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
The Dawn mission remains on course as the spacecraft continues to thrust with its ion propulsion system, patiently, persistently, and gently changing its orbit to keep its appointment with protoplanet Vesta in two years. Meanwhile, closer to mission control and in stark contrast, brave firefighters work hard to protect JPL and the nearby homes of many of its employees and others in the community.
The probe has continued in “quiet cruise” since the last log. During this month, engineers did give the robot a few extra tasks to ensure it remains healthy, but these were routine. When each such assignment was conducted the first time or two that Dawn was in space, they were treated as special activities, with even greater diligence than is normally applied to the unforgiving and complex undertaking of flying a spacecraft far from Earth. Now however, the commands for these activities are stored onboard well ahead of time along with the routine commands for thrusting, communicating with Earth, and carrying out all the other functions the spacecraft normally conducts without the mission control team devoting extra attention.
Included in the maintenance procedures were instructions to perform a sequence of movements of the mechanism that points ion thruster #1, to power off reaction wheel #2 and return #1 to service, and to operate the gyroscopes for about 4 days. For readers who do not have their copies of the Dawn operations manuals handy, some information about these 3 kinds of operations was provided in a previous log.
Another event that is now considered routine occurred on August 15. For the second time this year, a particle of space radiation struck a particularly sensitive electrical component on the spacecraft, depositing enough energy to interfere with the operation of a circuit. When this happened in January 2008, it caused Dawn to enter safe mode, interrupting its other activities. Thanks to software the team transmitted to the ship later that year, now the interplanetary explorer is immune to strikes in that formerly vulnerable location.
As Dawn continues its long (in space and in time) solar system journey to match orbits with Vesta and later with Ceres, both of which reside farther from the Sun than the probe has yet traveled, some readers may note a surprising trend in the statistics for the mission. The famously unimaginative ending of each of these logs reveals that Dawn’s distance from Earth has been diminishing since November 2008. Indeed, the probe’s maximum separation from its planet of origin occurred on November 10. Today, it is as far from Earth as it was on June 2, 2008. By January 2010, it will be as close as it was in March 2008. Is this progress?
Earth and Dawn, each following its own path, are both in orbit around the Sun. As grateful residents of the planet know, their world’s orbit doesn’t change very much. The planet keeps following the same nearly circular path around the Sun year after year after year. Today Earth is about 1.01 astronomical units (AU) from the Sun, and it never strays very far from its average distance of 1.00 AU. As Dawn has traveled independently of Earth, thanks to the push from its Delta rocket, its orbit has been farther from the Sun than Earth’s. The gradual effect of ion thrusting and the much more abrupt boost from Mars have caused that orbit to change considerably since then. To enter orbit around Vesta, Dawn will have to match the giant asteroid’s orbit around the Sun, ranging from 2.15 AU to 2.57 AU. Today, Dawn is 1.53 AU from the Sun and headed outward.
As we saw a year ago (that is, one Earth-orbit-around-the-Sun ago), objects at different orbital distances travel at different speeds. The probe, orbiting the Sun at a greater range than Earth, travels more slowly, because the Sun’s gravitational attraction diminishes with distance. So as Dawn heads slowly for Vesta, gradually spiraling away from the Sun, and Earth speeds around more quickly in its orbit, sometimes our planet moves closer to the spacecraft and sometimes it moves farther away.
In a continuing effort to offset the extraordinary cost of these logs with the handsome revenue from subtle product placements, we can refer to still another in the apparently endless line of Dawn clocks (many of which have been described in recent logs and all of which are available in the Dawn gift shop on your planet). On this clock, the minute hand is shorter than the hour hand. The motion of the former represents Earth, traveling closer to the Sun (at the clock’s center) and more quickly. Dawn is at the tip of the hour hand, moving more slowly in its larger orbit. (We’ll ignore for now that the hour hand should be growing in length, as the spacecraft recedes from the Sun.) Some of the time (such as between noon and about 12:30), the distance between the ends of the hands increases, but then the situation reverses; the faster minute hand begins moving closer and closer to the hour hand as the time approaches about 1:05.
Earth and Dawn are exhibiting the same repetitive behavior, albeit more complicated because of Dawn’s ever-changing orbital speed and distance from the Sun. They will continue to draw closer until January, when Earth, coming from behind, passes Dawn and moves on ahead. The explorer will not need to take note however, as its sights are set on the asteroid belt.
So for readers tracking the distances reported in each log, don’t despair. The continuously declining separation between Earth and its celestial envoy is a reflection of the elegant mechanics of the cosmos and not the result of inattentive engineers setting the spacecraft on the wrong path.
Next month, as Earth and the spacecraft continue their separate solar system dances, together with the Sun they will briefly make an attractive arrangement. On September 18, Dawn will be just as far from the Sun as it is from Earth, at 1.56 AU from each. Earth and the Sun will be 1.00 AU apart. The trio formed a very similar pleasing pattern last year. A triangle such as this, with two sides of equal length, is usually called “isosceles.”
Although there is nothing inherently significant for the mission about this alignment, we can use one more clock example to illustrate this isosceles triangle. In this case, we put Dawn at the center and Earth at the 12. (This clock may not be as useful for telling time as some of the others that are available, but it would still make a great gift.) The Sun would be next to the sixth little tick mark, where the minute hand would point at about 6 minutes and 15 seconds after the hour. (Note that this depiction of the geometry illustrates the angles and the relative separations of Dawn, Earth, and the Sun; hence, the clock may be any size. Several sizes are available in the gift shop, and we helpfully recommend the most expensive one.)
Not only is Dawn on course for the asteroid belt, on course for returning new and exciting discoveries from its enigmatic destinations, it is on a new and better course than it had been. According to inside sources, the vast team of writers specifically assigned to create the next log is already planning to explain what has changed and why. Just as all loyal readers, your correspondent is hoping for an interesting description of this improvement in the mission.
Dawn is 250 million kilometers (156 million miles) from Earth, or 620 times as far as the moon and 1.66 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 28 minutes to make the round trip.
Dr. Marc D. Rayman
10:30 pm PDT August 30, 2009
Shhhh! Dawn is in “quiet cruise,” and we do not want to disturb it. The indefatigable spacecraft is devoting most of its time to thrusting with its ion propulsion system, applying a gentle but persistent pressure to its trajectory around the Sun. With patience, it will reshape its orbit to match those of the mysterious and intriguing protoplanets Vesta and Ceres. In order to conduct its detailed explorations of each of these exotic worlds, Dawn will accompany them around the Sun, visiting with Vesta in 2011 - 2012 and rendezvousing with Ceres in 2015.
Today Dawn is 220 million kilometers (137 million miles) from the star at the solar system’s center. We can express this more conveniently by recalling the ruler so often used in describing interplanetary distances, the astronomical unit (AU). The average distance between Earth and the Sun, nearly 150 million kilometers (93 million miles), is defined to be 1 AU. (For comparison, Mars, whose orbit is not as circular as Earth’s, travels between about 1.38 AU and 1.67 AU.) Today Dawn is 1.47 AU from the star at the solar system’s center.
If the spacecraft stopped thrusting now, well, gosh, that would be a disappointment! But let’s calm down and think about it anyway. Its momentum would keep it going around the Sun in an elliptical orbit that ranged from about 1.38 AU to 1.84 AU. Vesta, the first stop on Dawn’s solar system itinerary, lives between 2.15 AU and 2.57 AU. Dawn has a lot of work left to do!
The task is far more difficult than simply enlarging its orbit to have precisely the same shape as Vesta’s. In addition, Dawn has to change the orientation of its orbit so it overlaps Vesta’s. Even that is not sufficient, however, because the spacecraft also has to be in the same place in its orbit that Vesta is in its. It wouldn’t be helpful to be on exactly the same path but be very far apart.
Still, let’s consider just the problem of changing the size of the orbit. If Dawn thrust all day today but stopped tomorrow, the one day’s worth of work would change the orbit by only about 0.0004 AU. For another perspective on the effect of the thrusting, compare the orbit size 2 paragraphs above with the orbit Dawn was in after the boost from Mars. The difference is the result of 7 weeks of powered flight, yet on the scale of these vast distances, it is so small it barely registers. Yes, there is indeed a great deal of work ahead.
Dawn will reach Vesta in about 2 years. The key to getting there is the combination of the extraordinary efficiency of the ion propulsion system with the patience and reliability of all systems. The probe’s persistence in quiet cruise will pay off with the excitement of its discoveries in the asteroid belt.
Now why is this deemed “quiet cruise”? It’s true that in space no one can hear you thrust, but that’s not the reason. Rather, it is considered quiet because the spacecraft is not engaging in any special routines or functions (assuming you don’t consider traveling for years in deep space to be special). Thrusting is Dawn’s most familiar activity. Even with the long coast from October 31, 2008 to June 8, Dawn has spent about half of its time since launch tirelessly adjusting its orbit. In contrast, the great preponderance of spacecraft coast all of the time (or nearly so), just as planets and asteroids do, simply going where their orbits take them.
