2010
Pushing ever farther into space, deeper into the asteroid belt, Dawn is continuing to progress smoothly on its solar system journey.
The spacecraft spends most days climbing away from the Sun atop its pillar of blue-green xenon ions. A day’s thrusting is enough to change the spacecraft’s speed by a very modest 7.3 meters/second (16.3 miles/hour). While such an effect would be entirely inadequate for an interplanetary mission as ambitious as Dawn’s, the extraordinary efficiency of ion propulsion allows the probe to thrust for much more than a day. Although almost all spacecraft coast most of the time, as do planets, moons, and asteroids, this explorer usually maintains a gentle pressure on its orbit, constantly changing it so that it can rendezvous with Vesta next year, leave in 2012, and then rendezvous with Ceres in 2015. Dawn has spent 60% of its time since launch patiently accelerating with the ion propulsion system. It has already managed to change its speed by more than 3.6 kilometers/second (8100 miles/hour), far exceeding the capability of most spacecraft, yet it has a great deal more thrusting ahead. (For a comparison with probes that enter orbit around Mars, visit a previous log.)
In contrast to conventional chemical propulsion systems, ion propulsion achieves its astonishingly high performance by using electrical power to create the thrust. Outfitted with the most powerful solar arrays ever carried on an interplanetary probe, Dawn converts sunlight into the electricity consumed by the ion thrusters. And yet even as its travels take it ever more distant from the luminous source of its electrical power, the effect of the ion thrust becomes greater each day, not less. At the beginning of the mission, a day of thrust yielded only about 6.5 meters/second (14.5 miles/hour). Now, more than 1.8 times farther from the Sun, the acceleration is greater, and by this summer, when Dawn is still farther from the Sun, it will climb to 7.6 meters/second (17.0 miles/hour) every 24 hours. The reason for the paradoxical increase is deceptive, yet simple.
Dawn’s solar arrays are so large that they can produce enough power to operate an ion thruster at the maximum throttle level (as well as all other spacecraft systems) even when twice as far from the Sun as Earth is. Therefore, the propulsion system will not have to be throttled to lower power until the probe is more than 2 astronomical units (AU) from the Sun, a distance it will reach this summer. In the meantime, because the arrays produce excess power, the thrust is independent of the distance to the Sun.
The acceleration depends on more than the thrust, however; the thruster pushes against the spacecraft, so the change in speed depends on the spacecraft’s total mass. A rocket engine (whether powerful and inefficient or soft and efficient) imparts a lower velocity to a more massive craft than to a less massive one. (Gravity, or the absence of it, is not relevant.) This is quite familiar from terrestrial experience. If you throw a baseball with the same force as you throw a shot put, the baseball will depart with a higher speed.
When Dawn began its mission in September 2007, it was about 1218 kilograms (2685 pounds). Since then, it has expended 140 kilograms (309 pounds) of xenon plus about 5 kilograms (11 pounds) of hydrazine from the reaction control system (the system that uses small conventional thrusters to aid in orienting the spacecraft in the zero-gravity of spaceflight). The ion thruster now is pushing against less mass, so the effect of the thrust is greater. As Dawn continues to expel its propellants, it will become still less resistant to the thruster’s efforts to change its speed. In summary, with the thrust staying constant and the mass decreasing, the acceleration is increasing.
The mass will always go down when the ion propulsion or reaction control systems are operated. Once the spacecraft is far enough from the Sun that it needs to reduce the throttle level, the availability of power, and hence the thrust, generally will decline faster than the mass, so the effect of the thrust will diminish. By the end of the mission, a day of thrusting will provide less than half of the change in speed that it does now.
Of course, engineers have been accounting for this since they began designing the project. The entire flight plan from Earth to Vesta (via Mars) and from Vesta to Ceres is based on how much the craft can accelerate throughout its mission.
