As NASA's Dawn spacecraft orbits and explores its second target, dwarf planet Ceres, to provide scientists with a window into the dawn of the solar system, mission director and chief engineer Marc Rayman shares a monthly update on the mission's progress. Learn more about the Dawn mission on the JPL Missions database.
Now in interplanetary cruise, the Dawn spacecraft is following a much more leisurely pace than the one it maintained during the initial checkout phase of the mission. While its daily schedule is not demanding, as it follows (and changes) its orbit around the Sun, it is separating from Earth at nearly 4 light seconds per day (more than 1.1 million kilometers, or 720 thousand miles, per day). Every 8 hours, the probe recedes from Earth by a distance equal to that between Earth and the moon.
The spacecraft has accumulated more than 1000 hours of thrusting with its ion propulsion system. Although far far longer than the overwhelming majority of spacecraft have operated their propulsion systems, this represents only a small fraction of the total thrusting required to complete its solar system journey. [Note to editors: This milestone may be of significance only to human readers. When translated for those who use different numbering systems or different time systems, it may not yield an interesting result. (For that matter, 1000 hours is not a special number when expressed in seconds, days, or millennia.)]
Most of Dawn's time is devoted to thrusting with its ion propulsion system, but each week the spacecraft stops for a communications session with controllers on distant Earth, during which it returns detailed data on the performance of its subsystems throughout the previous week. Reports of voltages and currents, temperatures and pressures, and myriad other parameters allow engineers to determine how well the ship has been doing and how to keep it sailing as smoothly as possible.
On January 14 shortly before 10:00 pm PST, a high energy subatomic particle, a "cosmic ray," traveled through one of the main panels of the spacecraft and then penetrated one of the electronics units. The energy it carried had been imparted to it through an unidentified cosmic process, and after the particle had traveled across vast distances, that energy was transferred to a small integrated circuit. Such an event is not all that uncommon on spacecraft, and Dawn is designed so that most space radiation does not interfere with its operation. The deposition of energy in this particular component however triggered the electronics to inform the software of a problem. To rectify the situation, other software correctly responded by resetting the computer in that unit.
In the last log, we saw that two master computers work together to oversee and control activities on the spacecraft. The computer that was reset in this case was neither of those; it was one of many auxiliary computers with more limited responsibilities. In addition to resetting the computer, software running in the main computer correctly reconfigured systems onboard to "safe mode." The spacecraft then awaited instructions from engineers on Earth (or, more accurately, in Dawn mission control on the top floor of JPL's whimsically named building 264).
A few hours later, when it was time for the weekly communications session, the Deep Space Network and mission controllers promptly recognized that the spacecraft was in safe mode. As with the safing in November, a small team gathered during the night to begin the diagnosis, and more team members joined after dawn. It did not take long to reach a conclusive explanation based on the error code stored by the software and other data downloaded from the spacecraft. The culprit was a cosmic ray.
By the time the detailed analysis of the safing was concluding, mission controllers were already commanding the spacecraft step-by-step out of safe mode, returning it to its normal flight configuration. Within a few days, Dawn was ready to resume work, and before the end of the week was thrusting with its ion propulsion system again. (The effect of having missed some thrusting that week is not significant for the mission.)
While Dawn will thrust during most of its interplanetary cruise, the flight plan includes some periods of coasting in addition to the normal weekly communications session. One such period was January 22 - 25. Most of this time was devoted to updating software in the main computer.
The main software resides in 4 locations (well, 4 locations on the spacecraft): primary and backup copies in the primary main computer and primary and backup copies in the backup main computer. Three copies of flight software 7.0 were installed on the spacecraft in November. As reported in the last log, the primary main computer was scheduled to receive its backup copy during this break in thrusting.
While it had been planned before launch to transmit 7.0 to the spacecraft in November, after the software was finalized, the need for one additional change was recognized. For technical reasons it was not necessary to change all the stored copies; rather, it was sufficient to modify (or "patch") only the version that was running. Making that change on December 12 advanced the software to 7.0.1.
As Dawn moves farther from its former home, a planet it is not scheduled ever to visit again, its capability to communicate with Earth diminishes. The large main antenna is strong enough that no changes are needed yet, but the smaller, auxiliary antennas do not permit the same level of performance that they did earlier in the mission. As explained last month and before launch, when Dawn enters safe mode, it uses 1 of these auxiliary antennas. Now that the distance to Earth has increased so much, the weaker signals require lower transmission and reception rates. Engineers decided several years ago to start the mission with safe mode programmed to use data rates that were too high to work throughout the mission but would allow simplified operations during the first months of the flight. Those months have now passed, so following that plan established before launch, the software was patched to use lower data rates whenever safe mode is called. The new values were put into in the primary and backup locations in the primary and backup main computers on January 23 and 24.
With these changes, Dawn is now using flight software version 7.0.2. If you want to update the software running on your spacecraft to this latest version, with the hottest new features, we have made special arrangements to provide a copy you can download for free. (Note: this special offer is not available to customers within 1 decaparsec of Dawn.)
Replacing or updating software on an operating spacecraft is complex, and there are many opportunities for problems to arise. To allow for time to understand and resolve any unexpected difficulties, more time was allocated than was expected to be needed. As it turned out, after the team's exhaustive preparations, the software uploads went perfectly, and there was plenty of time to spare. When the work was complete, the spacecraft simply waited until the scheduled time for the resumption of thrusting on January 25, and then it returned to powered flight.
Shortly after Dawn launched, we recalled the 50th anniversary of the launch of humankind's first satellite. As our spacecraft continues in journey through space, today we remember the 50th anniversary of another launch. The United States' first craft to reach Earth orbit, Explorer 1, departed from Cape Canaveral's Space Launch Complex 26A, almost exactly 500 meters (about 0.3 miles) southwest of complex 17B, from which Dawn began its climb to space. (We don't include a link here to any previous remarks about Dawn's launch site, as it was mentioned in more than 63% of the logs posted from April through September 2007. With so many past references, readers who flow forward in time are counted upon to remember or not to care about the launch location.)
Explorer 1's elliptical orbit around Earth carried it about 2550 kilometers (1585 miles) above the surface. On this 50th anniversary of the beginning of that mission, so important in the history of space exploration and space science (as well as the Cold War), Dawn is more than 26 thousand times farther from Earth, in its own orbit around the Sun. Despite the great difference in distance (as well as other comparisons in size, mass, capabilities, technological sophistication, scientific ambition, and more, all of which we spare readers from trying to absorb), the two projects have much in common. Both are part of humanity's efforts to help broaden our perspectives beyond our home planet, to apply both engineering and scientific knowledge as we seek to gain even more such knowledge, to undertake adventures that include great challenges but offer great rewards in which everyone can share. And although it may not feel this way now, with the first mission 50 years in the past and the second just starting, for most of humanity's future, both will be among the first tentative probes into the cosmic unknown.
Dawn is 67 million kilometers (42 million miles) from Earth or 175 times as far as the moon. Radio signals, traveling at the universal limit of the speed of light, take more than 7.5 minutes to make the round trip.
Dr. Marc D. Rayman
7:49 pm PST January 31, 2008
Dawn is climbing away from the Sun on a blue-green pillar of xenon ions as it begins a new chapter in its mission. After the remarkably successful initial checkout phase, the project is now in the interplanetary cruise phase.
When last we visited Dawn, it had superbly demonstrated that it was ready to fly the way it will for most of the mission. Although there will be many special activities during its journey to Vesta and from there to Ceres (and be sure to visit this site again to be among the best informed on your planet of what Dawn is doing), the probe will spend most of its time doing what it is doing today: patiently reshaping its orbit around the Sun with its amazingly efficient ion propulsion system.
Without taking any more time than was needed for some high fives (and a few high sixes for the more mathematically avant-garde) following the flawless execution of the test of long-term thrusting, the team turned its attention to more checkout activities. Ion thruster #2 was the focus of tests on November 13 - 15, and the excellent results matched those of thrusters #1 and #3. (The identities and locations of the thrusters were revealed in a previous log before such information could be publicized by more sensational, vulgar sources.)
November 19 was the first in-flight use of the spacecraft’s main antenna. (In fact, that antenna was used in some of the thruster #2 tests, in which the spacecraft was oriented so the antenna cast a shadow on the thruster. This permitted operation at the highest throttle level without overheating while still relatively close to the Sun. We recommend even to official Dawn scorekeepers that that not count as the antenna’s first use.) Because of the celestial geometry earlier in the mission, pointing the main antenna to Earth would have exposed sensitive components elsewhere on the craft to too much solar heating. All communications prior to the use of the main antenna relied on 1 of the 3 smaller antennas that do not emit as tight a radio beam. As the distance to the spacecraft increases, the smaller antennas will allow only very slow communications, while the main antenna, with a diameter of 1.52 meters (5 feet), will permit the return of many pictures and other scientific data even from distant Vesta and Ceres.
Much of the rest of the initial checkout phase was dedicated to updating software on the spacecraft. Many units onboard, both in engineering subsystems and in science instruments, incorporate their own computers, but the command and data handling subsystem contains the master computers. One main computer runs operations aboard the ship, and another computer supports it (and has a few other responsibilities of its own). Each of these computers has an identical backup, able to take over should the primary unit experience problems. With a planned 8-year mission in the forbidding environment of deep space, Dawn may face challenges that require more than such backup hardware. So each main and each support computer holds both a primary and a backup copy of the software. If radiation or some other problem corrupts one version of the software, the system can detect that and resort to another.
Engineers recognized well before launch that new software would need to be loaded during the initial checkout phase. As extensive as ground testing was, the team anticipated that the need for some updates would be identified once Dawn was operating in space. In addition, during the last few months before launch, when ongoing testing ferreted out bugs, only those changes that were essential for the beginning of the mission were made. Modifying complicated software is -- well, complicated; and seemingly simple changes can have unintended consequences. To allow thorough testing of the spacecraft’s capability to complete the complex and critical steps after separating from its Delta rocket, as described on September 21, late prelaunch changes to the software were kept to a minimum, and an improved version was planned for November.
Following the popular trend of giving software a snazzy name, the project denoted the latest suite “flight software 7.0.” We fully expect this to inspire new toys, movies, fashions, and even lifestyles (particularly among readers in the Pleiades), and Dawn’s marketing department is standing by to work with you.
On November 20, the new software for the support computers was radioed from Earth and installed separately on the primary and backup units in the command and data handling system. The software for each computer requires 67 files occupying about 135 kilobytes with a total of 22,800 lines written in the programming language C and in assembly code. The backup copies of the 7.0 software were transmitted to both computers on December 6. Each of these activities required great care, verifying that the computer memory remained healthy throughout, that no bits were lost or altered in transmission or storage, and that if an unrelated problem arose on the spacecraft during the process, the computers would be able to handle it, never being left in a vulnerable state. Each step was tested and verified repeatedly with the indispensable Dawn spacecraft simulator at JPL (down the hall from mission control and around the corner from the very popular Dawn ice cream freezer and the less popular Dawn fruit bowl).
Loading software into the main computer was still more complex than doing so in a support computer. To start running the new software, the computer would have to be rebooted. While that is a familiar and straightforward procedure for most terrestrial readers in the early 21st century, it is considerably more complicated when the computer is in control of a spacecraft in flight.