Dawn has already had plenty of time in space that was not quiet (readers of these logs have shared in some such times), and much more lies ahead, particularly when it is in orbit around its two very distant targets. For now though, the spacecraft’s focus is on its trek to Vesta. While its engineering colleagues will continue to be diligent in maintaining Dawn’s health and providing regular updates to its flight plan, the quiet cruise does afford the team more time to develop and refine plans for operations at Vesta. A great deal of work remains to prepare for that not-so-quiet time.
But really, how quiet is quiet cruise? In fact, Dawn is a hive of activity. For the probe to reach its targets in the asteroid belt, all engineering systems labor together. For the following discussion, reminders of the essentials of each of these systems are available by clicking here (note that this link works only for loyal readers), and another detail is available here.
For the ion propulsion system to keep emitting a steady stream of high-speed xenon ions, it must regulate the delicate flow of xenon from the main tank at more than 1000 pounds per square inch (for you readers on Earth, that’s about 70 times atmospheric pressure, and for you readers on Venus, that’s low pressure) to the thruster at roughly one millionth of a pound per square inch (around one ten-millionth of atmospheric pressure). It constantly ionizes the xenon and electrically accelerates it, requiring careful control of the high currents and voltages. The electrical power system delivers to the ion propulsion system the needed power; it supplies all other systems as well. It draws its energy from the solar array wings and converts it to the voltage its onboard customers need.
To keep the huge arrays pointed at the Sun and the ion thruster aimed in the direction needed to travel to Vesta, the attitude control system is always at work. Five times every second it takes a fix with its star tracker and computes the orientation, or “attitude,” of the craft in the void of space. If needed, it rotates the solar arrays, and it uses several different means to adjust the entire spacecraft’s attitude to achieve the required thruster pointing.
The telecommunications system continually transmits a radio signal through one of the small antennas, broadcasting a very wide beam that encompasses Earth so that mission controllers can listen in whenever they choose. The system is also unceasingly alert, listening for an almost imperceptible radio whisper from Earth in case human team members need to contact the probe between the weekly sessions that use the main antenna.
Every second the thermal control system reads more than 100 temperature sensors and decides which heaters to turn on or off to keep each component from becoming too cold or too hot.
The command and data handling system, including the main computers, is orchestrating most of this activity. It switches its attention 200 times per second as it communicates with other components. About 35 times every second it assesses all the data available from around the ship and selects some for storage and subsequent transmission to Earth through the main antenna. More than 200 parameters are checked each second so that if there are any problems, the software can take action, promptly issuing instructions to protect the craft and the mission, ensuring unexpected situations do not get out of control. And while performing these and many other functions on its own, every second the software checks to find out whether there is a stored command from mission control. Engineers formulate these directives many weeks in advance and place them onboard to be carried out at the precise second planned.
So quiet cruise really is not so quiet. And yet, this is what Dawn was designed to do. In the forbidding depths of space, all the systems work together in a near frenzy of activity to hold the stalwart ship steady, keeping it healthy and on course, as it maintains its sights on the distant horizon, where the asteroid belt beckons.
Dawn is 275 million kilometers (171 million miles) from Earth, or 710 times as far as the moon and 1.84 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.
Dr. Marc D. Rayman
12:30 am PDT July 28, 2009
Dear Dawnterested Readers,
Having completed the longest planned coasting period of its entire mission, Dawn is now back to its familiar routine. On June 8, the ion propulsion system was called back into action to propel the probe to its rendezvous with asteroid Vesta.
The spacecraft began its 7-month coast period on October 31, 2008. Since then, it had used its ion thrusters for a measurement of the solar array power, a small adjustment in its course to Mars (the gravitational effect of which provided a boost to its distant destination), and tests of the software remotely installed on the main computer in April. The accumulated thrusting during all of those activities added only about 10 hours to the mission’s log of 282 days when coasting commenced.
Now that the ship has resumed its powered flight, the spacecraft devotes most of its time to thrusting. With the utmost patience, like an artist perfecting each delicate detail in a grand masterpiece, Dawn gradually reshapes its orbit around the Sun. A full day of thrusting is enough only to change its speed by a modest 7 meters/second (less than 16 miles/hour).
Dawn thrusts all but about 6 to 8 hours per week, providing only a brief opportunity to turn away from the direction it needs to aim its ion thruster in order to point its main antenna at Earth. That weekly radio communications session affords the robotic explorer its sole contact with mission controllers. While it is thrusting, Dawn is programmed to broadcast signals from one of its small, auxiliary antennas, spreading its radio signal in a wide swath that encompasses distant Earth. Usually one of the exquisitely sensitive receivers of NASA’s Deep Space Network will listen in on the spacecraft for a few hours halfway through the week, capturing the extraordinarily faint transmission showing the spacecraft is sailing smoothly.
Spending so much time thrusting is possible thanks to the extremely frugal use of xenon propellant. The ion thruster expels only about 0.26 kilograms (10 ounces) in a day. So while Dawn would need nearly 4 days to accelerate from 0 to 60 miles/hour, it would consume little more than 1 kilogram (about 2.3 pounds) of its supply of xenon during that time.
As the probe climbs away from the Sun to reach the cold depths of the asteroid belt, the multiyear thrust profile is designed to make its solar orbit match that of Vesta. The current flight plan has it arriving at the massive protoplanet in September 2011, requiring it to thrust for more than 700 days along the way, the significant majority of the time.
Prior to resuming thrust, the spacecraft carried out a routine check of one its scientific instruments. All of the instruments designed to uncloak the secrets Vesta and Ceres hold about the dawn of the solar system spend most of the time during the interplanetary cruise switched off, waiting for their opportunities to go to work in orbit. Each instrument is powered on occasionally to verify its health. On May 27, the visible and infrared mapping spectrometer (VIR) was activated. On this occasion, VIR repeated the routines it first executed in space in October 2007. All of its mechanisms were exercised, and they operated smoothly. Instead of aiming at distant celestial targets, its visible and infrared detectors measured emissions from built-in lamps. VIR passed the 4-hour test with flying colors (some of which are outside the range of human vision).
The VIR operation was one of many assignments for the coast period, most of which have been described in logs since November 2008. With all activities completed successfully, the spacecraft set about thrusting right on schedule. On June 8, executing instructions already stored in its main computer, Dawn rotated to point thruster #1 in the required direction. It powered on the ion beam shortly after 11:59 am PDT. Any readers who happened to be in the vicinity during their own deep-space excursions would immediately have recognized the familiar scene: Dawn majestically perched once again atop a blue-green pillar of xenon ions, as its ambitious journey of exploration continues.
Dawn is 291 million kilometers (181 million miles) from Earth, or 775 times as far as the moon and 1.91 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.
Dr. Marc D. Rayman
5:00 am PDT June 28, 2009
Dawn’s mission continues to go very well, as the spacecraft nears the end of the longest coasting period of its astronomical journey. The deep-space member of the team has completed more special activities under the helpful guidance of the terrestrial contingent.
The previous log described the installation of software in the spacecraft’s main computer. (Known as flight software 8.0, this name was chosen as part of Dawn’s new outreach effort to increase public awareness of the number 8. Please consider yourself aware. The outreach plan for the designation of the subsequent version of the software is expected to be 12.5% better.) The software had been tested extensively in simulators on Earth, but the ultimate test is its performance in keeping the spacecraft running smoothly. Because Dawn devotes more time in space to thrusting with its ion propulsion system than doing anything else (this does not count the time it spends in eager anticipation of the exciting rewards to be garnered at its destinations Vesta and Ceres), it was important to verify that the capability to thrust was not compromised with the new software. Despite the assiduity with which 8.0 was developed, the possibility of a subtle bug being introduced could not be discounted.
A test well ahead of the planned resumption of thrusting early in June would allow time to rectify problems and still keep to the itinerary. Just as with the tests in the initial checkout phase of the mission, the objective was to show that all subsystems would work together (under control of the new software) to sustain stable thrusting.
On April 27, as it had done so many times earlier in the mission, the spacecraft executed a set of commands to rotate to point a thruster in a selected direction, perform the pre-start procedures, and undertake powered flight. With tests during the initial checkout phase plus all the thrusting in the interplanetary cruise phase designed to reshape the probe’s orbit around the Sun, Dawn had accumulated 123 days of operation with ion thruster #1 and 158 days with ion thruster #3. (The locations of the thrusters are explained in a previous log.) Thruster #2 has not had its turn yet for long-term thrusting, so it had been operated only for 22 hours. While much longer than the operating time for a propulsion system on a typical planetary spacecraft, for Dawn this is still considered brand new. Therefore, the 8.0 system thrust test used ion thruster #2, allowing it to accelerate xenon for the first time since November 16, 2007.
After thrusting for 4 hours, the spacecraft stopped and reoriented itself, aiming the main antenna to Earth again. On May 1, it conducted a similar exercise, differing only in the direction of thrust. These tests provided the final confirmation that 8.0 was ready to take the helm when the time came to resume propelling the craft to the asteroid belt.
These activities had a secondary benefit. During the initial checkout phase, some tests were conducted of how accurately thrusters #1 and #3 could be pointed. The equivalent test with #2 could not be run then because the spacecraft was too close to the hot Sun to point that thruster in the required direction, so it was planned for April 2008. That operation was interrupted by an event that led to safe mode being invoked. Because thruster #2 was not scheduled to be used until much later in the mission, the test was postponed to the coast period after Mars. The 8.0 system thrust verification test was executed in the directions needed to complete the thruster #2 pointing tests, thus accomplishing these additional objectives.