While the spacecraft will not reach Vesta until July 2011, the Dawn team has been hard at work developing the detailed plans for what it will do there. This month, they initiated the long process of formulating the specific instructions that will be radioed to the probe to carry out those plans, guiding it through all the steps it must follow to get into orbit, to perform the myriad scientific measurements that are planned, to transmit the results to Earth, and to remain healthy and productive in that distant and forbidding environment. The team is beginning with the “approach phase,” which commences in May 2011 and concludes when Dawn has completed thrusting its way to the first orbit from which it will conduct intensive observations in August 2011. (Of course, it will stop occasionally to peer at Vesta as it closes in on the enormous asteroid during the approach phase.) As work on each of the Vesta phases is completed, the team will turn its focus to the next, so by the time Dawn begins its approach, most of the instructions for its year at Vesta will have been prepared.
The commands will be checked and double checked just as they are for carrying out the interplanetary flight. For operating at Vesta so long from now, however, they also are being designed so that shortly before it is time to transmit them to the spacecraft, controllers can update them to account for the exact trajectory the spacecraft is on and other details that may change slightly.
In upcoming logs, we will describe some of the highlights of the plans that are being readied for when the ship reaches its first celestial destination.
To help refine preparations for flying near Vesta and studying it from Dawn’s vantage point, scientists are taking advantage of the convenient alignment between Earth and the protoplanet to observe it with the Hubble Space Telescope on February 25 and 28. The venerable 200-inch (5.1-meter) Hale Telescope on Palomar Mountain in California will be used for other Vesta measurements in April.
While engineers and scientists focus their attentions on Dawn, the ship continues to grow more remote. As we saw in the previous log, after closing in on each other for 14 months, Earth and the spacecraft are now separating again, their independent orbits around the Sun carrying them farther apart. On February 28, Earth will be equidistant from Dawn and the Sun. Readers on that planet will be at the apex of a broad cosmic isosceles triangle, 0.99 AU from both the tremendous star that has governed the solar system for 4.6 billion years and the tiny probe that is quietly and patiently making its way to investigate unexplored alien worlds to help us understand the dawn of the solar system. The third leg, between Dawn and the Sun, will be 1.84 AU long.
Although quite undetectable with all but the most sensitive radio receivers of the Deep Space Network, those who are share in the profundity and the passion for the exploration of the cosmos may wish to gaze upon the spacecraft with their minds’ eyes. It has been farther from Earth before, and other spacecraft have been much farther still, but while it is at the same distance as the Sun, it presents an occasion to reflect upon humankind’s achievements. Dawn’s milestone represents much more than the opportunity to gain fascinating new insights into the solar system and an exciting adventure to reveal vistas previously unseen. At the same distance as the Sun, it symbolizes the extraordinary success of science and engineering. At the same distance as the Sun, it compels us to mediate upon what humankind can accomplish when we are inspired to translate our grand ambitions into action. At the same distance as the Sun, it reminds us that some feats that once were even beyond imagination may be achieved. While physically we are confined to the vicinity of our home planet, the power of uncounted millennia of thoughts, hopes, and dreams combined with persistence, discipline, and tremendous cognitive effort allows us to have an extension of ourselves as far as the Sun. A spacecraft as far as the Sun is a triumph of humble creatures bold enough to reach out into the universe.
To aid in contemplating the nature of such grand undertakings, some readers may wish to peer in the direction of the invisible, distant craft. It is 5.5 degrees northwest of Mars, an easily identified ruby among the gems of the evening sky. (For reference, 5.5 degrees is 11 times the diameter of the Moon or about the width of 3 fingers held together at arm’s length.) It is roughly 2/3 of the way from Mars to the bright star Pollux. As the month ends, your correspondent (reporting on location from Earth) plans to contemplate his view of the Sun during the day and the sky near Dawn in the evening.