On November 26 and 27, the main computer’s memory was checked, its health was verified, and the updated software was sent from mission control to the Deep Space Network stations in California, Spain, and Australia (the work took long enough that all 3 communications complexes were required), and then 17 million kilometers (11 million miles) to the main computer. The 2.6 megabytes of flight software 7.0 for that computer consists of 591 files totaling more than 410,000 lines of C and assembly code. After loading all the software, operators began preparing the spacecraft for rebooting, scheduled for November 28.
Whenever the main computer reboots, it commands the spacecraft into “safe mode.” Other conditions can trigger this mode as well, including separation from the rocket on September 27. The probe does not know whether safe mode was planned, as it would be with the installation of new software, or was called in the process of dealing with a problem when ground controllers were not available to intervene.
Most of the characteristics of safe mode remain as they were on launch day, but there are some differences now. Dawn no longer has to wait for xenon to stop spinning, and it does not have to deploy its solar arrays. It still points one face to the Sun, the only easily identifiable spatial reference throughout its flight in the solar system. For the first few weeks of the mission, the relative locations of the Sun, Earth, and spacecraft required Dawn to sweep its safe mode radio signal like a searchlight that would periodically illuminate Earth, as explained before launch. For the rest of the mission, from Dawn’s vantage point well outside Earth’s orbit around the Sun, the planet and star always will appear close enough together that the spacecraft can use the broad beam of an antenna that points at the Sun in safe mode, instead of an antenna at right angles to it. While extremely weak (as we will see in a log early next year), the faint radio whisper that would reach Earth would still be loud enough to be heard. In October, controllers modified safe mode so it would employ this other antenna.
After the reboot preparations were completed on November 27, most team members went home to get some rest for the activities scheduled for the subsequent 2 days. After they commanded the reboot and Dawn established itself in safe mode, the Deep Space Network would need to capture the radio signal, they would have to verify that the software was operating correctly, and then the long process of bringing it out of safe mode to normal operations would begin. Exquisitely detailed plans for each step had been formulated with great care, but when the team left for the day, they did not know that a single surprise lay in store.
Around 10:00 pm PST on November 27, the spacecraft’s main computer rebooted, just half a day before operators planned to command it, and Dawn entered safe mode. As soon as the Deep Space Network detected the corresponding change in the radio signal and the small night-shift team in mission control realized what had happened, a different plan, known dryly as “anomaly response,” was put into action. Some team members were called back in to JPL and spent the entire night investigating this unexpected event; some others were in occasional contact by telephone or Internet. As many team members as possible were not disturbed, so they would be fresh the next day to pick up after the anomaly team’s overnight work. (In the same vein, although your correspondent was among those who went to JPL, he opted not to contact you right away; rather, he chose to let you read about it now, under more leisurely circumstances.)
The team first used the trickle of data from the distant probe to verify that it was indeed safe. Then, proceeding with great diligence, given the unanticipated behavior of the computer, they downloaded certain diagnostic files. All indications were that Dawn was quite healthy, with no apparent signatures of a lingering problem. By the middle of the day on November 28, engineers had determined that the new software (automatically loaded when the computer rebooted) was doing well and the spacecraft was ready to resume its work. That day and the next had been planned already for the time-consuming transition from safe mode to normal configuration, so most of the team followed those plans while others continued analyzing why the reboot had occurred. On November 29, all steps were complete, just as they would have been had the reboot been commanded the previous morning.
Even now the investigation into the unplanned reboot continues with simulators. Meanwhile, the mission has progressed very smoothly. The backup main computer received its primary copy of 7.0 on December 7 and its backup copy on December 14. The backup copy of the primary main computer’s software will be loaded in January during a hiatus in thrusting.
While flight software 7.0 had been tested extensively before being sent to the spacecraft, ever-prudent mission controllers had planned to conduct one additional test, this time on the spacecraft. Because Dawn will spend most of its life thrusting with the ion propulsion system, engineers wanted to verify that the new software did not introduce any bugs that would interfere with this essential capability. Under the guidance of the main computer, all systems operated well during the “thrusting validation” on November 30.
In the subsequent 2 weeks, as they were finalizing plans for the beginning of interplanetary cruise, the team conducted another round of instrument tests. Following the excellent results of the first set of tests in October, this month the science camera and the visible and infrared mapping spectrometer were pointed at specific targets to allow more thorough characterizations of their functions and capabilities. Observations included Saturn (too distant to appear as more than a bright spot to Dawn’s camera, but still useful for tests), Arcturus, Vega (of special significance to your correspondent and his wife), and other stars and star fields. While the primary science camera was operated for the first time in October, the backup had its first in-flight exercises on December 10 and 12.
As the instrument and outreach teams find time, more views from the cameras and spectrometers will be posted at http://dawn.jpl.nasa.gov/multimedia/index.asp. (Certain ones may take longer, while 3-way negotiations drag on among the Dawn project, as-yet unnamed celebrities in the imaged star systems, and tabloids in those regions.)
Several lovely scenes have been captured. While the brief encounter with Mars is still 14 months away, arrival at Vesta is 2.5 years after that, and rendezvous with Ceres takes place in 2015, the excellent instrument tests whet our appetites for what will be revealed. Readers are particularly recommended to see the image of a star field in Cepheus as well as the Carina Nebula (known to some readers as NGC 3372 and to others as “home”), captured in a calibration image of the fine cluster of bright stars NGC 3532, a popular sight for observers in Earth’s southern hemisphere. Eta Carinae is a massive, unstable star in the immense star-forming nebula, and it has displayed a highly variable brightness since it was cataloged 330 years ago. For a time in the 19th century, it outshone all but one of the stars in Earth’s nighttime sky, despite being much much farther away than most. It has faded and brightened several times since then, sometimes being too faint for naked-eye observation.
Dawn’s initial checkout phase was remarkably productive and has served extremely well to certify the systems on the spacecraft and in mission control for interplanetary cruise. On December 14, in addition to loading the backup copy of 7.0 into the backup main computer, controllers radioed to the probe all the instructions and data it would need for the first 37 days of the next mission phase. The files were stored for use beginning on December 17.
At 12:01 pm PST on December 17, Dawn obediently began executing the instructions to reconfigure in preparation for long-term cruise. Three hours later, it initiated the prestart sequence for the ion propulsion system. While preparing for thrusting, the spacecraft also turned to aim ion thruster #3 in the required direction. This took the main antenna away from Earth-point, but the commands directed the craft to switch to one of the smaller antennas with broader coverage, allowing the team at JPL to monitor progress using signals received at the Deep Space Network complex in Spain.
After the spacecraft’s fine performance during initial checkout, engineers observing the beginning of thrusting expected no less. During initial checkout, the ion thrusters were started a total of 16 times and accumulated about 11 days 14 hours of thrust. (Some of the thrusting was for tests of the ion propulsion subsystem, and some was for tests of other subsystems or the entire Dawn system while thrusting.) With this experience, there was little reason to be concerned, but prudence dictated verification that Dawn got underway smoothly.
Telemetry confirmed that thrust began on schedule at 4:08 pm, as Dawn began propelling itself deeper into space, farther from Earth and the Sun. When the spacecraft turned its main antenna away from home, it set its sights elsewhere, on uncharted worlds, as it embarks on the next phase of its extraterrestrial expedition.
Dawn is 27 million kilometers (17 million miles) from Earth or 70 times as far as the moon. Radio signals, traveling at the universal limit of the speed of light, take 3 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 pm PST December 17, 2007
Now more than halfway through its 80-day initial checkout phase, the Dawn spacecraft continues to please its fans in mission control and throughout much of the rest of the universe. The project team has maintained the intensive pace described in the last log, and the sole team member in deep space has performed extremely well.
Dawn excelled in what may be the most important test of this part of the mission, an essential step in preparing for the rest of its voyage. The probe will spend most of 2008 - 2011 patiently using its ion propulsion system to change its orbit around the Sun to match Vesta's solar orbit. After more than half a year orbiting the enormous asteroid, the explorer will devote the majority of the subsequent 3 years to ion-propelled travel to dwarf planet Ceres. While its interplanetary journey will include many other activities, gentle thrusting will be the most common. (The most interesting of these activities will be covered in future logs, so to advertise your product in one of those logs, contact our representative on your planet now!)
Dawn must be able to sustain thrusting week after week, month after month, year after year, with only occasional interruptions, ranging from a few hours to a few months at a time. In a typical week of its interplanetary flight, Dawn will thrust for about 6 days 16 hours. (The exact duration will vary from that by as much as about a day. It will depend upon a number of details, including the schedule for Dawn’s use of the Deep Space Network, NASA’s amazing system for communicating with spacecraft throughout the solar system.) It will stop thrusting long enough to rotate so that its main antenna points to distant Earth, to transmit engineering information stored since the previous communications session (to allow engineers to assess its health and performance), to receive any new commands, and to rotate back to point the ion thruster in the required direction. Then it will settle in for nearly another week of thrusting.
While the craft was designed to be able to accomplish just such a routine, one of the principal objectives of the 80-day initial checkout is to verify its readiness. On November 2, mission controllers radioed all the instructions Dawn would need for a typical week of interplanetary operations. On November 5, right on schedule at 4:00 pm PST, Dawn began following the steps to start ion thruster #3. It turned to point that unit in the direction engineers had specified, and, with all systems configured, began thrusting at 5:07 pm. Throughout the subsequent week, as it emitted a high velocity beam of xenon ions, it recorded information on the operation of its systems and executed other programmed maintenance activities. Following the plan perfectly, it stopped thrusting on November 12 at 1:53 pm PST.
In the interest of full disclosure, 4 additional points should be made: 1) During most of the time in the mission that Dawn will thrust, Earth will not have contact with it, but this test included frequent contact. Dawn transmitted information on its health, so if a problem developed, it could be diagnosed promptly. Still, the craft was instructed to store all the relevant data for transmission during one 6-hour period (on November 12), just as it will have to for most of the mission, so engineers could verify that all the data buffer sizes and recording rates were adequate. This also shows the operators what their view of spacecraft telemetry will be when contact really is only once each week. 2) There are only 2 points that should be disclosed, not 4. 3) The previous point is not correct.
After terminating ion thrust, Dawn turned to a new orientation and transmitted the information it had been storing since November 5. The mission operations team will not check out the main antenna until later this month, so this test used 1 of the 3 smaller antennas, each of which still provides an adequate signal this early in the mission (as the probe has not receded too far from Earth).
Following a 6-hour communications session, the spacecraft repeated the steps of a week before: it turned to the direction needed for thrusting as it initiated the steps required to start the ion thruster. Another 4 hours of thrusting was adequate to demonstrate that it could execute the repetitive pattern.
This test was not designed to verify the performance of any one subsystem; rather, it was intended to show that all subsystems could work together as one integrated system. All engineering subsystems onboard played a role: command and data handling, electrical power, attitude control, reaction control, ion propulsion, thermal control, and telecommunications. Overviews of these subsystems were presented in the September 17, 2006 log. (Upon reviewing that material, and in preparing for the midterm exam, it may be worth keeping in mind that any system may be viewed as a subsystem at the next level up. So the use of “subsystem” or “system” is often a matter of context or personal preference.)