Thrusting at this time in the mission does not help reach the targets (if it did, Dawn would not spend so much time coasting), but the short thrust trials did not modify the trajectory appreciably. The ion propulsion system has changed the spacecraft’s speed by about 1.87 kilometers per second (4180 miles per hour) so far in the mission, and this latest work added only 2.3 meters per second (5.1 miles per hour) to the total.
Following these successful tests as well as several weeks of other operations, engineers were sufficiently pleased with 8.0’s admirable performance in both its software and its outreach functions that they were ready to commit to using it. In April, the software had been installed only in the primary location in the primary computer. On May 11, mission controllers powered on the backup computer and installed the primary and backup copies of 8.0. On May 13, they loaded the software to the backup area of the primary computer, so now 4 copies of 8.0 are stored onboard. Updating the software, even in a backup location, is a delicate operation, but all the procedures went according to plan. That brought the work on 8.0 to a successful and satisfying conclusion.
For the ion propulsion system to operate with its extraordinary efficiency, a gentle flow of xenon gas to the thruster needs to be accurately controlled. At full throttle, this amounts to slightly more than 3 milligrams per second (less than 10 ounces per day), and by the end of the mission, when limited electrical power necessitates a low throttle level, the flow will be reduced to less than 40% of that rate. A sophisticated labyrinth of tubes, electronically controlled valves, and other components feeds the propellant from the main tank to the selected thruster. Along the way, xenon is held temporarily in a pair of small, low-pressure tanks (each known as a “plenum” and the pair as “plena”). The main tank is pressurized to more than 1000 pounds per square inch (psi, and note we will spare readers the metric units for pressure), but one of the plena is charged to less than 67 psi and the other is never raised above 37 psi. Pressures will be still lower at lower throttle levels.
The ion propulsion system’s computer controller relies on pressure and temperature sensors on each plenum as it regulates the flow of xenon. The controller interprets any change in the pressure as a change in the amount of gas moving through the system, so a shift in the behavior of a sensor would cause the controller to adjust the rate at which xenon is delivered to the operating thruster.
The pressure sensors are designed to remain stable throughout the mission, but in order to keep the ship in good shape, ever-skeptical engineers want to determine how much (if at all) the devices have drifted in their measurements since they were calibrated well before launch. So here they are confronted with a conundrum: how can they verify the sensors on the distant spacecraft are giving accurate readings?
We solicit here ideas from readers for how to accomplish this “plenum pressure transducer calibration.” Please submit your ideas to the Dawn project before the next paragraph is written, and the best idea will be implemented. What a thrilling reward that will be for some lucky reader! Be sure to include thorough calculations, assessment of risks to the spacecraft and mitigations for those risks, a complete list of commands, predictions for subsystem telemetry, and any other pertinent details.
Thank you very much to all those who proffered suggestions; we received a surprising number of innovative methods, a few of which are even compatible with physical laws and the capabilities of the Dawn spacecraft. We now have an effective solution. (The winning entry came from a reader on Earth who does not want the prestige of being named here, but we offer our most sincere congratulations!)
By bringing the plena to a pressure that is ascertained without the use of the pressure sensors, engineers can compare the sensor readings with the independently determined pressures. So they commanded certain valves to open, thus allowing the xenon in these tanks to vent into space. Each plenum will evacuate to nearly 0 psi, so the report from the pressure sensors will provide a direct measurement of how much their behavior has changed, and any discrepancy can be accounted for by straightforward updates to the software.
The procedure of connecting the plena to the cosmic vacuum is known as a blowdown, perhaps connoting a gale-force wind of xenon as it begins rushing out of the system. The reality is somewhat different. When the valves were opened on May 4, the force from the zephyr of xenon leaving the spacecraft was less than 3% of the famously light thrust achieved when the xenon is ionized and accelerated at full throttle. The unionized xenon pushed on the spacecraft with about one tenth of the force you would feel holding a penny. As the plena pressures fell, the force diminished still further.
At the conclusion of the 8.0 system thrust test, one plenum held 90 grams (about 3 ounces) of gas and the other held 50 grams (less than 2 ounces). (For comparison, the main tank carried 425 kilograms, or 937 pounds, at launch and now holds 350 kilograms, or 772 pounds.) The expenditure of this small amount (about what is used in half a day of thrusting at maximum throttle level) is well within the mission’s overall xenon budget. The valves will be left open for several weeks to allow plenty of time for the xenon to find its way from the plena through the maze of hardware to space. Well before thrusting begins on June 8, the valves will be closed and the plena repressurized.
In the meantime, Dawn will pass another milestone in its solar system adventure. As we have seen in previous logs, the nature of orbits is that at greater distances, objects travel more slowly. Dawn and Earth both orbit the Sun, but the spacecraft, being more remote than the planet, travels more slowly. On June 3, Dawn will complete its first revolution around the Sun (a “Dawn year”) since its September 27, 2007 launch. Earth, along with its residents (constituting a small but enthusiastic fraction of our readership) as well as the moon and all other natural and human-made satellites in orbit around it, needs about 365 days for a revolution, so it will have made 1.68 loops while Dawn was in its first.
You may be tempted to think that Dawn takes longer to round the Sun than Earth because it has to travel a greater distance to do so in its larger orbit, but you are encouraged not to yield to this simple explanation. Earth has moved more than 1.58 billion kilometers (984 million miles) since letting go of the probe. In the same time, Dawn has traveled only 1.32 billion kilometers (817 million miles). When farther from the Sun, objects travel more slowly because the gravitational pull they need to resist is weaker. As it climbs atop its ion beam to still greater heights, Dawn will go even more slowly as it seeks to match orbits with its more remote destinations. Since the spacecraft departed from Earth, Vesta has completed only 0.44 revolutions, traveling 1.00 billion kilometers (622 million miles), and even more distant Ceres has made only 0.40 loops, covering 994 billion kilometers (617 million miles).
To keep our subscription fees low, we include here another subtle product placement for one of the many clocks in the Dawn gift shop, several of which have been featured in logs over the past 5 months. On this model, the Sun is at the center and there is a separate hand for each of these orbiting objects. Although they were not aligned at launch, suppose they were, with each hand pointed toward the 12. (Because of the northern hemisphere perspective with which astronomy was developed on Earth, our standard view of the solar system depicts the orbits going counterclockwise. Here we will ignore that familiar perspective, as our focus is on how many rounds each body completes, not the direction of their travel.) Since launch, Earth would have gone all the way around once and continued on clockwise to just past the 8. The spacecraft will return exactly to the 12 on June 3. Vesta will have leisurely advanced just beyond the 5, and Ceres will not quite have made it even to that number yet.
As Dawn continues to push outward from the Sun, with its sights set on alien worlds deep in the asteroid belt, we hope readers will continue to follow its progress. With (some of) the profits from the first Dawn Ponzi fund, we have added RSS feeds for these logs and other features at the Dawn website. To learn more, and to be among the first of your species to subscribe to automatic updates, visit http://dawn.jpl.nasa.gov/RSS/index.asp.
Dawn is 305 million kilometers (189 million miles) from Earth, or 845 times as far as the moon and 2.01 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 am PDT May 25, 2009
The upgraded Dawn spacecraft is now traveling in a new direction in its orbit around the Sun. The mission continues to go smoothly during this long coasting period, scheduled to conclude in June, when powered flight with the ion propulsion system will resume.
Dawn has many computers in its onboard crew and one that serves as the captain of the ship. This primary computer’s software had been unchanged since February 2008. That last modification involved only a small patch, making version 7.0.3 of the software. That was preceded by a small update in January 2008 and one in December 2007. Prior to this month, Dawn had had only one complete in-flight replacement of its main software, in November 2007.
In January 2008, engineers began working on the next major upgrade in software. It was scheduled for the coasting period after the Mars gravity assist, and here we are! To maintain the sense of majesty that fuels the passions of those who yearn to explore the cosmos and long to discover its secrets, the software was given a name to suit its important role in this grand endeavor: OBC flight software version 8.0. Demonstrating an irrepressible sense of humor, though, most members of the team enjoy referring to it by its screwball nickname, “eight oh.” Sometimes the public may wonder how such serious and challenging work can be accomplished amidst such antics in the Dawn section of JPL’s building 264!
Software 8.0 contains 34 changes. These range from improvements to make more efficient use of the extremely low rate of transmission of data from the spacecraft when it uses one of its small auxiliary antennas (as it does in safe mode and in some other circumstances), to methods to expedite recoveries from certain causes of safe mode, to increasing the robustness of the handling of some conditions that, while highly unlikely ever to arise, could be very serious. A few bugs, inevitable in software of this complexity, also were fixed. Indeed, readers who are about 7 weeks behind in these logs are just now learning that the bonus instrument calibrations during the crucial and successful gravity assist at Mars were interrupted by the combination of two bugs, neither of which by itself would have been sufficient to affect the activity. One of them, which prevented the backup star tracker from being called into use when needed quickly, was corrected in this version of the software.