This will be the last chance to peer toward the spacecraft while it is this close. For the rest of its mission, and effectively forever, Dawn will be farther from Earth than the Sun. Yet it will remain eternally tied to the planet by virtue of the unique human thirst for knowledge, spirit of adventure, and insatiable yearning to know the cosmos, all of which propel it beyond distant horizons.
Dawn is 0.96 AU (144 million kilometers or 90 million miles) from Earth, or 395 times as far as the Moon and 0.97 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 16 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 pm PST February 24, 2010
Dear Plausible Dawniabilities,
Patiently and reliably continuing with its interplanetary voyage, Dawn is now flying in a new configuration and, from the perspective of those readers who may be on Earth, in a new direction.
The spacecraft still spends most of its time gradually changing its orbit around the Sun by thrusting with its ion propulsion system. The probe is outfitted with 3 ion thrusters, assigned the heartwarming names thruster #1, thruster #2, and thruster #3. (The nomenclature and locations of the units were divulged in a log shortly after launch, before such information could be distorted and used unethically by others.) The ship only uses 1 thruster at a time. All 3 were tested during the 80-day initial checkout phase of the mission, and when the interplanetary cruise phase commenced in December 2007, it was thruster #3 that was responsible for pushing the spacecraft away from the Sun. It performed flawlessly, but engineers plan to share the workload among the thrusters over the course of the 8-year mission, so thruster #1 was called into action in June 2008. By that time, stalwart #3 had been operated in space for 158 days. (For those readers who have just returned from an enjoyable excursion back to that log, the apparent discrepancy between the 158 days of operating time given here and the 149 days presented there is not an error. The smaller value is the operating time in the interplanetary cruise phase. Thruster #3 had accumulated about 9 days of operation during the initial checkout phase.)
Thruster #1 was in service until this month. Although it remains in excellent condition, engineers transmitted instructions in December for the spacecraft to reconfigure for use of a different thruster after its weekly communications session on January 4. By that time, #1 had thrust for almost 318 days. With its famously efficient use of xenon propellant, all that maneuvering consumed only 84.6 kilograms (187 pounds), yet it imparted 2.2 kilometers/second (4900 miles/hour) to the spacecraft.
Now it is #2’s turn. It had barely more than 1 day of total running time in space prior to this month, having been used only for some tests in November 2007 and April and May 2009 . Now 2010 will be its year to shine (with a lovely blue-green glow). In addition, as we will see in the next log, for the entirety of the mission, thruster #2 will have the distinction of providing the greatest acceleration to the spacecraft of any of the thrusters.
There is much more to the ion propulsion system than the thrusters. As explained in more detail in an earlier log, the system also includes 2 computer controllers and 2 units that draw as much as 2500 watts from Dawn’s solar arrays and converts the power to the currents and voltages the thrusters need. Controller #1 and power unit #1 are used for both thruster #1 and #3, so those electrical devices have already worked extensively during the mission, although most of their operation still lies ahead. For now, though, controller #2 and power unit #2 are in charge.
Although thruster #2 and its associated components have spent most of their time in space unpowered, they all are now performing just as smoothly as the other ion propulsion system elements did when they were in use.
Most of the artistic depictions of the spacecraft in flight happen to show it using thruster #2, the one nearest the main antenna. So the next time you see such an image, probably even at the top of this very page, you might consider that it is very much the way the spacecraft would look right now if you could see it.
Of course, Dawn is much too far from Earth to be seen by human eyes, even aided by the most powerful telescopes. But it has recently come nearer to the planet than it had been for nearly 2 years. As we have discussed in many logs (see, for example, November 2008), Earth and Dawn move independently through the solar system. Just as the hands of a clock sometimes move closer together and sometimes farther apart, Dawn and Earth sometimes approach each other and sometimes separate.
Some readers may not be at all surprised that even as the probe is receding from the Sun well over 2 years after launch, blazing a trail through the asteroid belt, constantly changing its own orbit (unlike most spacecraft, which coast most of the time, just as planets do), it is no farther from Earth than it was just 5 months after launch. They are excused from reading the material below. Others, however, may find this discussion helpful in thinking more about why this occurs. It is not important for the mission, but it may be satisfying for those who wish to direct a metaphorical gaze to the distant craft.