More than spacecraft subsystems were involved in this test. The full set of commands required to guide Dawn through a week would be too time consuming for the small mission control team at JPL to formulate and check without a broad suite of sophisticated software tools. Those tools -- another set of subsystems -- and the procedures for using them, also were part of this test.
While this test was an important success, much work remains in the 80-day checkout phase. Were it not for the need to characterize and test Dawn’s systems, thrusting would not be helpful during this part of the mission, as described on September 12. (That’s only 2 days after the wheel was invented on a planet in NGC 2099, a star cluster in Auriga. Although they won’t see this until after they have made a little more technological progress, we offer our congratulations now to those innovators, and we look forward to their joining the legions of future readers fascinated by the history of Dawn’s adventure.) The thrusting prior to mid December does not help propel Dawn to its destinations, but it does help prepare for the long periods in the mission in which such thrusting is required.
In addition to the tests described in recent logs, the Dawn team has been conducting many other activities, including testing reaction wheel desaturation during gyroless thrust vector control, changing hydrazine catalyst bed thermal control set points, setting the safe mode battery trickle charge rate in RAM to 175 milliamps, flying Ben’s radio-controlled blimp around Dawn mission control (boys will be boys -- and as engineers, we had fun figuring out how to improve it!), and loading Chebyshev thrust pointing files. While most such activities don’t lend themselves to more complete descriptions, they are part of maintaining steady flight.
Dawn is 13.0 million kilometers (8.1 million miles) from Earth or 34 times farther than the moon. Radio signals, traveling at the universal limit of the speed of light, take 87 seconds to make the round trip.
Dr. Marc D. Rayman
7:00 am PST November 13, 2007
Dawn’s checkout phase continues to go very well. The spacecraft is healthy as it and Earth travel their separate ways, separating at almost 1 light second (nearly 300,000 kilometers, or 186,000 miles) per day.
In our last report, thruster #3 of the ion propulsion system had been operated for 25 hours. Dawn’s mission control team at JPL commanded the probe to conduct additional tests on October 8 and 9, some focused on the thruster itself and others on how the attitude control system operated during thrusting. Both systems passed with flying colors (for the more literal-minded readers: flying - away from Earth at about 3.3 kilometers/second, or 7400 miles/hour; colors - the blue-green of a xenon ion beam).
Although operation of the ion thruster was an important success, it did not guarantee that the other thrusters would perform as planned. On October 10, the team’s attention shifted to thruster #1. (The thruster numbers are described in the previous log and can be found in the white pages in most binary star systems.) The system that feeds xenon propellant to the thrusters was prepared for operational use before the tests with thruster #3, but each thruster has to undergo individual preparation as well. To begin, thruster #1 was treated to the same ministrations #3 received on October 4 and 5.
Dawn will carry out most of its functions much farther from the Sun than Earthlings reside (although closer than most of our readers reside). On October 24, the spacecraft reached a greater solar distance than Earth ever attains in its annual elliptical orbit and will never again visit Earth’s part of the solar system. But still for a little while longer, it will be close enough to the Sun that careful monitoring is required to ensure that components remain at acceptable temperatures.
Although thruster #1 and #3 are mounted on different parts of the spacecraft and point in different directions, engineers had determined that to measure the thrust by using the Doppler shift, just as thruster #3 should not be aimed at Earth, neither should thruster #1 at this time in the mission. In both cases, it was clear that pointing the thruster toward Earth would cause other components to overheat. Pointing thruster #1 away from Earth however raised the possibility that another component would reach an undesirable temperature.
The plan was formulated to conduct the test with a software timer running onboard to stop it after a certain time had elapsed. That way, if the temperature of the component approached a limit that engineers did not want it to reach, or any other issues arose, and the team was unable to intervene because communications were interrupted (as can happen occasionally with systems as complex as deep-space communications), the spacecraft would stop the activity and return to the original orientation. The timer was to be reset by controllers at regular intervals until the data showed that the temperature would stabilize at an acceptable value.
On October 11, after instructing the spacecraft to rotate so that thruster #1 was directed away from Earth, mission control transmitted the commands to initiate ion thrusting. Electrical measurements transmitted from the spacecraft and Doppler computations performed by the navigation team all soon confirmed that the thruster was applying its well-known light touch. Thruster #1’s first operation in space was excellent.
After almost 8 hours of thrusting, the temperature of greatest interest was below the limit engineers had established, although it was still creeping up. While dealing with a temporary problem with communications between systems at JPL and the Deep Space Network, the small team monitoring the spacecraft on the night shift did not have time to reset the timer. The timer activated and, as designed, terminated the activity rather than let it continue without approval from mission control.
With some simple changes to parameters in the thermal control system, the test was restarted on October 23. It completed flawlessly on October 24 about 6:30 pm after operating thruster #1 at 5 different throttle levels for almost 27 hours.
The ion propulsion system delivers Dawn to its celestial destinations, but it is the scientific investigations to be conducted at Vesta and Ceres that make the journey worth undertaking. Dawn’s three science instruments were powered on and tested to verify that they were in good condition. (Some of the knowledge scientists will gain about the interior structure of the bodies will derive from exquisitely sensitive measurements of the gravitational pull they exert on the probe. These measurements take advantage of capabilities built into the telecommunications system, as we will see in a future log, and hence do not have a dedicated scientific instrument.)
On October 16, the gamma ray and neutron detector was activated. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. To infer the atomic composition of the outermost parts of Vesta and Ceres, GRaND includes a sophisticated suite of 21 sensors to measure the energies of gamma rays (a highly energetic form of light) and neutrons (subatomic particles). The instrument converts some of the power it receives from the spacecraft’s electrical power system into about 1000 volts for its detectors, and all power supplies and other electronics proved to be operating correctly. (Electrical power is distributed to most systems onboard at about 30 volts, and devices that need other voltages are responsible for making the conversion.)
While GRaND (and the other instruments) are much much too far from any solar system bodies to verify how well they will work at Vesta and Ceres, scientists have alternatives. Cosmic rays, high energy radiation that pervades space, constantly impinge upon the spacecraft. GRaND can detect some of that radiation directly. In addition, it senses some of the byproducts of interactions between cosmic rays and nuclei of atoms in the spacecraft. While the signals that were observed were not as grand (yes, the pun is an easy one, but readers should get accustomed to it, as it is expected to recur throughout Dawn’s mission) as those expected at Vesta and Ceres, they were sufficient to demonstrate the instrument is healthy and ready for further operations. GRaND remained powered on and measuring cosmic rays through October 22, when it was turned off in preparation for other spacecraft activities.
The visible and infrared mapping spectrometer (VIR) was the center of attention on October 17. (The instrument’s acronym was chosen because, as its Italian developers of both sexes recognized, “vir” is Latin for “man.” [Editors in Virgo, please take note: To maintain the good relations we have worked so hard to establish with locals, you may substitute the explanation that the name was chosen because stars in your constellation often are known by abbreviations that include “Vir,” such as Alpha Vir, R Vir, and 109 Vir.]) Objects as warm as the spacecraft (or, for that matter, as warm as many readers) emit so much infrared that if VIR’s detector were at that temperature, their own infrared radiation would interfere with planned measurements. Therefore, VIR incorporates an electronic cooling system which brought the infrared detector to about -191°C (-312°F). In the absence of nearby targets to observe, VIR tests use internal lamps that produce both visible and infrared light.
VIR’s cover was opened and closed, demonstrating that the mechanism controlling it works correctly. In addition, the instrument has an internal mirror that can be moved to make small changes in the direction VIR looks, and that scan mechanism was verified.
With all tests showing VIR to be fully healthy and ready for future operations, the flight team conducted the first checkout of a science camera on October 18. Two such cameras are onboard, and following standard practice on JPL’s planetary spacecraft, only one is checked out at a time. While it does not need to operate as cold as VIR’s detector, the camera requires its detector to be well below normal spacecraft temperatures, and the device cooled to below -69°C (about -93°F). The camera’s filter wheel, which allows pictures to be taken in color and even in near infrared, was exercised, and the instrument cover was opened and closed.
The camera took 102 images during the test. For this first set of instrument tests, no special spacecraft pointing was used; instead, the orientation of the spacecraft chosen for other purposes was maintained. Nevertheless, the camera captured images of the star field in the constellation Cancer that happened to be in its line of sight, and that was sufficient to demonstrate that it was working extremely well. The instrument's view of the sky is shown in the figures at http://dawn.jpl.nasa.gov/technology/fc.asp. (As readers in the vicinity of the star HIP 42243 may recall, the images were acquired during the Dark Matter Bacchanalia. As a further demonstration that the camera works as expected, and to the probable relief of the participants, no evidence of the festivities is apparent.)
As with VIR, all of the camera’s optics, electronics, detectors, and mechanisms are in excellent condition and ready for the mission ahead. More tests will be conducted with each of the instruments during the coming years, but all indications from their first opportunities to operate in space are that they are in fine fettle and will yield the exciting scientific data for which they were designed.
Following a wonderfully successful week of instrument tests, the Dawn team returned to propulsion tests the week of October 22. As well over 0.20% of readers know, Dawn relies on ion propulsion for reaching its destinations. Indeed, ions will propel the craft past Mars, on to Vesta, into orbit around that massive asteroid, from one orbit to another to allow the science instruments to gather data from different perspectives, out of Vesta orbit and back into orbit around the Sun, through more of the asteroid belt to dwarf planet Ceres, into orbit there, and through a series of orbits as at Vesta. Such a mission is far beyond the capability of conventional chemical propulsion; yet Dawn also has a reaction control system powered by the conventional propellant hydrazine.
The reaction control system is planned to be used only to help in stabilizing or changing the spacecraft’s orientation. Even if the entire 45.6-kilogram (101-pound) supply of hydrazine were devoted to changing Dawn’s velocity, the effect would be less than 0.1 kilometers per second (220 miles per hour), quite insignificant compared to the ion thrusting plan of about 11 kilometers per second (nearly 25,000 miles per hour). Despite its much lower efficiency, there are some dire (but highly unlikely!) cases in which the higher thrust of the reaction control system may prove critical. In those contingencies, patience may not be a virtue. As well over 0.19% of readers know, ion propulsion, efficient though it is, changes the trajectory only gradually. Should serious spacecraft control problems arise during the final approach to Mars or while in orbit at one of its destinations, engineers want to be prepared to execute a rapid change in the trajectory.
On October 22, the spacecraft’s capability to execute a hydrazine-based maneuver was verified. Dawn turned to a new orientation and fired 2 of its hydrazine thrusters simultaneously for 2 minutes. (Other hydrazine thrusters fired in other directions during this time as needed to keep the spacecraft stable.) The maneuver expended about 51 grams (1.8 ounces) of rocket propellant and changed the probe’s speed by about 8 centimeters per second (0.18 miles per hour). Although the boost in speed was quite modest, it was sufficient to demonstrate that the system could perform an emergency maneuver. While having confidence in such a capability, your correspondent expects (and invites you to share his hopes) that future occasions to write about the use of the reaction control system to change the trajectory will arise only while rhapsodizing about the differences between ion propulsion and conventional propulsion.