The process of loading software, particularly into the main computer, is complex and delicate. The spacecraft constantly relies upon that computer for keeping the solar arrays pointed at the Sun to generate electrical power for all systems, maintaining components at the correct temperatures despite the extreme conditions of spaceflight, aiming the antenna at Earth, and performing a great many other functions vital to Dawn’s operation. Should the loading of the new software go awry or introduce new bugs along with the intended changes, the consequences could be less than felicitous. A substantial effort was devoted to careful and thorough testing of the software in computer simulators on Earth, and the techniques and procedures for installing it on the most important computer, the one in deep space, were tested and rehearsed extensively as well.
As explained in December 2007, Dawn has a primary main computer, with primary and backup copies of the software, and an identical backup computer, also with primary and backup copies. On April 10, mission controllers verified that the backup copy in the primary computer and both copies residing in the backup computer were pristine, uncorrupted by radiation or any other anomalies since they had been loaded. If a problem developed while loading 8.0, the team was confident the spacecraft could resort to familiar and healthy software if need be.
On April 13, the new software was transmitted to the spacecraft. Dawn receives commands from large antennas of the Deep Space Network at 2000 bits per second. While this would seem agonizingly slow for our terrestrial readers located in the 21st century, that is the maximum rate for NASA’s interplanetary probes. Most sets of instructions bound for the spacecraft are short enough that they consume no more than a few minutes of transmission time (regardless of how long it takes the signals to travel to the spacecraft). The file containing the software was so large that the time required to transmit it was far longer than the nearly 18 minutes needed for radio signals (which, as regular readers know, travel at the universal limit of the speed of light) to traverse the vast distance to the craft. So, during the loading of the software, Earth and Dawn were connected by an uninterrupted bond, a radio link 320 million kilometers (199 million miles) long. Even as Dawn was accepting the signals, still more were being sent. An unbroken line of 0’s and 1’s, each bit 150 kilometers (93 miles) long, stretched through space, connecting the probe to its erstwhile home.
In addition to 8.0 onboard (and verifying that it did indeed arrive intact, with no dropouts, no errors, no miscommunications whatsoever), engineers conducted other checks and reconfigured systems to prepare for running the new software. Additional preparations and final verifications were conducted on April 14.
With all stations in mission control early that afternoon reporting their subsystems were ready, the final “go” was given, and the command to reboot the computer for the first time since November 2007 was transmitted. In addition to causing the computer to run the new software, this triggered the spacecraft to enter safe mode, as expected. (Somewhat less directly, it also triggered pizza to be delivered to mission control a few hours later.) Because safe mode does not use the main antenna but rather one of the smaller, auxiliary antennas, the operations team scheduled this delicate operation to occur under the watchful eye of the 70-meter-diameter (230-foot-diameter) radio antenna of the Deep Space Network complex near Canberra, Australia, rather than one of the 34-meter (112-foot) antennas. With the larger antenna, engineers could more quickly verify the new software was operating correctly and begin the long process of returning the spacecraft to its normal configuration.
The operations team took advantage of these activities to make another planned change. Safe mode employs all 4 of the reaction wheels, components which can be electrically spun faster or slower to help stabilize the spacecraft or rotate it. In normal operations, only 3 are used; wheel #2 had its turn to be dormant starting in October 2008. In commanding Dawn out of safe mode, controllers kept wheel #2 on and powered off #1.
By April 16, the spacecraft was fully reconfigured, and engineers continued to be happy with the functioning of the new software. While a great deal of work had gone into preparing for a wide variety of problems that could have occurred because of loading the software or that could have been more troublesome had they cropped up by coincidence during this sensitive operation, it all proceeded quite smoothly indeed.
We will report in the next log on installing the backup copies of the software and on further verifications that 8.0 is up to the job of continuing the mission that had been progressing so well with 7.0.3.
If you wish to download 8.0 for use at home (or on your spacecraft), you may obtain a copy directly from the main computer on Dawn. The computer is inside the box labeled “CEU A,” conveniently located under the -y panel of the spacecraft. Don’t forget to bring your own cable, and please be careful not to damage the delicate solar cells on the nearby wing.
Engineers have already begun work on the next version of the software, this one having the sleek and chic sobriquet “nine oh.” It will be sent to the spacecraft in 2010; and at that time, we surely will have occasion to link back to this log, so readers are advised to retain a copy for reference. (Some may wish to preserve this log purely as an investment. Dawn financial analysts predict that when it is cited in next year’s description of 9.0, its value might rocket to as much as 2% more than you paid for it.)
As it resumed normal operations with its new software, Dawn reversed one component of its course through space. Since August 8, 2008, it had been falling toward the Sun. Thanks to the principles of orbits, it was not in danger of reaching that star or even getting close enough to be singed. The ship is in an elliptical orbit, whose innermost point was reached on April 17 (at about 5:07 pm PDT, for those of you who wondered what seemed to change around that time). The probe was 205 million kilometers (127 million miles) from the Sun. We may recall a measurement unit more convenient than kilometers or miles, the astronomical unit (AU), in which 1 AU is the average distance between Earth and the Sun. With that handy ruler, Dawn was 1.37 AU from the star. After passing through that low point in its current orbit, momentum began carrying it farther from the Sun again.
Thanks to the effects of the ion thrusting in the next few years, it will continue to follow an outbound path until late 2012, when once more it will temporarily approach the Sun. It will never again be as close to the solar system’s center as it was this month, for the craft will have to climb to more than 2.2 AU to reach Vesta and to more than 2.9 AU as it orbits Ceres.
In preparation for its work there, on March 31, the spacecraft conducted another test of its primary science camera, which has performed flawlessly during the mission. It will be used not only to acquire exciting views of the alien surfaces of the unexplored worlds it is bound for, but also to help refine the location of those faraway and mysterious bodies so Dawn’s navigators can guide the probe into orbit. To plan for the most productive observations, the team needs to understand the detailed performance of the instrument under the full range of conditions in which it might be used. Can the camera point as close to the Sun as will be desired, without unwanted light compromising exquisite details in the pictures? Well, it is quite a coincidence that you, loyal reader, just now began thinking of that particular question, because the purpose of this test was to answer it.
The spacecraft started by pointing the camera 90 degrees from the Sun. It took a set of pictures, then rotated to aim a little closer to the Sun and took more, continuing through a range of 60 degrees. To imagine this, let’s take advantage once again of one of the many exotic Dawn clocks available in the Dawn gift shop (be sure to visit the shop on your planet soon to see the exciting new line of fashionable Dawnderwear). In this clock, the spacecraft is at the center, and the Sun is at the top, where the 12 would be. To begin, the craft pointed the camera toward the 9. After taking its pictures, it turned by 2.5 degrees, or about the same angle the second hand would move in just over 0.4 seconds. It stopped again and obtained more images. It continued with this pattern until it was directed at the 11, which was the closest it could point safely to the hot star at its current distance.
As with most clocks in the gift shop, this one might not be the best choice for something as mundane as telling time. Allowing time for each turn, stabilizing at each position, and then acquiring all the pictures, it took Dawn 12 hours to rotate from the 9 to the 11, or 6 times slower than the hour hand moves. This long test yielded 410 images at 25 different angles from the Sun, allowing engineers to determine if stray light could find its way to the camera’s detectors by any circuitous path, reflecting off components within the complex camera. Once again, the unit performed superbly. Even with exposures as long as 100 seconds, unwanted light did not reach appreciable levels.
Now flying under the control of new software and confident its camera will operate under all necessary conditions, Dawn is headed deeper into space again. Still more tasks lie ahead before it resumes its familiar routine of sustained thrusting with the ion propulsion system. The subsequent log, which we boldly predict will follow this one, will describe the next series of special activities.
Dawn is 315 million kilometers (196 million miles) from Earth, or 855 times as far as the moon and 2.09 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 am PDT April 26, 2009
Dawn continues to coast quietly and calmly in its orbit around the Sun, keeping its main antenna pointed to faraway Earth. The mission control team has given the spacecraft relatively few assignments in recent weeks, providing time to prepare for a busier future. To ensure the distant craft remains healthy and safe, operators transmitted instructions for conducting routine maintenance, activities that are familiar to the probe now that it has been on its deep-space journey for more than 1.5 years. Perhaps what is most noteworthy and satisfying since the last log is not what Dawn did, but rather what it did not do.
At some unknown time in the past, at some unknown location in space, under specific conditions and processes we can only speculate about (albeit under the influence of physical laws that are well understood), a high energy particle embarked on its own deep-space journey. On March 21 at 1:32 am PDT, its path and Dawn’s intersected. The particle penetrated the spacecraft and reached an electronics unit inside. The energy that had been given to it elsewhere in time and space was transferred then and there to a miniature electronic component in a circuit within that unit.
The circumstances of this event were very much like those that occurred on January 14, 2008. As a reminder for the reader who has not memorized the log describing that incident (you know who you are, and we do as well, but your secret remains safe under the terms of our readers privacy agreement), the chain of events led to the spacecraft entering “safe mode.”
While safe mode is the desired response to a wide range of unexpected or problematic conditions, Dawn’s engineers recognized that it was not necessary when space radiation hit that particular device. They reexamined the extensive analyses that had been conducted before launch and performed new studies as well, concluding that the mission could be interrupted by more such events because of the susceptibility of the component that was hit by the radiation. It was not possible to establish with high confidence how frequently it could occur (much less to know specifically when it would occur), but the data suggested that it might happen often enough that it would hamper the mission, interfering with too many activities and ultimately consuming too much of mission controllers’ limited time.