Unlike clock hands, Dawn does not travel in a circular path. Following the initial push away from Earth by the Delta rocket that carried it from Cape Canaveral into space, its orbit around the Sun was elliptical (see the second row of the table here). Its path has changed a great deal since then, principally because of the extensive thrusting (but also because of the gravitational boost from Mars).
Although elliptical orbits distort the picture a little, the essentials of the clock analogy are valid, so let’s imagine this alignment by considering the same clock we have used twice before, most recently last month (For readers who now have more clocks than room to display them, we promise that this will be that last reference to a clock from the Dawn gift shop, at least until your clocks’ warranties have expired.) With the Sun at the center, Earth is at the tip of the shorter hand and Dawn at the tip of the longer one. On January 25, the star, planet, and spacecraft were aligned as closely as the hands of the clock would be at 6:32:16.
When positioned that way, the Sun and Dawn were nearly in opposite directions from Earth’s vantage point. Suppose you were on Earth on that date and wanted to look in the direction of the spacecraft. You would have put the Sun at your back and Dawn would have been less than 6 degrees from your line of sight, equivalent to being in the center of a (different) clock, having the 12 at your back, and instead of looking at the 6, shifting your gaze almost to the next tic mark. (The positions constantly change, and by the middle of February, you would need to readjust your gaze to the 7, still keeping the Sun at the 12.)
Although the alignment is the result of the motion of both Earth and the spacecraft, from the terrestrial perspective, with our deceptive sense of cosmic immobility, it seemed that Dawn had been moving closer to us. Now it seems to be moving away.
Dawn reached its greatest distance from Earth so far in the mission on November 10, 2008. [Note: We had decided that it was unnecessary to include a link to that paragraph, thanks to our encouragement therein for readers to memorize it. According to our new consultants, Prescient Telepaths ‘R Us, you are the sole reader who did not commit it to memory. Therefore, in our goal to make every customer happy, we are pleased to include the link specifically for you. Enjoy!] At that time, it was 2.57 astronomical units (AU) from Earth. Since then, while its orbit has carried it closer to the Sun and then farther again, the distance to Earth has been declining the entire time. The spacecraft and its planet of origin finally moved to their closest point on January 18, when their travels brought them to 0.80 AU from each other. (It occurred at about 2:00 am PST, so if you sleep deeply, you may have missed it.) The minimum distance did not occur at exactly the same time as the nearly linear arrangement because the orbits are not as simple as the circular motion of the clock hands.
The last time they were this close was on March 11, 2008. They will never be so near each other again. Earth follows the same orbit around the Sun year after year, but with Dawn constantly changing its trajectory, pushing deeper into the solar system, the next time it and Earth are aligned on the same side of the Sun (in August 2011), the explorer will be much farther away. Indeed, if all goes according to plan, it will be in orbit around Vesta by then, beginning to reap the rewards for its long expedition through the cold depths of space, as it explores a distant and alien world that waits silently for its first visitor.
Dawn is 0.82 AU (123 million kilometers or 76 million miles) from Earth, or 345 times as far as the moon and 0.83 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 14 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 pm PST January 30, 2010
Dear Dawnters and Sons,
The Dawn mission continues to go smoothly, as Earth’s distant envoy carries out its interplanetary journey. Although the craft still devotes most of its time to the slow but efficient reshaping of its orbit around the Sun to match Vesta’s, controllers gave it some extra assignments since the last log to ensure its systems remain healthy and to prepare for its studies of Vesta.
Dawn usually interrupts ion thrusting once a week for about 8 hours to point its main antenna to Earth. On November 30, however, instead of resuming thrusting, it dutifully followed different instructions that were stored onboard.