The spacecraft has been in space almost 4 weeks now. Three days into its flight, when it was between 964,000 and 968,000 kilometers (599,000 and 601,000 miles) from Earth, Bill Dillon managed to capture portraits of Dawn among the stars. The faint smudge visible at 2.5 times the distance to the moon is Earth's last glimpse of its robotic ambassador to the cosmos. Although barely visible, indistinct, and unimpressive, what it represents is so much more: humankind reaching out from its tiny home into the vastness of space. Somehow this simultaneously recalls both our insignificance in the universe and our yearning to do our noble best.
Dawn is 7.90 million kilometers (4.91 million miles) from Earth or almost 21 times farther than the moon. Radio signals, traveling at the universal limit of the speed of light, take nearly 53 seconds to make the round trip.
Dr. Marc D. Rayman
9:30 pm PDT October 24, 2007
Joining an elite club among spacecraft, Dawn successfully fired up its xenon ion propulsion system on October 6. This important milestone in Dawn’s 80-day checkout phase followed ongoing work by the mission operations team members to become accustomed to flying this new spacecraft, as they continue monitoring telemetry, adjusting onboard parameters, and conducting special activities to keep the spacecraft performing smoothly.
The ion propulsion system (in the interest of environmental responsibility, we will conserve keystrokes in this log by using the acronym IPS) will be used to climb away from the Sun, pass Mars, rendezvous with Vesta, maneuver into different orbits around it to provide the science instruments with varied views of the alien world, leave orbit, push still deeper into space to dwarf planet Ceres, and orbit it for the same scientific scrutiny. All systems on the spacecraft are complex and important (the relative importance was considered on September 17, 2006 and October 29, 2006), but the IPS has been the focus of the Dawn team in recent days.
While the 3 ion thrusters are the most familiar part of the IPS, they are not its only elements. The system includes 2 computer controllers (only 1 is used at a time). When mission control selects 1 of the 112 throttle levels (each corresponding to a certain power consumption and thrust), the operating controller translates the command into the currents and voltages that must be applied to parts of the thruster and the flow rate of xenon propellant to the thruster. The controller also provides the principal communications between the main spacecraft computer and the rest of the IPS, accepting commands and reporting on the IPS performance. While the controller is the brain of the system, the 2 power units (again, only 1 is used at a time) are the brawn. Following instructions from the controller, a power unit receives power from the solar arrays at about 100 volts and converts it to more than 1000 volts for use by the thruster.
On October 2, a controller and a power unit were activated and verified to be operating correctly. When that was complete, those devices were turned off and the other ones were powered on and checked out.
The 3 ion thrusters are mounted on different parts of the spacecraft. The one located along the central axis of the spacecraft (at the “bottom” in many depictions) is known affectionately as thruster #3, and that was the focus of this past week’s tests. The thruster near the visible and infrared mapping spectrometer (on what might be considered the “back” of the spacecraft) is thruster #1. Given that sophisticated nomenclature, the astute reader might presume that the thruster near the main antenna (on the “front” of the spacecraft) is thruster #2. As our readers are well known all to be astute, it will be no surprise that indeed that is its designation.
Each thruster is mounted to a gimbal system that allows the direction it is pointed to be changed by a few degrees. The angle is not large; the total range corresponds to less than the movement of the minute hand of a clock in 3 minutes. While the purpose of IPS is thrusting is to propel the spacecraft in its orbit (around the Sun now, and later around Vesta or Ceres), the attitude control system uses the thrust as one of its means to control the spacecraft’s orientation by slowly swiveling the thruster. The gimbal for thruster #3 was commanded to execute a preprogrammed set of movements on October 2, and its health was verified.
Some of the components in the thrusters are very sensitive to contamination, particularly water. While every effort was made to prevent air, and its normal inventory of water vapor, from becoming trapped in the system while on Earth, it is inevitable that some stray molecules of water would be in the stainless steel lines that deliver xenon to the thrusters and in the thrusters themselves. To reduce the presence of contaminants, several days of activities were devoted to purging the system by baking it out. Around noon PDT on October 2, mission controllers transmitted commands to raise some parts of the spacecraft to about 50°C (approximately 120°F). The temperatures were restored to normal values 30 hours later.
As most space enthusiasts know, October 4 was the 50th anniversary of the launch of Sputnik 1 and, quite remarkably given the different calendric system, the 5,000,050th anniversary of the first mission to the eventual site of the Tribute to Coincidence. The Dawn project recognizes with great admiration those missions and all others that have ventured into space in the pursuit of knowledge and the spirit of exploration. As the Dawn team prepared for the day’s activity, 7 days after launch, the spacecraft was almost 2300 times farther from Earth than Sputnik 1 was at its maximum range. Yet Dawn’s journey is still just beginning, and its travels should take it more than 250 times still farther from home.
To ionize its propellant, the thruster bombards the xenon atoms with electrons, as explained on December 28, 2006. The device that emits electrons was heated for several hours on October 4 as another step in preparing the thruster for operation.
The last operation before thrusting, undertaken on October 5, was to ionize xenon inside the thruster but not accelerate the propellant, obliging the IPS to do almost everything required for normal thrusting.
Because the thrust is so gentle, there are no sensors on board that directly measure it. To verify that the thruster performs as expected, the remarkable accuracy of the techniques of deep-space navigation are employed. With measurements of the change in the frequency (or pitch) of the radio signal, engineers can calculate the change in the spacecraft’s speed. This capability relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Although like all spacecraft built by humans, Dawn’s speed is only a tiny fraction of the speed of light, with the astonishing sensitivity of the Doppler measurements, the gradual effect of the thrusting can be sensed. With the spacecraft coasting away from Earth at more than 3.3 kilometers per second (7400 miles per hour), the radio measurements can detect changes smaller than 0.5 millimeters per second (less than 6 feet per hour). Snails, take note.
Others take note that the speed relative to Earth is most assuredly not the speed relative to the Sun. The spacecraft is in its own orbit around the Sun, and at this point in the mission, it is traveling at about 33.1 kilometers per second (74,000 miles per hour) in that orbit. This writer and others on his homeworld are orbiting the Sun at 29.8 kilometers per second (66,700 miles per hour). The difference is the speed at which Dawn is leaving Earth.
The Doppler effect applies only for motion toward or away from the observer; movement across the line of sight does not change the pitch of the signal. Therefore, to maximize the effect in the test of the IPS, the thruster should propel the spacecraft toward or away from Earth. With the present relative positions of Earth, our favorite interplanetary probe, and the Sun, when thruster #3 is pointed toward Earth, the incident sunlight, in combination with the heat generated by the operation of the thruster itself, would cause the unit to overheat. With the thruster pointed directly away from Earth, the temperature is fine. That has the curious consequence of engineers choosing to propel the spacecraft toward Earth during the first thrust test.
The objective of all the thrusting during the first 80 days of the mission is not to change the spacecraft’s trajectory but rather to evaluate the performance of all systems and prepare for the thrusting after this checkout phase. As the effect of the ion propulsion only becomes significant after long intervals, the short thrust periods for testing do not cause important changes in the trajectory.
On October 6, the mission control team instructed Dawn to turn to point thruster #3 away from Earth. Following that, and after one final verification that all onboard systems were healthy and ready for the next step, the command to initiate thrusting was sent. The drama was captured in the stirring name of the file that was transmitted to the spacecraft: dz002e.scmf. (Our readers who are not versed in neutron star orbital opera may not fully appreciate the drama of that name and are requested to accept that others may find great passion in the command file name.)
In the silent depths of space, far from its designers and controllers, connected to Earth only by the faintest whisper of a radio signal, Dawn dutifully executed the programmed steps. The craft had no appreciation of the hopefulness of its terrestrial handlers as it began emitting a bluish beam of xenon ions at 6:07 pm PDT.
When data revealing the thruster’s electrical currents and voltages showed up in mission control, the excitement remained controlled but was clearly rising much much faster than the gradual acceleration of the spacecraft. Experienced team members, huddled around the monitors in mission control, kept in mind that while starting thrusting was wonderful progress, success required sustaining it. Within 5 minutes though (in fact, shortly after 4.5 minutes for team members who also worked on Deep Space 1), the enthusiasm could no longer be contained, as all indications were that Dawn was quite content to keep thrusting.
Much of the joy was in sharing the success with colleagues who have worked very very hard together for years, each perhaps with his or her own personal motivations and rewards, but each contributing to a common goal of pushing the frontiers of space exploration. Still more of the happiness is in sharing the accomplishment with supportive family members and friends -- and loyal readers! While, like launch, this is but one more step in Dawn’s very long journey to unlock the secrets of Vesta and Ceres, it is an important one, and the many feelings of having a probe in powered flight in deep space are all -- well, perhaps “out of this world” is the best descriptor.
The test sequence operated the IPS at throttle level 28 (in the range from 1 to 112) for nearly 12 hours. Next, it throttled up to level 49, and then it pushed still higher every 4 hours after that, operating at levels 70, 91, and finally 112. Thrust was commanded off on October 7 at 7:12 pm as the test completed successfully. In about 25 hours of thrusting, the acceleration amounted to 3.6 meters per second (8 miles per hour), truly negligible compared to the spacecraft’s speed relative to Earth or the Sun.
After the checkout phase, except in special circumstances, Dawn always will use the highest throttle level it can. As it travels farther from the Sun, eventually its enormous solar arrays, the most powerful ever used on an interplanetary spacecraft, will not produce enough power to permit operation at level 112, so the IPS will be throttled down. That is why it is necessary to certify operation over a range of throttle levels.
In addition to the Doppler measurements to reveal the thrust, engineering data were collected on the performance of the IPS, attitude control system, electrical power system, thermal control system, and all other onboard participants in the thrusting. More tests are ahead for thruster #3 as well as the other thrusters and other spacecraft systems.
Dawn is 3.21 million kilometers (2.00 million miles) from Earth or more than 8 times farther than the moon. Radio signals, traveling at the universal limit of the speed of light, take more than 21 seconds to make the round trip.
Dr. Marc D. Rayman
10:00 pm PDT October 7, 2007
The Dawn project welcomes you to deep space! Dawn is operating smoothly on the fourth day of its 8-year adventure. Like new parents, its extremely proud and greatly sleep-deprived Earthbound mission operations team is carefully monitoring its every move.
Launch had been targeted for September 26, but during its last few days on Earth, Dawn continued to be subjected to the vagaries of the weather on that dynamic planet. The Delta II 7925H-9.5 rocket had been scheduled to have its second stage filled with propellants on September 23. The nitrogen tetroxide was pumped in before bad weather prevented further activities at Cape Canaveral's Space Launch Complex 17B, so Dawn waited patiently and safely inside the protective payload fairing, or nose cone, of the rocket. On September 24 a delicious blend of hydrazine and unsymmetrical dimethylhydrazine (together known as Aerozine-50) was loaded as the countdown resumed, targeted for launch on September 27 at the 7:20 am EDT opening of the launch window.
This writer arrived at JPL at 11:30 pm PDT on September 26. The security guards, although recognizing him (and his car), diligently verified his identification in the chilly autumn evening and received his enthusiastic greeting, “We're going to the asteroid belt tonight!” Upon hearing “All right!!” your loyal correspondent was ready to head into mission control.