The team, therefore, formulated a change to the software in one of the spacecraft’s auxiliary computers that would allow it to accommodate another radiation hit without the subsequent cascade of events culminating in safe mode. While changing the software is challenging and time consuming, the project chose to undertake the work to avoid a repeat of the incident, judging the certain cost of the change to be less expensive to Dawn's mission than the possible cost of subsequent incidents.
On July 22, 2008, the new software (along with all the instructions to patch it in) was radioed to the remote spacecraft. Since then, it has been available should a radiation strike energize the component as it had 6 months earlier.
The modified part of the software had its first use in flight with the radiation impact on March 21, when nature repeated itself. The electronics were unchanged, of course, so the circuit responded the same way it had 14 months earlier, informing the software of a problem. This time though, the software responded simply by storing a short message to transmit along with all the other spacecraft health and status information during the next scheduled contact with Earth (by coincidence, later that day). The unnecessarily dramatic activation of safe mode is no longer part of the outcome, thus allowing the robotic probe and its human support team to continue their work without interruption or distraction.
While Dawn happened to be quiescent when this radiation impact occurred, it might not have been, given the inherent unpredictability of the timing. It might not be when the next uninvited particle strikes the same component. Now, with the clear demonstration of the effectiveness of the software patch, it won’t matter. Controllers were gratified to receive the message on March 21 reporting the spacecraft's nonchalant response to the radiation, validating their decision to modify the software last year.
The spacecraft will be busier in the coming weeks than it has been since it was deflected by Mars last month. The next log, already available for readers in the future [Note to those readers: please send a copy to the author in his present to save him the time of writing it], will cover some of the upcoming special activities and will recognize another milestone (if not million-milestone) on this journey of scientific discovery and adventure to distant and unexplored worlds.
Dawn is 327 million kilometers (203 million miles) from Earth, or 875 times as far as the moon and 2.19 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 36 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 am PDT March 29, 2009
Dear Dawnlight Saving Times,
Now boosted into a new solar orbit courtesy of Mars, Dawn continues its interplanetary journey. The spacecraft is healthy and coasting, keeping its main antenna pointed to Earth, as it will for most of the next 3 months. After that, it will resume its familiar routine of devoting most of the time to gently thrusting with its ion propulsion system, with only a short period each week for communications.
Following the last log, as the probe succumbed to the gravitational pull of the red planet, its trajectory gradually began to change. Flying true to the plan, Dawn swooped close to Mars and then left it behind on a new course, having taken advantage of Mars’s gravity.
The spacecraft plunged to within 542 kilometers (337 miles) of the planet, reaching that lowest altitude at 4:27:58 pm PST on February 17. The last time it had been so close to another solar system body was on September 27, 2007, as it left Earth to begin its long journey to the asteroid belt. In the intervening time, it traveled alone (although always accompanied by the good wishes of space enthusiasts on its home planet and throughout the cosmos) for 1.06 billion kilometers (661 million miles).
The targeting of the encounter was well within acceptable limits. Before embarking on the mission, long before launch, engineers chose 500 kilometers (311 miles) as a convenient initial target altitude for planning purposes. With the extraordinary capability of the ion propulsion system, Dawn easily could accommodate significant deviations from the plan. To accomplish its mission, the probe needed to fly anywhere through a window shaped liked a croquet wicket, extending from more than 750 kilometers (466 miles) above the planet down to 300 kilometers (186 miles). (The lower limit was chosen for safety, maintaining a comfortable distance from the tenuous atmosphere and other threats the spacecraft was not designed to handle.) The width at the bottom of the wicket was almost 670 kilometers (415 miles).
In an effort to broaden our readership beyond only those sentient beings in the universe who share in the passion for the exploration of the solar system, the November and January logs included some material for anyone passionate about archery. We compared Dawn’s flight by Mars to shooting an arrow at a target 47 kilometers (29 miles) away. The objective was to hit a small region just outside the 30-centimeter (1-foot) red bull’s-eye. For the readers who have joined us because of that topic, we return to it once again here. (We do not plan to expand upon the croquet theme in future logs but hope our new readers attracted by references to that sport will remain with us anyway.)
Our original target altitude of 500 kilometers corresponded to about 17.2 centimeters (6 3/4 inches) from the center of the bull’s-eye. Using navigational data from January, we predicted Dawn would fly by at 543 kilometers (337 miles) above the surface, the arrow sinking into the target about 17.4 centimeters (less than 6 7/8 inches) from the center, just outside the red circle. Now we know that that prediction was in error by the equivalent of about 40 micrometers (less than 1/600 of an inch). The arrow was off from that expected location by about the thickness of a hair.
The location of the original target was at the 11:00 position, but the base of the wicket-shaped window extended from where the hour hand would be at 10:51:02 to 11:03:49. (The wicket is not as symmetrical as official croquet rules might require.) As a reminder, because we are concerned only about the hour hand, the tick marks adjacent to the 11:00 position correspond to 10:48 and 11:12, so this wicket is very narrow. In January, it appeared our arrow was headed for about 11:03:42. In fact, the actual trajectory took the spacecraft through the window at the 11:03:40 position.
The gravity assist was extremely accurate indeed. The archers hit their target and won the big prize: the continuation of the mission of exploration in the asteroid belt, seeking answers to questions about the dawn of the solar system. In addition to the gravity assist, which was essential to the success of the mission, the operations team had devised a plan to acquire some bonus data to aid in the calibration of the science instruments, as described last month.
In order to point its instruments at their calibration targets, the probe oriented itself for a short time in such a way that light reflected from Mars reached its “star tracker”. This unit (with a mystifying name, whose origin is lost in the dim mists of time) tracks stars in order to help the attitude control system establish the spacecraft’s orientation (or “attitude”) in the zero-gravity of spaceflight. The tracker’s camera images stars and its internal computer recognizes patterns, much as you might recognize some of the lovely constellations visible from your planet and use them to orient yourself at night. When Mars light entered the star tracker, the camera was dazzled, temporarily unable to see the stars. If you reside on a planet with a large moon, you may have experienced a similar phenomenon. It is much harder to see stars when the bright moon interferes.
Engineers had anticipated this behavior. Because they knew the star tracker likely would be unable to provide useful data to the attitude control system for a while, gyroscopes had been powered on well beforehand. Using these spinning masses, attitude control can sense turns and keep track of how the attitude changes even when the star tracker is not yielding accurate information.
As another preventive measure, commands stored on board temporarily precluded protective software, known as fault protection, from seeing any alerts indicating that the star tracker was not able to produce valid data. Because they expected the data to be invalid, engineers did not want fault protection to respond under the mistaken impression that the tracker was unhealthy.
As Dawn approached Mars, with instruments powered and beginning their calibrations, the light reached the star tracker, but it performed better than expected. After reaching its minimum distance, as the spacecraft rotated during its ascent, the star tracker’s line of sight moved closer still to Mars. Almost 2 minutes after the closest approach, the device finally was overwhelmed with light and reported that it could not track stars, declaring itself to be nonoperational and causing a software flag to be hoisted to alert interested parties. As planned, attitude control relied on gyros and fault protection remained blind to the alert.
About 11 minutes later, as the spacecraft’s attitude continued to change, the star tracker’s view moved far enough from Mars that the unit once again could discern stars. When it recognized patterns, it reported its data to attitude control, which readily used them. All was well, and the tracker had functioned better than anticipated, identifying stars with Mars closer to its line of sight than anticipated.
Another 7 minutes after that, as Dawn’s momentum continued to carry it away from Mars, the stored command to restore fault protection’s ability to see any star tracker problems was executed. The star tracker was indeed working well, but the compulsive reader will note that the chronology above does not include lowering the virtual flag that was raised when the tracker announced it had stopped tracking stars. A software bug, hardy enough to elude the operations team and survive the rigors of the deep-space environment, prevented the flag from coming back down when the tracker resumed normal operation. Although attitude control was making good use of the data, when fault protection saw the flag, it fulfilled its function; treating the tracker as if it were unhealthy, the protective software deactivated the unit.
Fault protection’s next step was to try to use the backup star tracker. Another bug, identified last year while the spacecraft was in flight, deprived the second tracker of the time necessary to complete its activation routine. (That bug has been fixed in a new version of the software scheduled to be transmitted to the spacecraft before ion thrusting resumes in June.) With no star tracker available, fault protection correctly followed the plan its designers had given it: it interrupted the calibrations, powered off the instruments, and put the spacecraft into “safe mode,” awaiting instructions from Earth on what to do next.
The operations team, following events on distant Earth (delayed by the more than 19 minutes it took radio signals to cross the separation), observed the signature of safe mode. There was no urgency in responding, however, because the instrument calibrations could not be restarted, and the crucial gravity assist was unaffected by the spacecraft’s activities. Mars would sling Dawn in the intended direction without regard for the probe’s actions.
The cause of the “safing” was quickly determined, and the team returned the spacecraft to its normal operational configuration within about 2 days. In addition, all the bonus calibration data that the instruments had transferred to the spacecraft’s main computer before they were deactivated were transmitted to Earth.