The spacecraft began the 5 days of special activities by activating the gamma ray and neutron detector (GRaND). Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. It will reveal the atomic constituents of the surfaces of Vesta and Ceres. GRaND’s measurements of space radiation this month showed it to be in excellent health. After a week of smooth operation, it was deactivated on December 7.
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
Dawn continues to make steady progress through the solar system as it maintains a gentle pressure on its orbit around the Sun. It has spent 95% of the time since the last log thrusting with its ion propulsion system, stopping only briefly each week to communicate with the mission control team on distant Earth.
The probe is on an exciting journey to unlock secrets from the dawn of the solar system ensconced in the mysterious worlds Vesta and Ceres. And yet there is one aspect of this expedition that likely is much less exciting than some readers may expect.
Dawn entered the main asteroid belt on November 13. As it ventures ever deeper into this vast collection of material between Mars and Jupiter, it may be tempting to think of the spacecraft constantly dodging asteroids. In some science fiction movies, the huge rocky bodies are so close together that highly skilled piloting is required to avoid catastrophes. Now Dawn is guided by some of the most proficient interplanetary fliers this side of Pluto, but the reality is that accidental impacts are exceedingly unlikely. Space is big, and as plentiful as asteroids are, the distances between them are tremendous.
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
Dawn continues to make steady progress on its journey through the solar system. The spacecraft has devoted another month to thrusting with its ion propulsion system, ever with its sights set on its rendezvous with Vesta in July 2011. While it will have other assignments along the way, propelling itself to the giant protoplanet deep in the main asteroid belt remains its principal responsibility.
The asteroid belt consists of innumerable objects in orbit around the Sun between Mars and Jupiter. (Dawn is aiming for the 2 most massive members of the belt.) Just as with a ball of cotton or a cloud, while there may appear to be a clear border when viewed from a great distance, a more careful examination reveals it to be less distinct. There is no sharp edge to demarcate the boundary. For example, although most asteroids remain between the two planets, the orbits of some bring them closer to the Sun than Mars. We can adopt a part of one common definition in which, to be designated as a resident of the main asteroid belt, an object’s orbit can bring it no closer to the Sun than 1.666 astronomical units (AU). It is not coincidental that this is the greatest distance that Mars travels from the Sun. (Earth and its inhabitants never reach more than 1.017 AU from the solar system’s gravitational master.)
As with Earth, Mars, and asteroids, Dawn’s orbit around the Sun is elliptical. The principal difference is that the ship is constantly changing its course by emitting a high velocity beam of xenon ions. (It has racked up more than 10,000 hours of powered flight, with much more thrusting ahead.) In a lovely solar system dance in February, Dawn briefly partnered with Mars for additional assistance on its way as well. As we saw in the last log, the spacecraft’s orbit grows larger as the mission progresses, bringing the explorer ever closer to its first destination. On November 13, it will enter the asteroid belt as its silent flight takes it past 1.666 AU from the Sun. It will remain in the belt for the rest of its mission and well beyond. Dawn will become a permanent inhabitant of that part of the solar system, the first emissary from Earth to take up residence in the main asteroid belt.
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.› Learn more about the Dawn mission
Dawn is celebrating the second anniversary of leaving its home planet by engaging in the same function it has performed most of its time in space: with the utmost patience, it is using its ion propulsion system to gradually modify its orbit around the Sun.
In its interplanetary travels, the spacecraft has thrust for a total of about 389 days, or 53% of the time (and about 0.000000008% of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 103 kilograms (228 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 2.62 kilometers per second (5870 miles per hour). As previous logs have described, because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished only one-fifth of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by most spacecraft. (For a comparison with probes that enter orbit around Mars, refer to a previous log.)
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.
|Minimum distance from the Sun (AU)||Maximum distance from the Sun (AU)||Angle from Earth’s orbit|
|Dawn’s orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sept. 27, 2009||1.42||1.87||6.2°|
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.› Learn more about the Dawn mission