The countdown continued smoothly until shortly before launch when a ship was discovered to have entered a restricted zone in the waters east of the launch site. This required an unplanned hold.
The Delta rocket does not account for the changing position of the launch pad in space as Earth rotates, so a launch delay would place the spacecraft on a different trajectory. Most interplanetary missions have launch windows of only 1 second because they have too little maneuvering capability to compensate for the altered trajectory of the rocket. Dawn's ion propulsion system gives it much greater flexibility, so its launch window on September 27 was 29 minutes long. That proved to be more than enough to allow the Coast Guard to invite the ship to depart and then continue to ensure that no one would be at risk of being harmed as the launch vehicle flew overhead.
The countdown resumed, no other glitches occurred, the rocket roared to life, and Dawn's voyage began at 7:34:00.372 am EDT. It was propelled off the launch pad not only by nearly 890,000 pounds of thrust (which grew within 1 second to about 1,070,000 pounds) but also by the enthusiasm of the people who designed and built it, those who will fly it and will analyze the data it returns, and the vastly greater number of people who share in the yearning to know the cosmos.
The rocket and all downrange tracking systems performed extremely well, and Dawn's ride to space was very much what had been foretold in prophecy. This was the 76th consecutive successful launch of a Delta II. Following separation from the third stage at 8:36 am, Dawn went to work, and the Deep Space Network at Goldstone, California began receiving its radio transmissions at about 9:43 am.
Since then, the mission operations team at JPL has kept it company constantly, albeit from an increasingly remote location. Even as the cheers of hearing from the probe were echoing in mission control, the team began a prompt assessment of Dawn's health. It was evident quickly that it was in good condition, and operators were pleased to see that the myriad problems they had trained to handle were now little more than a fond recollection from simulations.
Upon conducting more detailed analyses of Dawn's telemetry, engineers found that it handled itself quite admirably, operating completely on its own, in space for the first time. As it was programmed to do, it dealt with the few minor unexpected conditions it encountered with the skill of a seasoned pro.
Over the subsequent days, the team gradually reconfigured the spacecraft subsystems to prepare for the extensive testing and checkout scheduled to conclude in mid-December. By the time this report was filed, the team had sent 148 sets of commands to Dawn and had scrutinized thousands of measurements of temperatures, pressures, voltages, currents, data buffer volumes, valve and switch positions, and many many other parameters. Now the spacecraft is ready to be put through its paces before it begins its ion propelled voyage past Mars and then on to the uncharted and distant worlds Vesta and Ceres.
After years of planning, designing, building, and testing, the Dawn mission is underway. While the fulfillment of its scientific objectives remains well in the future, the craft finally is in space, and a far far more exciting and challenging phase of the project is beginning.
Dawn is 1,158,000 kilometers (720,000 miles) from Earth or 3 times farther than the moon. Radio signals, traveling at the universal limit of the speed of light, take almost 8 seconds to make the round trip.
Dr. Marc D. Rayman
8:30 pm PDT September 30, 2007
The countdown is underway for Dawn’s liftoff on September 26 at 7:25:00 am EDT.
This is the second time our hero has been within a few days of launch, and with a full 20-day launch period still ahead of it, confidence is high the mission will get underway soon. Now the Dawn project is ready with a new flight profile to allow the probe to leave Earth months later than planned and yet still keep its interplanetary appointments on schedule.
Because this new flight plan begins with a different launch, we present here an update to the July 5 log, accounting for the changes. Dawn’s intention was to launch in June or July, and the postponement was because of circumstances beyond our control; nevertheless, we understand the difficulties this can cause our readers. Therefore, for those readers who have the July 5 log tattooed on either themselves or a relative, we have arranged with our favorite fine tattoo and taxidermy emporium for a discount on this log. (Certain restrictions may apply; this offer void in galaxies with less than the cosmic abundance of deuterium or tattoo ink.) If weather or other minor glitches delay the launch by a few days, we will not publish another update.
In the last log we began on the launch pad with the entire Delta II 7925H-9.5 rocket, including its passenger. Together, they are 285,581 kilograms (629,592 pounds), and we followed the plan for the delivery of the 1218-kilogram (2685-pound) Dawn to space. For a launch prior to October 10 (a date chosen based on the sophisticated mathematics of interplanetary trajectory design, not it being your correspondent’s birthday), the rocket and spacecraft will spend about 62 minutes flying together. For launches on October 10 or 11, one phase of the launch, the coast in Earth orbit, will be 2 minutes 40 seconds longer. Should launch need to occur on October 12 - 15, the coast will be 4 minutes 16 seconds longer than for launches in the beginning of the launch period. In all cases however, the relative timing of other events during the flight of the Delta rocket will remain as described in the previous log.
During their shared flight, the rocket is in control. Following separation from its conveyance to space, Dawn has three primary objectives: 1) get sunlight on its solar arrays, 2) establish contact with mission control at JPL, and 3) revel in the beginning of a remarkable mission of exploration. Most of what it does to accomplish the first two steps also will be standard procedure for the spacecraft throughout the mission when it encounters a problem and needs to enter “safe mode,” in which it will await instructions from Earth. Of course, detaching from the launch vehicle is anything but a problem. Engineers have taken advantage of their extensive work developing the directions Dawn will follow to reach its safe configuration by having it execute nearly the same program as soon as it is flying independently in space. Future logs are sure to have reason to discuss safe mode again.
The Delta does not provide electrical power to the spacecraft (even though it rides in the first-class section), so Dawn carries a large battery. While on the rocket, as few of the probe’s components as possible are turned on. Its computer and a few other devices are operating, heaters are activated as needed, and some data are recorded, but mostly the craft simply waits for the signal that indicates it and the third stage have parted ways. Conserving energy (a responsibility familiar to readers on Earth) is vitally important.
Now one might be tempted to conclude that with the longer time from liftoff to separation for an autumn launch than a summer launch, Dawn’s power reserves will be more critical. Readers are urged to avoid this temptation with their utmost resolve! When the two solar arrays are folded, the outermost panel on each side is oriented so the solar cells point out. The arrays are so powerful that even with only 1 of the 10 panels exposed to the Sun, enough electricity is generated to satisfy all of Dawn’s needs (except thrusting with the ion propulsion system) when at Earth’s distance from that brilliant orb. While the battery will have been partially drained during the last few minutes on the launch pad and during ascent, the intermittent exposure to the Sun in the course of the “barbecue roll” during the quiet coast in Earth orbit will provide sufficient power for all systems that are running and still have enough extra to recharge the battery. By the time the barbecue ends and the second stage begins preparing for its second burn, Dawn’s battery should be fully charged.
Because the craft will be returning a tremendous bounty of rich scientific information from distant Vesta and Ceres, its radio system is powerful. It does not have a mode in which it can transmit at low power, so the transmitter remains off until the solar arrays can provide essentially endless power.
When the third stage releases Dawn, it will leave the spacecraft spinning slowly, with xenon propellant spinning inside in the opposite direction. In addition, the springs that push the spent stage and the eager spacecraft apart are likely to impart a slightly unbalanced push, so Dawn is expected to be turning slowly around all axes. After the computer determines that Dawn has separated, it waits 8 minutes 20 seconds for the friction between the xenon and the spacecraft to lower the spacecraft’s spin rate enough that it can be stabilized by the attitude control system. Known to its friends as ACS, this system is responsible for controlling the spacecraft’s orientation.
After waiting the prescribed time, software directs ACS to begin using its sensors to determine the direction and rate of the spin. Then ACS commands the small rocket thrusters of the reaction control system to fire, gradually stopping the unwanted rotations. The process of bringing the attitude under control can take as little as 1 minute or as long as 15 minutes, depending upon the imbalance in the separation forces and details of the xenon behavior.
Once the spin is fully controlled, it is safe for Dawn to deploy its large solar arrays. Each wing is divided into 5 panels, which are stacked against each other and secured to the spacecraft by cables during launch. To release the wings, small heaters press against the cables, causing them to weaken and break. When they are no longer restrained by the cables, the wings unfold under the gentle urging of springs. With its wings folded, the spacecraft is 1.84 meters (6 feet 1 inch) wide. When they open, the two wings span 19.74 meters (64 feet 9 inches) tip to tip. The software provides 12 minutes 47 seconds to allow the cables to release and the arrays to extend to their full reach.
Although ACS remains in control throughout the solar array deployment, after the computer has allowed the programmed time to elapse, it requests ACS to perform another stabilization, now with the new, much larger configuration of the spacecraft. ACS may report back that this is complete in as little as 1 minute or as long as 15 minutes.
Just as when a teneral dragonfly spreads wide its new wings for the first time, these intricately patterned marvels must be pointed at the Sun. Up to this time, Dawn has paid attention only to itself, without regard to the external universe. (Of course, it continues coasting away from Earth with the energy given to it by its recent companion, the Delta rocket.) Supported on a short extension from each corner of the boxy body of the spacecraft is a pair of solar cells, just like those on the arrays. But these cells are not intended to meet Dawn’s electrical needs; instead, ACS uses them to find the location of the Sun. This is not very different from using your eyes to find the Sun, a particularly appropriate analogy both for dragonflies and for those readers who have eyes that allow them to see in all directions simultaneously. Once it has established where the Sun is, it rotates with its thrusters to point the arrays in that direction. Depending upon the orientation the probe happens to be in prior to this activity, it can take as little as 1 minute and as long as 18 minutes to locate the Sun and complete the turn.
As soon as light from the solar system’s master, the star at the center, reaches the arrays, the battery begins to recharge again, and all of Dawn’s electrical needs for the rest of its 8-year mission will be satisfied by the energy the solar cells receive from the Sun.
The computer waits another 4 minutes after the arrays are fully illuminated by the Sun to make sure all systems remain stable, and then it activates its power-hungry radio transmitter. It should take about 4 minutes 30 seconds for the transmitter to warm up and begin sending radio signals, reporting on the status of all systems.
The spacecraft is well prepared to resolve a wide range of problems as it progresses through the list of tasks to complete between separating from the Delta and powering on its radio. If it has not been delayed by correcting any anomalies, the entire sequence could take as little as 32 minutes 37 seconds and as long as 77 minutes 37 seconds; otherwise, this could stretch to well over 3 hours. In mission control at JPL, the operations team, taking a cue from one of the virtues Dawn will display as it traverses the solar system, will remain patient. Nevertheless, everyone will look forward to verifying that it is starting its long journey in good health.
But Dawn’s radio signals may not reach Earth quite yet. Without information on where that planet is, the spacecraft cannot know where to point its antenna. (For most of the mission, Dawn will know where it is in relation to Earth and other solar system bodies, but at this early stage, having just begun its flight, such information will not yet be available onboard.)
After it has finished directing its solar arrays at the Sun, the spacecraft begins a roll around the line between it and the Sun, turning once per hour, perhaps appearing like an exotic and lazy windmill. Given the direction of its departure from home, the Sun and Earth are at about right angles from Dawn’s perspective. So as it makes its slow spin, it uses an antenna pointed at the same right angle to the solar arrays. The antenna sweeps out a broad beam, like a wide searchlight sending its signal out to anyone who happens to see it.