The gamma ray and neutron detector (GRaND) obtained excellent measurements of both gamma rays and neutrons from Mars. The instrument has been operated a number of times in flight to measure high-energy radiation that pervades space as it strikes the spacecraft, but, unlike the other science sensors onboard, it can detect bodies only when it is very close to them. Dawn's other instruments have observed distant planets and stars several times already, but that is not possible for GRaND, even with its suite of sophisticated detectors. Mars is the only occasion in the mission for it to observe a specific, well-characterized object. It was powered on in January in anticipation of this opportunity.
GRaND obtained a thorough set of data as Dawn traveled down to its lowest point, despite being over the night side of the planet part of that time, because it does not require illumination by the Sun to "see" its subject. It acquired additional valuable data as the spacecraft receded from Mars. This was GRaND's only measurement of a planetary body; and, apart from being quite brief, it was performed in much the same way it will be when Dawn orbits each of its protoplanetary destinations. It was especially good fortune that space radiation levels were relatively low during the encounter with Mars, making the radiation escaping from the atmosphere and surface particularly clear for GRaND. Scientists will compare GRaND's data with measurements of gamma rays and neutrons by the Mars Odyssey spacecraft (which has been studying the planet for more than 7 years), helping them prepare for interpreting the unique observations it will make of Vesta and Ceres to reveal many of the atomic constituents of those protoplanets.
The visible and infrared spectrometer’s data were not stored in the spacecraft’s main computer memory before the safing because the camera’s data had priority. One of the images the camera acquired is shown here.
With the benefit of an excellent gravity assist, the Dawn project is gratified to have Mars behind and Vesta now so clearly ahead. Having obtained even more than was needed from the red planet, Dawn is ever more eager to press on to its destinations in the asteroid belt.
Dawn is 4.1 million kilometers (2.5 million miles) from Mars. It is 336 million kilometers (209 million miles) from Earth, or 915 times as far as the moon and 2.26 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 37 minutes to make the round trip.
Dr. Marc D. Rayman
3:00 am PDT March 8, 2009
P.S. While the spacecraft is not concerned with changes between standard time and daylight saving time, its human colleagues are. Your correspondent, however, subscribes neither to the spacecraft’s indifference nor to the more traditional rigid adherence. He does follow the convention of advancing clocks, as we do today, but rather than setting his clocks back with everyone else late in the year, he saves that extra hour. Every 24 years, that should give him one extra day.› Learn more about the Dawn mission
Dear Charles Dawnwins,
Dawn continues to close in on Mars, ready for the gravitational slingshot that will help it on its expedition to the asteroid belt and its quest to gain insights into the evolution of the solar system. The ship remains on course, with the latest calculations of its trajectory being very close to those described in the last log. The spacecraft will streak about 549 kilometers (341 miles) above the planet at 4:28 p.m. PST on February 17.
While Dawn and Mars move in their separate orbits around the Sun, they are approaching each other at the stately pace of 2.56 kilometers/second (5720 miles/hour). Gradually, the gravitational pull of the planet will grow as the distance shrinks, and the spacecraft’s path will start to change, beginning the boost we seek. By about noon PST on February 14, the tug from Mars will have grown to be the same as the famously gentle thrust from Dawn’s ion propulsion system. When Dawn is closest to Mars, speeding past it at 5.31 kilometers/second (11,900 miles/hour), the red planet will be exerting 34,000 times greater force than the blue-green xenon beam generates.
Just as a swing speeds up as it approaches the bottom of its arc and slows down as it rises again, Dawn accelerates as it gains on the planet and decelerates as it climbs away. Unlike a swing, though (at least the kind your correspondent was allowed to play on as a youngster on Earth), Dawn will not retrace its path; it will not descend again after ascending from the vicinity of Mars. It is in its own orbit around the Sun and will move too swiftly by Mars for the planet to capture it into orbit.
To illustrate Dawn’s arc, let’s use the somewhat arbitrary speed of 4 kilometers/second (9000 miles/hour) as a reference. Over the coming days, Mars will attract Dawn, and the craft will have accelerated to that speed about 34 minutes before its closest point, still 5500 kilometers (3400 miles) away from the planet. After dipping still lower and falling still faster while it approaches Mars, the spacecraft will slow down as it departs. It will have decelerated to the same speed of 4 kilometers/second at 34 minutes after its closest encounter. Within a few days, the speed will have dropped to today’s value of 2.56 kilometers/second again. The arc of Dawn’s acceleration toward Mars matches the arc of its deceleration, displaying the same kind of symmetrical motion as a swing.
Where then is the effect of the gravity assist? Because Dawn is in orbit around the Sun, it is the distortion of that solar orbit caused by Mars that provides the advantage to the mission. In the previous log, we saw how the benefits of the gravitational interaction with Mars could be described as changes in the spacecraft speed. The 1.1 kilometers/second (2500 miles/hour) that represented the change in the shape of the orbit means that Dawn’s incoming speed and outgoing speed relative to the Sun are not the same, even though, as we saw above, they are the same relative to Mars. Before it reaches Mars, the probe will be traveling around the Sun at less than 25.5 kilometers/second (57,000 miles/hour). Thanks to the boost from Mars, the speed after the encounter will be more than 26.6 kilometers/second (59,500 miles/hour). The difference is precisely the effect described in the previous log.
The reshaping of Dawn’s orbit is different from the reorienting of the plane of the orbit, also explained in the previous log. The plane change from the gravity assist, of vital importance to the mission, does not manifest itself as a difference in the speed of the spacecraft around the Sun; it is a difference in the direction of motion. Nevertheless, the effect can be described as being equivalent to a change in speed (actually, in velocity), as it was in the last log. (For interested readers for whom these points are not already evident, please click here to go to the log that clarifies it. [Editor, I have not written such a log yet, although I will write an explanation in the future. In the meantime, please insert an acausal hyperlink for readers who do not want to wait.])
As Dawn plunges toward Mars, it will be coming in over the northern hemisphere, reaching above 60° latitude. As Mars bends the orbit, changing the direction the craft travels around the Sun, Dawn will depart over the southern hemisphere. Leaving the planet behind on its new path around the Sun, it will be above 60° south latitude.
The focus of the Dawn team’s work for the Mars encounter has been to achieve a trajectory that would deliver the probe to the target above Mars at the right time and traveling at the correct angle to accomplish the needed gravitational boost. As long as the spacecraft will be in the vicinity of such a familiar solar system site, albeit briefly, the team decided to take advantage of the occasion to calibrate the instruments that are designed to elucidate the nature of Vesta and Ceres. As mentioned at the end of the previous log, there is little Dawn can learn about Mars that is new. Indeed, it is flying higher and faster than spacecraft there right now. In addition, its instruments are intended for the initial examination of previously unexplored worlds, not the detailed investigation of a planet that already has been scrutinized by spacecraft for decades. That very scrutiny, however, means that there is a valuable database for use in comparing observations by Dawn’s instruments, helping to prepare them for their assigned tasks in the asteroid belt. Dawn’s observations at Mars, like the other, more distant measurements it has made of stars and planets since launch, help prepare for the real rewards of the mission. (To recapture the thrill of some of the previous in-flight observations, visit the logs of October 24, 2007, December 17, 2007, February 29 of any year, or April 22, 2008.)
Each instrument will acquire data to be used in comparisons with observations made by similar instruments on spacecraft dedicated to the study of Mars. Rather than using Dawn to learn about Mars, our focus is on using Mars to learn about the performance of our instruments. This will be helpful in doing science at Vesta and Ceres and in navigating there. As we will see in subsequent logs, the positions of Vesta and Ceres are not known accurately enough that Dawn could rendezvous with them using conventional radio navigation techniques alone. To improve the navigation, the craft will take images of the bodies as it is closing in on them, and analysis of those images by the Dawn navigation team will help pin down the location of the target. Tests at Mars will contribute to characterizing the camera not only for science, but for this separate function as well.
If Dawn’s encounter had represented a unique opportunity to conduct vital new science, the plans would have been different. As just one example, Dawn will not attempt to acquire the highest resolution visible images that it might be capable of achieving. Instead, by smearing the view of Mars across the camera’s line of sight, engineers will provide a relatively uniform illumination for the camera’s detectors, providing interesting engineering data that cannot be obtained when observing the pinpoint light of stars.
In preparation for the encounter, on January 20 controllers activated the gamma-ray and neutron detector (GRaND), which, despite its name, is a very modest (but wonderfully capable) member of the on-board instrumentation. The last time GRaND was operated was in April. It remains healthy and is continuing to work well, sensing the effects of cosmic radiation impinging on the spacecraft.
Dawn is approaching Mars from outside the planet’s orbit around the Sun. Therefore, from the spacecraft’s point of view, Mars and the Sun are close together, and Mars would appear to be a thin crescent. This geometry precludes directing its instruments toward the planet well before it arrives. Designed to operate in the more distant asteroid belt, the instruments would not be able to tolerate the heating from the Sun. Therefore, although GRaND is able to detect space radiation, there would be nothing for the other instruments to do while awaiting their calibrations at Mars.