Antennas at the Deep Space Network (DSN) complex in Goldstone, California will be ready to detect Dawn’s transmissions and pass the data on to JPL. Had the launch occurred in the summer, Dawn would have begun transmitting its signals in view of DSN and European Space Agency antennas in Australia. Now, following its longer travel time from Florida, the coast in orbit will carry it farther east, so Goldstone has the privilege of being the first to communicate with the spacecraft.
The DSN station should be able to receive signals during about half of each rotation of the spacecraft, or about 30 minutes every hour. It is impossible to predict where Dawn’s antenna will be pointed when it begins transmitting, so it might be aimed at Earth immediately, or it could take as long as 30 minutes until the spacecraft’s rotation brings it around to start the half hour of terrestrial coverage.
With all these steps, the time from liftoff to the receipt of the first radio signal may be as little as about 1 hour 35 minutes or as long as 2 hours 50 minutes even if Dawn encounters no surprises along the way, and more than 4 hours if it does. If you are entering your planet’s friendly betting pool on when Dawn’s data first will light up the computers in mission control, you are advised to consider that the likelihood that all circumstances will conspire to yield the shortest possible time is extraordinarily low. That time is more a theoretical minimum than a practical guide, and although mission control will be ready, no one there will be expecting signals that early.
Once controllers see the data, they will begin evaluating the spacecraft’s condition. Over the course of the subsequent few days, they also will review the data it stored during launch and begin configuring it for further operations. One of them will try to find the time to write another of these logs as well.
Meanwhile, Dawn will continue racing away from Earth. In less than 2 hours 45 minutes from liftoff, it will be more than 35,800 kilometers (22,200 miles) high, passing the ring of satellites in geosynchronous orbit, and thus will be more remote than the great majority of spacecraft launched in Earth’s half century of probing and utilizing space. It will go beyond the most distant point in the moon’s elliptical orbit less than 29 hours after leaving the launch pad, as it travels farther from home than humans have ever ventured. Yet that is but the very beginning of Dawn’s journey.
Distant though it will be, it may be possible for terrestrial observers with capable telescopes to glimpse the probe in the first week or two of its travels. (Other spacecraft have been imaged not long after they left Earth. See http://www.jpl.nasa.gov/releases/98/ds1palomar.html for what this former member of the Deep Space 1 team considers to be the best portrait ever made of that craft.) It would be very faint, perhaps no more than a speck amidst a sea of distant stars between the constellations Auriga and Gemini near right ascension 6 hours 20 minutes and declination +28.5°. [Note to self: before this is posted, remember to insert a wonderfully clever remark here that connects Dawn to a charioteer and the twins, the figures represented by these constellations.] These approximate coordinates will change if Dawn’s launch does not occur on September 26 at the opening of the window. For a launch at a later time that day, the position will move to slightly higher right ascension. The dependence upon the day in the launch period is a little more complex. Throughout the launch period, the farthest from this location would occur for a liftoff at the end of the launch window on October 15. That would shift the coordinates to approximately 7 hours 28 minutes and +26°, within Gemini. For anyone interested in trying to observe the spacecraft, please visit JPL’s HORIZONS system, and change the target body to (no surprise here) “Dawn” to find its exact location.
Even before the navigation team gets a good fix on Dawn after launch and enters the trajectory data into HORIZONS, observers in Hawaii may get a view of Dawn's early light. With a launch on September 26 at the opening of the launch window, the spacecraft will exit the shadow of Earth 1 hour 19 minutes after liftoff (2:44 am Hawaii-Aleutian Standard Time, or HST). At that time, the spacecraft will not yet have deployed its solar arrays, so it may not be very bright, but its relatively small size at that time should be somewhat compensated for by its relative proximity to Earth. It will be about 68° above the horizon when it comes into the sunlight, and will pass directly overhead 13 minutes later.
Dawnophiles in Hawaii, Alaska, and the Pleiades may be treated to a particularly attractive alignment shortly after that. As viewed from the first two of those locations, Dawn will appear to pass less than 1.5° north of the center of that familiar star cluster at about 3:14 am HST when it is less than 18,000 kilometers (about 11,000 miles) from the surface. (Note: “it” refers to Dawn. The Pleiades, in contrast, will be more than 430 light years from Earth, or more than 200 billion times farther than Dawn.) By then it likely will have opened its solar arrays, presenting a much larger target for the Sun to illuminate.
Observers are advised however that, depending upon the spacecraft's progress in the many steps described above, the arrays may already be pointed straight at the Sun by the time it transits the Pleiades, so the reflection would not be directed toward Earth. The Dawn project is confident no one's eyes will be damaged from direct exposure to this view; indeed, the spacecraft may be quite dim. It is possible however that before the spacecraft has completed aiming its panels at the Sun, terrestrial spectators could see a brief bright reflection or “flare,” a phenomenon familiar to amateur satellite observers. We will not know until we receive reports from witnesses.
If liftoff is delayed to later in the launch window, the views described here will occur later by less than the change in launch time. More details on where to look are posted here, and the Dawn navigation and outreach teams will be standing by to update the information as soon as possible after liftoff. We will do our best to give some fortunate observers the opportunity to see Dawn as it recedes into the depths of space. If you obtain any images, we will be interested in seeing them and would appreciate your sending them to the Dawn Education and Public Outreach Team.
If all goes according to plan, this will be the last log written when Dawn is bound to Earth. We hope readers throughout the cosmos join in wishing the explorer well as it gets underway for a journey that offers new knowledge, excitement, the rewards -- and the risks -- of facing the unknown, and the spirit of adventure that compels humankind to undertake such bold quests.
Dr. Marc D. Rayman
September 21, 2007
Now less than two weeks from its planned September 26 launch, Dawn is eagerly awaiting the beginning of its fantastic adventure. It has been here before, but the opportunity to take the fast track out of Florida in June or July was consumed by delays in the readiness of the launch vehicle, adverse weather conditions, and problems with launch vehicle tracking facilities. After spending a very quiet, if not boring, 7 weeks at nearby Astrotech, the spacecraft returned to the launch pad on September 11.
While Dawn remains unwaveringly committed to its mission to explore Vesta and Ceres, the first part of the trajectory from Cape Canaveral’s Space Launch Complex 17B to the alien worlds is different for an autumn departure from what it would have been in the summer. Lacking an interplanetary version of mapquest.com, the Dawn team has developed a new route. It begins with a different launch profile, so right now -- at this very instant! -- you are reading an updated version of the June 23 log accounting for those changes. (The preceding sentence may not apply to readers capable of advanced time travel, but this sentence does -- or did -- or perhaps will.)
In addition to some changes in times, altitudes, and velocities, readers who have the details of the earlier version of this log committed to memory will notice that the mass of the third stage is different. As with most missions, the Dawn team finished designing the launch trajectory before the spacecraft was fully assembled. To ensure that the parameters loaded into the rocket’s guidance computer would be correct, engineers planned for the maximum possible spacecraft mass. As explained at the beginning of the log we are updating, when Dawn was tested for its stability at 50 rpm, it was better balanced than had been expected. That meant it was not necessary to add as much mass to it as had been considered possible, so Dawn’s final mass was less than anticipated. By then, it would have been too time consuming to update the Delta’s computer parameters, so ballast masses were installed on the third stage. This brought the combined third stage and spacecraft mass to the value used in all the trajectory calculations.
With the delay in the launch and the need to redesign the guidance program for the different launch conditions anyway, engineers were able to take advantage of their knowledge of the final spacecraft mass. So the 5.34 kg (11 pounds 12 ounces) of ballast were removed from the third stage, and the new plan benefits a little from the lower mass by commanding the rocket to impart slightly more energy to the spacecraft.
With that background, now that launch is so close, let’s have a preview of what is planned during this important event. Much of the work on the design of the spacecraft focused on ensuring that it is prepared for the acceleration, vibration, noise, heat and cold, and other conditions it will experience during the ride to space. And yet for all that effort, as well as the spectacular sights and sounds for observers, this is the shortest phase of the mission. During it, Dawn will be a polite passenger, patiently recording data and awaiting its chance to begin flying on its own in space to undertake its mission of discovery deep in the solar system.
This log has many more numbers (readers are encouraged to quantify this) than most, and hence will be of special interest to our friends the Numerivores, who reside in the “quadruple quasar” Q2237+0305. Others need only follow well enough to gain a sense of how dynamic Dawn’s departure from home will be, in great contrast to the more leisurely pace of its interplanetary flight.
In a previous log, we saw that to leave the launch pad, the Delta rocket will use its liquid-fueled first stage and 6 of the 9 solid rockets strapped to its side. Thirty seconds later (L + 30 seconds) it will exceed the speed of sound. The solid motors burn out at about L + 77 seconds when the rocket is at an altitude of about 24 kilometers (15 miles), and the remaining 3 motors ignite less than 2 seconds later. Three of the spent motors separate at L + 80.5 seconds, and the other 3 are jettisoned 1 second later as the rocket continues its ascent. The remaining 3 motors burn for 77 seconds, and when they are released at L + 2 minutes 39.5 seconds, the rocket will be nearly 73 kilometers (117 miles) high and traveling 10 times the speed of sound. The first stage’s main engine continues firing on its own until L + 4 minutes 23 seconds, and then the rocket coasts for 14 seconds. After 8.5 seconds of the coast, having lofted Dawn to 130 kilometers (81 miles), the first stage separates.
When the second stage engine is commanded to life 5.5 seconds later, the rocket is traveling at 6.1 kilometers per second (3.8 miles per second, or almost 14,000 miles per hour). At an altitude of 135 kilometers (84 miles), the shroud that shielded Dawn from the dense atmosphere below is no longer needed, so it is ejected. Now 4 minutes 41 seconds from liftoff, Dawn has its first view of space. The second stage continues climbing and accelerating until it reaches the altitude and velocity to be in a low orbit. At L + 8 minutes 58 seconds, the stage stops firing.
Let’s take advantage of the hiatus in orbit to consider the timing of all the events during launch. The overwhelming majority of spacecraft our species [Note to extraterrestrial editors who repost these reports: change the previous two words to “humankind.”] sends beyond the atmosphere remain gravitationally tied to Earth. They accompany the planet on its endlessly repetitive travels around the Sun, and except for the few that are designed for scientific observations of the cosmos, the orbits of these satellites are mostly unrelated to the rest of the solar system. Where Earth is in its orbit, and where other members of the Sun’s retinue are, generally do not matter. Such is not the case for Dawn (and other interplanetary probes).
The entire launch sequence is timed so that Dawn will depart Earth at a carefully chosen point in the solar system. For each possible launch day, extensive analysis has established the mathematically optimal plan for reaching Vesta and Ceres, distant worlds that beckon and that Dawn seeks to unveil. The analyses account for the gravitational effects of the Sun and all planets, and the resulting strategies (modified somewhat from the mathematically perfect solutions) include times that Dawn will thrust with its ion propulsion system and times that it will coast. As reported in another log, many years of exquisitely gentle thrusting allows the indefatigably patient probe to reshape its orbit around the Sun to rendezvous with its destinations. As we will see in logs after launch, the first 80 days of the mission will be devoted to checking out the spacecraft systems and preparing for the long journey ahead. At L + 80 days, the thrusting needed to follow the flight plan begins, and the timing of the launch sequence is arranged so that Dawn will be at the correct location in the solar system, about 27 million kilometers (17 million miles) from Earth, at that time.