These instruments will be activated on February 17 using instructions already stored onboard. Dawn will be pointing its main antenna to Earth, so the mission control team can observe telemetry, but there is no plan to send additional instructions then. The visible and infrared mapping spectrometer will be powered on at about 9:47 a.m. PST, and the primary science camera will be commanded on at about 1:21 p.m. PST. (The backup science camera has been used for other tests in flight, but it will not participate in the activities at Mars.) All the times presented here are as measured on the spacecraft. Mission controllers will have to wait more than 19 minutes, as radio signals traverse the great distance to Earth, to observe the associated telemetry. In essence, their entire view of events will be delayed by this “one-way light time.” (The Dawn project remains ready to advise the FCC on the use of such a system to provide a guaranteed delay in live broadcasting.)
At 1:57 p.m. PST, the spacecraft will begin turning to prepare for its calibration activities. The maneuver will move Earth out of the radio beam from the main antenna, so the spacecraft will switch to 1 of its 3 auxiliary antennas. Each of these antennas can emit a much broader beam, allowing communications over a wider range of orientations. The cost of spreading the signal over a much greater area is that when it is received at Earth, it is significantly weaker, so only a very limited amount of telemetry can be sent. For the subsequent day and a half, controllers will use this reduced flow of information to monitor Dawn’s work. Meanwhile, the instruments will attempt to record neutrons, gamma-rays, and ultraviolet, visible, and infrared light, all from Mars, all providing a bonus to the mission. (Dawn has previously conducted infrared observations of Mars. At that time the spacecraft was about 100,000 times farther from the planet than it will be when it swoops by next week.)
At 3:09 a.m. PST on February 19, Dawn will begin turning back to sight its main antenna on Earth once again. For more than a day, it will radio engineering data it stored during the time it could not transmit at high speed. At first, a Deep Space Network (DSN) antenna near Madrid, Spain, will receive the signals. As Earth rotates, the 34-meter (112-foot) dish will no longer be able to point to Dawn (as viewed from Madrid, Dawn will set in the west, just as the Sun, the moon, stars, and other celestial objects do), so a DSN antenna near Goldstone, California, will take over. After about 8 hours, the Goldstone facility will hand the responsibility over to a DSN antenna still farther west, near Canberra, Australia, which eventually will pass the baton once again to Madrid.
At 7:48 a.m. on February 20, the spacecraft will rotate again to bring its camera to bear on Mars. By then, the view of distant Mars will be similar to what the spacecraft will have as it navigates to Vesta, once again providing an opportunity to prepare for the visit to that mysterious world. At 11:28 a.m., it will turn away from Mars for the last time and resume transmitting data back to Earth 45 minutes later. The precious capabilities of the DSN are shared among all interplanetary spacecraft, so Dawn will return to more intermittent tracking on Friday. The return of data will be completed the following week.
All data that Dawn collects at Mars are considered a bonus of the mission. Indeed, when the mission was conceived, its launch was to be in 2006, and the mission to Vesta and Ceres then did not require the assistance of Mars. It was only in 2005, when the launch was rescheduled to 2007, that trajectory designers added Mars to the itinerary. The gravitational deflection is essential to the success of the mission, but the activities during the visit to Mars are secondary to the mission’s principal objectives.
Even before Dawn has paid a visit to Mars, engineers are already busy planning the details of the spacecraft’s next assignments. Following its brief divertissement at the planet, it will continue to coast in its orbit around the Sun until June. With Mars helping to reshape its orbit, there is no need for ion thrusting for a while. In the next few logs, we will follow along as Dawn engages in other work to be completed before the resumption of its most familiar function of reaching for the asteroid belt atop a blue-green pillar of xenon ions.
Dawn is 1.1 million kilometers (680 thousand miles) from Mars. It is 348 million kilometers (216 million miles) from Earth, or 910 times as far as the moon and 2.36 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 pm PST February 12, 2009
Dawn continues on course for its pas de deux with Mars on February 17. The planet's gravity will gracefully assist the spacecraft on its way to rendezvous with its intended celestial partners Vesta and Ceres in the more distant asteroid belt. Even the extraordinary capability of its ion propulsion system would not be sufficient for Dawn to complete its celestial dance without the help of Mars.
In the last log we saw that the mission operations team was preparing to adjust the probe's flight path to keep it on target for next month's flyby. Just getting to the vicinity of Mars is not sufficient, as the passage by the planet is only one short segment of a very long itinerary. Indeed, choreographing Dawn's trajectory is a complex matter of finding the most efficient route through the solar system to travel from the moving platform on which it started (Earth) to encounter Mars in just the right way to reach Vesta at the proper time to complete its work there before it has to begin the trek to Ceres to meet it on schedule, aided during most of the journey by the ion propulsion system. Dawn must arrive at Mars on time, traveling in the correct direction and at the necessary location, for the gravitational slingshot to yield the desired effect. Flying the spacecraft through that "window" at Mars is like threading a celestial needle.
Early this month, engineers determined that the craft's interplanetary route was so close to the one planned originally that it was unnecessary to refine it. In part, this is a reflection of how accurate the first (and now only) trajectory correction maneuver (TCM1) -- where do normally undemonstrative engineers come up with these incredibly cool names?) was when it was executed on November 20. In addition however, Dawn does not need to be aimed as accurately as a typical spacecraft might for such a gravity assist. The ion propulsion system will be used so extensively between Mars and Vesta to continue modifying the probe's orbit around the Sun that it has enough leverage to compensate for a significant range of deviations from the mathematically optimal bending of its course by Mars's gravity.
To understand the targeting at the red planet, at the request of readers on planets of all colors throughout the constellations Sagitta and Sagittarius, we shall extend the analogy of the archery problem described in an earlier log. We can follow both the spacecraft and the arrow as each travels to its goal. Of course, while the arrow's path ends when it produces a satisfying thunk at the target, Dawn's target is a region of space near Mars that is much more penetrable. There are other important differences between the two problems, not the least of which is that Dawn has thrust with its ion drive most of the way to Mars. (For the complete list of caveats that apply to the analogy, contact the Dawn legal department.)
When Dawn was fired into space, aiming for the window near Mars was analogous to shooting an arrow at a target 47 kilometers (29 miles) away. In the center of the target is a red circle 30 centimeters (almost 1 foot) in diameter, representing Mars. Of course, we don't want our arrow to hit the red bull's-eye! Rather, our goal is a spot about 2.2 centimeters (7/8 of an inch) outside the circle, near the 11:00 position.
By the time of TCM1 2 months ago, Dawn had traveled 880 million kilometers (550 million miles), corresponding to the arrow having sailed 39 kilometers (24 miles) from our bow. After flying that tremendous distance, our projectile was headed for the bull's-eye itself, so we applied a tiny adjustment to put it on course for the real aim point.
By January 15, when the mission operations team had scheduled TCM2, Dawn had put more than another 110 million kilometers (70 million miles) behind it, so our arrow would have streaked another 5 kilometers (3 miles) closer to its target. Even with so far still to go, the aim is so good that no further correction is needed.
Although controllers will continue to monitor Dawn's trajectory and refine the predictions of its course, as long as the arrow flies true, it will reach its target. With the latest estimate, instead of hitting the mathematically optimal spot, it will strike less than 2 millimeters (a little over 1/16 of an inch) farther from the bull's-eye. It won't quite land at the 11:00 position, but it will be less than a third of the way to the tick mark just after the 11:00 marker, where the hour hand would be pointing at about 11:04. This is comfortably within Dawn's allowance for accuracy.
These coordinates correspond to the spacecraft passing about 543 kilometers (337 miles) above the reddish surface of the planet rather than the original plan of 500 kilometers (311 miles). While that may seem like a large difference, the effect of a gravity assist largely depends on the distance from the center of a planet, not the surface. In that context, instead of passing about 3896 kilometers (2421 miles) from the center, Dawn will pass 3939 kilometers (2448 miles) from that reference location. (The arrow strikes 17.4 centimeters, or less than 6 7/8 inches, from the middle of the bull's-eye rather than 17.2 centimeters, or 6 ¾ inches.) The mission easily could accommodate much larger deviations from the original plan.
And what does the difference mean for the overall journey to the asteroid belt? Dawn's 4-month period of coasting between Mars and the resumption of ion thrusting will be shortened by about 4 days. As the spacecraft will thrust for most of the 2.5 years from Mars to Vesta, powering up the ion drive a few days earlier is virtually inconsequential.
Over the coming weeks, navigators will continue to gather data on Dawn's route through space to refine their computations, and some of the numbers above likely will change a little. It might be tempting to think that if we had truly perfect knowledge of the spacecraft's position and velocity at some time then we could predict the encounter details correspondingly accurately. After all, the gravitational forces from the Sun and Mars and even other planets and asteroids, far though they are from Dawn, are very well known indeed. It turns out, however, that there are other forces acting on the spacecraft as well. They are like a soft but variable breeze blowing on the arrow we have aimed at the distant target, making the already challenging archery problem still harder.