The second and third stages linger in Earth orbit so that following the ascent from Cape Canaveral, they are properly positioned to propel Dawn to reach its required location nearly 3 months later. If launch occurs before October 10, the pause in the second stage’s firing will last about 42 minutes 37 seconds. (Because the solar system is constantly rearranging itself, launches near the end of the launch period will slightly require longer intervals. Maintaining a constant interval for most of the launch period is a degree of flexibility enabled by the ion propulsion system and was chosen to reduce the vast volume of work required to design the autumn launch trajectories in the short time available.)
Had the launch occurred in June or July, Dawn would have departed in a very different direction, which would have been reached with a much shorter pause in Earth orbit. To deliver its precious payload to the new starting point for the interplanetary journey, the Delta rocket now needs 33 minutes 30 seconds longer to travel from the launch pad to its target than it would have on July 8. To keep its temperature comfortable during this extra time, the rocket performs a “barbecue roll,” allowing all parts to receive equal exposure to the hot Sun, warm Earth, and cold space. After turning to the programmed orientation, the second stage begins rolling at the lazy rate of 1 revolution every 6 minutes. During the 29 minutes 50 seconds of barbecuing, Dawn basks in the beautiful glow of its home planet for the last time. These are some of the final quiet moments before it goes to work on a journey of nearly 8 years and 5.1 billion kilometers (3.2 billion miles).
The second stage engine reignites at L + 51 minutes 35 seconds while at an altitude of 179 kilometers (111 miles) and operates for 2 minutes 39 seconds. Fifty seconds later, to finish its contribution to Dawn’s mission, the second stage fires 4 small rockets pointed around its circumference to spin the third stage and spacecraft to 48 rpm. (Unlike the first and second stages, the third stage is stabilized by gyroscopic rotation, like a spinning bullet or football.) This is when the spacecraft’s balance becomes most important. The second stage separates at L + 55 minutes 8 seconds.
For the next 37 seconds, the spinning assembly continues following the orbit the second stage left it in, and then the final burn of the Delta begins. The third stage fires for 86 seconds, and during that time it exceeds “escape velocity” so that it has enough energy to break free of Earth’s gravitational hold. When the solid motor burns out, it is only at an altitude of 275 kilometers (171 miles), but Earth is too weak to slow the rapidly receding craft enough to bring it back. (Pause here for a moment of awe: 80 days later, the spacecraft will be about 100 thousand times farther from Earth.) Unlike a ball you might throw that goes up and then comes down, the Delta will have thrown Dawn so hard that it will never fall down. It will be in its own orbit around the Sun, traveling at 11.46 kilometers per second (7.12 miles per second, or 25,600 miles per hour) relative to Earth. With the third stage spent, for the rest of the mission, onboard propulsion will be achieved only with ions.
When the second stage spins the spacecraft, the xenon propellant stored inside does not immediately spin up to 48 rpm, just as when you rotate a glass filled with a liquid, it takes a while for the liquid to catch up with its container. (We recognize that some readers live on planets without liquids, but the analogy applies to gases as well. In fact, the xenon on Dawn is maintained at a temperature and pressure that create a special state called “supercritical,” in which it bears some similarity to a gas and some to a liquid. Amazing though its properties are, supercritical xenon should not be confused with superheroes that may bear similar names.) The friction between the rapidly spinning spacecraft and the xenon inside it causes the spacecraft’s spin to slow down and the xenon’s spin rate to grow. The Dawn project has invested a great deal of effort over the past 2 years to understand the detailed behavior of the xenon while the spacecraft is spinning. This has involved both sophisticated analysis techniques as well as spin tests with a tank of exactly the same shape and size as Dawn’s filled with a fluid with properties similar to those of xenon’s. Based on this work, engineers can predict how quickly the spacecraft and xenon will change each other’s spin rates.
After the third stage has finished firing, it remains securely attached to Dawn for another 4 minutes 50 seconds. Although the stage is stabilized by spinning, the spacecraft does not operate that way; yet by this time, they would be spinning together at 46 rpm, too fast for the latter’s control system. Therefore, starting 5 seconds before separation, the third stage activates a surprisingly simple system to slow its rotation rate. Wrapped around the Delta are two cables, each 12.15 meters (39 feet 10 inches) long. At the end of each is a 1.44-kilogram (3-pound-3-ounce) weight made of aluminum and tungsten. When the cables are released, the spin causes them to unwind. As they carry the weights farther and farther out, the spin slows down because of the same principle that makes an ice skater spin faster by pulling her arms in or slower by extending them to her sides. After 4 seconds, when they are fully unwound, the cables unhook from the spacecraft. With their weights still attached, they enter independent orbits around the Sun; perhaps one of them will be studied by a future solar system archeologist.
The values of these “yo-yo” weights are chosen carefully and are accurate to about 1 gram (0.04 ounces) in order to achieve the required change in spin rate. The slight reduction in mass of the third stage because of the removal of ballast required a tiny change in the yo weights. Eschewing both diet and exercise, technicians opted for surgical removal of 7 grams (0.2 ounces) from each one.
Even with a 204-kilogram (450-pound) third stage (which was 2224 kilograms, or 4903 pounds, before it began expending its propellant) and a 1218-kilogram (2685-pound) spacecraft, the small yo-yo system halts the spin and even reverses it, leaving Dawn rotating at 3 rpm in the opposite direction from its original spin. About 1 second after the cables have separated, the attachment between Dawn and its rocket is severed, and springs push them apart.
Only 62 minutes 1 second after liftoff, while 1021 kilometers (635 miles) above their home world, the Delta bids the spacecraft farewell. The third stage, its raison d'être fulfilled and having no further purpose, continues on its own through the vast emptiness of the solar system. But its disconnection from Dawn triggers sensors on the spacecraft that alert the central computer to the separation.
Spinning slowly at 3 rpm in one direction, with xenon inside spinning at 39 rpm in the opposite -- the original -- direction (because the propellant still lags behind its container), Dawn waits for 8 minutes 20 seconds. That is long enough for the spacecraft and xenon each to slow the other down, and after that, Dawn’s systems are ready to go to work.
In the next log, shortly before launch, we will see what the spacecraft plans to do as mission control waits to hear from it. That update of the July 5 log also will have a special suggestion for our readers in Hawaii, Alaska, and near the Pleiades on how to catch Dawn’s early light less than 2 hours after liftoff.
Dr. Marc D. Rayman
September 12, 2007
NASA is preparing (again) to bring Dawn to the Florida skies as all systems are gearing up for a September 26 launch. This new date is later than had been planned just a few months ago; nevertheless, as we shall see, in the most important sense, this genuinely is not a delay for Dawn’s mission of adventure, discovery, and the search for answers to exciting and important scientific questions. Earth’s next interplanetary ambassador remains on schedule for its engagements.
Following the decision in July to reschedule the launch, a complex story described in the previous log and occasionally told with other scary stories around campfires or microscopic black holes in elliptical galaxies, the first priority was to move Dawn from Cape Canaveral’s Space Launch Complex 17B to a safe location so NASA’s bold new Mars explorer, Phoenix, could launch from nearby 17A. After the protective payload fairing (the nose cone) was removed from the Delta II rocket, the spacecraft and mated third stage were detached from the second stage on July 22. They were transported back to the Hazardous Processing Facility clean room at Astrotech Space Operations for safe storage. Since then, the spacecraft has had a very leisurely summer vacation, with little to do but allow technicians and engineers to maintain its readiness while it enthusiastically anticipates the autumnal beginning of its deep space voyage. Meanwhile, the first and second stages of the rocket remained on the launch pad, where they had a delightfully close view of the launch of Phoenix on August 4. Soon Dawn and the third stage will be reunited with the rest of the launch vehicle.
One of the keys to success in space exploration (and in some other challenging and complex endeavors of the human species) is careful planning and preparation for contingencies. In one example of that, as soon as the crane used to erect Dawn’s launch vehicle malfunctioned on May 30, engineers at JPL, Kennedy Space Center, and United Launch Alliance began detailed preparations for the possibility of a launch in September or October. Had the work not begun as early and as intensively as it did, it is quite likely that it would not have been possible to complete it in time for launching after Phoenix. Even with its uniquely capable ion propulsion system, Dawn cannot conduct its exploration of both Vesta and Ceres if it does not launch before late October 2007.
In working out the complex strategies for planning launches, engineers use a great deal of jargon, such as “C3 = 11.4 km2/s2” (C3 equals 11.4 kilometers squared per second squared), “-2? RLA dispersion for a 95% PCS” (negative two sigma right ascension of the launch asymptote dispersion for a 95% probability of command shutdown), or “This project is so cool, I can hardly believe we get paid to do this” (OK, perhaps this last isn’t really jargon, but it is part of the Dawn parlance). Terms that are particularly pertinent to our discussion here are the “launch period,” the interval of days on which a launch may occur, and the “launch window,” the range of time on any one day in the launch period during which a launch may take place.
Most interplanetary missions have brief launch periods during which they must take off or pay the price of a significant change in their itineraries. As explained in previous logs, Dawn’s travels are unusual because of the extraordinary capability of the ion propulsion system, which provides a thrust of susurrant gentleness that is more than compensated for by its virtually tireless persistence. With so much maneuvering capability, even though its launch was deferred by months, Dawn’s scheduled arrivals at asteroid Vesta and dwarf planet Ceres are effectively unchanged. The delay in launch does not necessitate a delay in accomplishment of Dawn’s goals, so in many important ways, from an overall mission perspective, the postponement of the launch is inconsequential.
Indeed, a complex combination of myriad factors, including the positions of Earth in its orbit on candidate launch dates in 2007, of Mars in its orbit in the first few months of 2009, and of Vesta in its orbit late in 2011, makes the new launch more favorable for the mission. Although still a challenge of astronomical proportion, this will make it slightly easier for Dawn to complete its assignment. This may be translated to slightly greater resilience in keeping its alien appointments should it encounter difficulties on its voyage through the unforgiving and remote depths of space.
Now Dawn will follow a completely different launch trajectory and take a different path to Mars. Had it launched in July, the spacecraft would have used Mars early in April 2009 to boost it along its way to Vesta. In the new plan, it will swing by the red planet in February 2009. Mars is one of the easiest destinations in the solar system to reach, and Dawn could travel there more quickly if that were its sole objective. Much of the ion thrusting prior to Mars however is designed to aim the craft so that when it reaches Earth’s neighbor, the planet’s gravity slings it in the most effective way to help it in its long flight to distant Vesta. Leaving Earth in September or October lets Dawn gain greater benefit from its brief visit to Mars.
Despite remaining with Earth throughout the summer of 2007, Dawn’s new flight profile will allow it to catch up with the old one. The spacecraft will be in the same place in the solar system in the summer of 2009 as it would have been had it launched in June or July. The mission after that will be quite similar to what it would have been with the earlier launch. The principal difference is that to accomplish the mission with the later launch, Dawn will consume a little less of its xenon propellant, so more will be available in case the probe needs to perform unplanned thrusting.