Light, as insubstantial as it feels to our corporeal readers, has momentum, so as the Sun shines on Dawn, it constantly modifies the trajectory. (The force from the light is far greater than from the solar wind, the stream of charged particles that our star blows into space. The effects of the solar wind are too tiny to matter.) The constant nudge from the Sun's light is small, but it needs to be incorporated into the targeting, and that is difficult because the size of the effect depends on the reflective properties of the spacecraft. Light bouncing off a polished piece of metal provides a different force from light absorbed by a dull black thermal blanket. As the spacecraft changes its orientation, different components are exposed to the light from the Sun, and the angles at which they are illuminated change. Navigators are able to account somewhat for the solar pressure, but the small uncertainty in it contributes to the uncertainty in the encounter details.
Dawn itself is responsible for an even larger variability in its own trajectory. As we have seen before, the spacecraft is equipped with reaction wheels, which are part of the attitude control system. (To achieve a certain mystique about their work, engineers use the term "attitude" to describe the orientation of the probe in the weightlessness of spaceflight; the system also happens to have a very sanguine attitude about its work.) By electrically spinning the wheels faster or slower, the spacecraft can control its attitude (and sometimes even its outlook). The reaction wheels are used to start and stop turns as well as to hold the craft's attitude steady, right now, for example, keeping the main antenna pointed to Earth. The solar pressure not only alters the spacecraft's trajectory but it can rotate the spacecraft as well, just as wind can both propel and turn what it blows against. Attitude control automatically changes the wheels' speeds to counteract this effect. The consequence, however, is that the units gradually speed up as they hold their own against the incessant solar pressure.
The wheels cannot increase their speeds indefinitely, and they lose effectiveness when they reach their limit. To allow them to slow down, attitude control calls upon its colleague the reaction control system to fire small thrusters, which use the rocket propellant hydrazine. The force essentially reverses the influence of the solar pressure that has caused the wheels to spin up, thus allowing them to spin down without the spacecraft ever having to change its attitude. Indulging their usually secretive poetical impulses, engineers refer to this process as "desaturating the reaction wheels."
In addition to slowing the wheels, the thrust from the hydrazine pushes the spacecraft, modifying its trajectory. During the current phase of the mission, the thrusters are operated twice a week to desaturate the reaction wheels, so engineers need to quantify the ramifications of firing the reaction control thrusters prior to the encounter with Mars. Here again, while the phenomena are understood, there remains some level of uncertainty in how accurately they can be predicted, given imperfect knowledge of the actual effect of the solar pressure and details of how the spacecraft will operate its 6 thrusters to reduce the speeds of the 3 operating wheels.
The uncertainty in the change in the spacecraft's speed from the desaturation of the reaction wheels is around 1 millimeter/second (12 feet/hour). Except to our readers in the planetary nebula NGC 6543 (sometimes known as the Snail Nebula), this speed may seem very slow, but consider a simplified example of how this affects the calculation of exactly where Dawn will be as it sails past Mars. Suppose the planetary encounter is 5 weeks away, and all present and future trajectory parameters are known perfectly except the result of the desaturation scheduled for 4 weeks before the encounter. (This example disregards all the other desaturations yet to occur, treating them as if they would be known exactly.) The best prediction of the effect on the spacecraft's trajectory of that desaturation still a week away is included in the analysis to determine where Dawn will be when it reaches Mars, but that prediction is off by just 1 millimeter/second. That speed, when maintained for the more than 2.4 million seconds in the 4 weeks between the desaturation and the flyby of Mars, leads to a distance of more than 2.4 kilometers (1.5 miles). So, if the actual effect of the desaturation is 1 millimeter/second different from what is included in the computations, Dawn's actual position will wind up 2.4 kilometers away from the calculated position. Of course, all other parameters are not known perfectly, and there are more desaturations before Mars, so the actual encounter conditions can change by much more than in this illustration. Nevertheless, there has been great effort to establish how unpredictable all the factors are, and engineers are confident the trajectory is well within the required window to achieve the needed assistance from Mars.
The powerful tug exerted by the planet will bend Dawn's path by about 78 degrees. To picture that angle, suppose Mars is a dot at the center of a clock, and Dawn flies toward it (or, more accurately, toward the required window very nearby) from the 12. If the planet had no gravity, the spacecraft would continue in a straight path, exiting the face of the clock by the 6. Instead, the probe takes a sharp turn at the center of the clock and heads out between the 3 and the 4. (This is not the same clock used in the discussion of solar conjunction in the previous log. Be sure to check out the full selection of celestial timepieces in your planet's Dawn gift shop.)
The deflection from Mars changes Dawn's orbit around the Sun (as does thrusting with the ion propulsion system). To enter orbit around Vesta, Dawn needs to match its orbit around the Sun to the one that Vesta is in, and the Mars encounter is designed to help accomplish that, bringing Dawn's plane into closer alignment with Vesta's. (We saw in the previous log that part of the journey requires changing the plane of Dawn's solar orbit.) The gravity of Mars will alter Dawn's orbital plane by about 5.2 degrees, a seemingly modest angle. Yet, if it were up to the spacecraft to accomplish such a change on its own, it would require a velocity change of more than 2.3 kilometers/second (5200 miles/hour).
Thanks to the design of the Mars encounter, there is another benefit as well. In addition to tilting Dawn's orbit around the Sun, Mars changes its shape, enlarging the elliptical orbit and sending the probe farther from the Sun.
In considering the size of solar system orbits, it often is convenient to measure lengths with the "astronomical unit," which is simply the average distance between the Sun and Earth. So 1 astronomical unit (AU) is 150 million kilometers (93 million miles), conveniently helping your correspondent locate not only how far he lives from the Sun, but also how far he works from it.
If Mars had no gravity and thus could not divert Dawn on its travels, the craft's current elliptical orbit would take it as close as about 1.23 AU to the Sun and as far as 1.69 AU away. (For comparison, Mars orbits between about 1.38 AU and 1.67 AU.) As we saw in a log last summer, Dawn is temporarily heading in toward the Sun now. Without the effect of Mars, and with no additional ion thrusting, our interplanetary robot would reach that minimum distance in June 2009, and by early May 2010 it would have swung out to the greatest distance, only to begin falling back again as it followed its elliptical loop.
After the boost from Mars, if Dawn undertook no further thrusting, it would come no closer than 1.37 AU to the Sun, reaching that distance in April 2009. Then it would head out to 1.84 AU, arriving there in April 2010. In this new orbit, Dawn still will be a long way from Vesta (which never dips even quite to 2.1 AU from the Sun) and still farther from Ceres (which travels out to 3.0 AU), yet it will be much closer than it would have been without Mars's help.
Reshaping Dawn's orbit, quite a separate effect from reorienting it, would require more than 1.1 kilometers/second (2500 miles/hour). The combination of these two benefits is equivalent to the planet imparting about 2.6 kilometers/second (5800 miles/hour) to the spacecraft.
Dawn's unique propulsion system allows it to change its own speed by well more than this during its mission. Yet the famously gentle ion thrust means it would take quite a while to achieve these changes, and the mission itinerary, fit between the September 2007 launch and the February 2015 arrival at Ceres, does not afford enough uncommitted time. There are other technical reasons as well that making these changes only with its built-in capabilities would be impractical. The encounter with Mars is a free way to get significant help.
Is it really free? Well, in these difficult economic times, there is a cost we are obligated to divulge in the interest of full disclosure. The changes to Dawn's orbit come at the expense of Mars's orbit. Just as when you throw a ball forward, you feel a "reaction" force backward, in pushing the spacecraft one way, Mars reacts by moving the other. Mars exerts a force on Dawn, but Dawn exerts an opposite force on Mars. As the planet's mass is nearly 600 million million million times that of the spacecraft, the effect on our probe is far larger than on the fourth planet from the Sun. The cost of helping Dawn is that Mars will slow in its orbit enough that after 1 year, its position will be off by about the width of an atom. Adding up the growing deficit, it would take 180 million years for Mars to be out of position by 2.5 centimeters (1.0 inches). That is the cost, and, on behalf of Dawn and all who share in the eager anticipation of the mysteries it will reveal in the asteroid belt, we express our gratitude to Mars for its upcoming sacrifice!
The control team's best estimate now is that Dawn's closest encounter with Mars will occur at about 4:28 pm PST on February 17. In the next log, we will cover some of the spacecraft's activities during its short visit, but we conclude this one with a note about the plans. Many spacecraft have visited this intriguing planet already, and it has been studied extensively from orbit and from the surface for years and years. During its brief passage, Dawn cannot learn much that is new. On the other hand, Mars is so well characterized and familiar that it provides a useful reference for calibrating Dawn's scientific instruments. As we will see, the calibration plan precludes obtaining the highest resolution images that might otherwise be attempted. Such a strategy may seem surprising to all those who appreciate spectacular pictures of Mars, but, of course, there are already many many such images, and future missions will produce even more that will continue to captivate and inspire us.
The instrument calibrations will be a nice bonus, but if Dawn ended up flying by Mars and conducting no calibrations whatsoever, as long as the trajectory targeting were correct, everyone on the project would consider it a success, because the gravity assist is all that is required for the mission to forge ahead to its goal of unlocking exciting secrets deep in the asteroid belt. Dawn's focus is on preparing for the unknowns of Vesta and Ceres rather than the knowns of Mars.
Dawn is 4.6 million kilometers (2.9 million miles) from Mars. It is 358 million kilometers (222 million miles) from Earth, or 900 times as far as the moon and 2.41 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 pm PST January 27, 2009