To account for the new launch date and path to Mars, Dawn’s departure from Earth will be very different from what it would have been in June or July. Instead of launch windows in the middle of the afternoon, now Dawn will launch closer to dawn. On September 26, the launch window is 7:25 am EDT to 7:54 am EDT. (To simplify coordination among the many organizations around the world participating in the launch, liftoff is scheduled on the whole minute. The capability to round off the time this way is another benefit of the ion propulsion system’s flexibility, and it should be particularly appreciated by all Dawn enthusiasts on planets whose clocks don’t have second hands, including our newest readers, members of the Honorable Minority of Antipunctualists in the Horologium supercluster of galaxies.)
If unfavorable weather or other fortuities prevent launch (possibilities with which all loyal readers are exceptionally familiar) on September 26, launch windows during the rest of the launch period are:
Sept. 27: 7:20 - 7:49 am EDT
Sept. 28: 7:14 - 7:43
Sept. 29: 7:09 - 7:38
Sept. 30: 7:03 - 7:32
Oct. 1: 7:12 - 7:31
Oct. 2: 6:55 - 7:24
Oct. 3: 6:49 - 7:17
Oct. 4: 6:44 - 7:13
Oct. 5: 6:41 - 7:10
Oct. 6: 6:38 - 7:07
Oct. 7: 6:35 - 7:12
Oct. 8: 6:34 - 7:12
Oct. 9: 6:33 - 7:11
Oct. 10: 5:43 - 6:23
Oct. 11: 5:42 - 6:22
Oct. 12: 5:13 - 5:54
Oct. 13: 5:13 - 5:57
Oct. 14: 5:16 - 5:58
Oct. 15: 5:18 - 6:00
Mortal readers are encouraged not to waste time trying to discern a pattern in either the time of the opening of the launch windows or the window durations. The underlying reasons for these values are manifold and complicated, and to avoid violating statutes in some spiral galaxies on publishing dangerously boring text, the explanations will be omitted. Let’s look briefly at just one relatively simple observation: the 19-minute window on October 1 is shorter than all the others. Under certain circumstances that are unlikely but possible, an earlier launch window opening on that day would make Dawn pass close enough to the moon less than 28 hours later that the gravitational deflection of the spacecraft could only be compensated by significantly more ion thrusting than planned. Rather than take the small risk of incurring this minor complication in the mission, the window was shortened. On all other days in the launch period, as the spacecraft departs Earth in roughly the same direction, the moon will be elsewhere in its orbit so it will not cause as much interference in the trajectory. Still, the moon’s gravity is included in all analyses.
Demanding as it is, there is more to replanning the mission than designing new trajectories for the rocket and the spacecraft. Because the geometry for the departure from Earth has changed so much, the Dawn operations team has had to redesign many of the activities scheduled during the early part of the mission. The location of the spacecraft relative to Earth and the Sun will be quite different from what had been planned, so onboard instructions for how to orient in order to achieve certain objectives must be modified. While Dawn itself has spent an unusually quiet and leisurely time at Astrotech waiting to be reunited with its rocket, mission controllers have been very busy indeed developing new plans and the corresponding sets of commands for the first few months of the mission.
To maintain proficiency for launch, the team also completed another set of simulations of the final 16 hours of countdown, launch, and the first day or so of flight. Most of the week of August 27 was devoted to a slightly shortened version of the ORTathon that was conducted early in June. Differences from the first ORTathon included not only the launch time and trajectory, but also a new set of fiendish surprises injected by the simulation supervisor and a modified (and, in this participant’s careful analysis, a superior) selection of snack food in mission control.
One regrettable consequence of the changed launch conditions is that the timelines presented in the June 23 and July 5 logs will not apply for the new launch period. This threatens a lucrative deal negotiated with importers of the stone-engraved versions of these logs on icy asteroids in most irregular galaxies. Therefore, in the coming weeks, when the relevant analyses are completed and the new data are available, those logs will be reposted with the only changes being those that are essential to bring them up to date for the new launch plan.
Dr. Marc D. Rayman
September 3, 2007
There are two ways for a spacecraft to leave its launch pad: climbing on a blazing tower of powerful flames accompanied by a thunderous announcement of its departure or suspended securely and gently on the crane that hoisted it there in the first place. Now Dawn has the opportunity to experience both, with the former to be in September and the latter this month. The spacecraft and third stage of the Delta rocket are being prepared now for removal from the second stage. They will be transported to the clean room at nearby Astrotech Space Operations before being returned to the launch pad in less than two months.
For more than a year, Dawn had been planning for a launch between June 20 and July 10. As is well known to readers of these logs in galaxies of all shapes, Dawn’s ion propulsion system affords it a flexibility in its trajectory unavailable to missions that use conventional propulsion. One of the important consequences of this is that, as we will see in more detail in the next log, the postponement of the launch to September does not change the scientific objectives or plans; even the dates for arriving at Vesta and Ceres are essentially unaffected. Dawn’s launch period was chosen based on the readiness of the project to begin the mission and on the schedule for the use of Cape Canaveral’s Space Launch Complex 17, from which Delta II rockets get their start to space.
In addition to Dawn lighting up the Florida skies, NASA has planned to launch an exciting Mars lander, Phoenix, this summer on a Delta II. Phoenix’s launch period starts August 3 and, like most planetary missions, without ion propulsion, if it were unable to launch within a few weeks, it would have to wait a very long time for the correct planetary alignment to recur.
There are two pads at the complex. Phoenix has been scheduled for pad A, and Dawn’s departure will be from pad B. The pads are less than 175 meters (575 feet) apart, and prudence dictates that a rocket not launch from one when a spacecraft -- an extremely precious resource -- is mounted on a rocket at the adjacent one. In addition, there are some shared facilities for the two pads, including the environmental control systems for the protective enclosures for the delicate hardware. (This also is why Dawn must be removed from 17B while Phoenix has its turn at the starting gate.)
These considerations led to the plan to end Dawn’s launch period early enough that there would then be sufficient time and resources to devote to preparing Phoenix for its launch.
Following a delay of 10 days in the production of Dawn’s rocket components earlier this year, the beginning of the launch period was rescheduled for June 30. The time required to repair a crane at 17B at the end of May and beginning of June pushed the earliest attempt to launch Dawn to July 7. There was much less flexibility in the closing of the launch period because of the need to preserve the schedule for Phoenix, but creative launch teams began looking into ways to launch Dawn a bit later than July 10.
Launching a mission into space requires much much more than the simultaneous cooperation of a rocket, a spacecraft, and the weather.
The Delta’s flight is fully controlled by its onboard computers. Even with this launcher’s extraordinarily high record of success, it is important for engineers to receive telemetry from the rocket during the major events of its short mission so they can verify that it performed correctly. This allows every launch not only to benefit from the successes of the previous ones but also, if necessary, to avoid those rare problems that do occasionally affect rockets. Each mission, with its unique trajectory, requires downrange tracking at certain locations. Antennas at many ground sites are available to track the vehicles, but often some portions of the flight are not within view of any of these stations. In such cases, ships or aircraft are used.
For the June 20 - July 10 launch, NASA had planned to use an aircraft (known as a P-3 Orion, a name sure to appeal to readers from Betelgeuse to Rigel) in the southeast Atlantic Ocean to receive data from the second and third stages. As the end of Dawn’s launch period changed, conflicts with prior commitments for that aircraft led to the decision to use a ship instead.
Tracking ships are leased by NASA from private owners and are outfitted with a system for acquiring the signals from the rocket. The principal burden this system carries is its name: ocean-going test and evaluation transportable resource (known to rocket scientists and lutrine dyslexics as OTTR, and pronounced “otter”). Standard telecommunications systems on the ships relay the data collected by the OTTR back to controllers via communications satellites.
After all preparations had been made for its voyage, including installing the OTTR hardware and vaccinating the crew members for yellow fever, the 57-meter (186-foot) vessel left its California port on June 6, as the Dawn operations team was conducting its ORTathon. The next day, the OTTR demonstrated that it was operating as expected by tracking a Delta II carrying an Italian spacecraft from Vandenberg Air Force Base in California to low Earth orbit. The ship was projected to be in position by July 4 to track Dawn’s launch on July 7.
On June 19, the OTTR’s transport reached the entrance to the Panama Canal. While awaiting its turn to transit the canal, the ship’s chief engineer conducted an inspection of the engine and discovered a problem in 1 of the 12 cylinders. The ship had been scheduled to stop in Puerto Rico, so arrangements were made to repair it there. Well before it left harbor at San Juan on June 29, it was apparent the OTTR would not reach its destination by July 7.
As soon as the OTTR’s transportation problems arose, NASA began working on alternate plans. As part of its normal set of equipment, a U. S. Air Force jet known as Big Crow (a modified KC-135, derived from the Boeing 707 design) has systems that could receive the Delta II telemetry. The plane was undergoing scheduled maintenance at that time in preparation for an appointment that would make it unavailable after July 9. Plans were formulated to have Big Crow track the launch should it occur on July 7, 8, or 9, with the expectation that the OTTR would be in position by about July 10.
Besides the time required to repair the engine, there were two other obstacles to the OTTR’s providing support for Dawn’s launch. Headwinds and rough seas prevented the ship from making progress at its expected speed. In addition, Dawn’s rocket would follow a different trajectory almost every day of the launch period, thus changing the required location for the ship to put the OTTR within view of the rocket’s flight. When the decision was made in May to switch from the Orion to the ship, some of the ascent trajectories were quickly replanned to ensure that the shift from one day to the next was within the ship’s daily travel range. But now the ship, already behind schedule, was fighting the uncooperative conditions of the Atlantic as it chased a target that moved each day.
Meanwhile, for a time, weather seemed to be a threat to Big Crow as well. It had to conduct a flight to verify all systems before it began the multisegment trip to Ascension Island, its base station for the daily flights to the coordinates for tracking Dawn’s launch. Bad weather delayed the check flight, but finally it was completed, and the bird flew to its temporary island roost in good time.
Weather certainly proved to be a hindrance at Cape Canaveral. Predictions of afternoon thunderstorms made Dawn’s July 7 launch appear unlikely, and prospects in subsequent days did not look much better. Formulating an accurate forecast of the weather conditions was essential. Two days before launch, the second stage of the Delta is loaded with its propellants. One of the two propellants is highly corrosive, and once the second stage has been exposed to it, the stage remains useful for only about 37 days. After that, this part of the Delta would have to undergo an extensive refurbishment or replacement, either of which would consume many months and be very expensive. There would not be enough time to have a restored or new second stage before late October, after which the changing alignment of the solar system would no longer allow even Dawn’s powerful ion propulsion system to accomplish the planned mission for many years.
With Dawn’s launch continuing to creep up on the opening of Phoenix’s launch period, it was essential not to load second stage propellants until favorable launch conditions were foreseen. Once the second stage was filled, poor weather, an unauthorized incursion of a boat or aircraft into the launch vehicle safety zone, a balky valve, a misbehaving sensor, or any of the other myriad glitches that can lead to a launch scrub could create a serious dilemma. If NASA made a subsequent attempt to launch Dawn, that would deprive Phoenix of some of its precious launch opportunities. If Phoenix were given priority so that it would have all of the planned chances to launch during its limited period, that would impose a very long and expensive delay on Dawn, after which its scientific goals would be compromised. Therefore, NASA was very diligent in waiting for all conditions to be satisfactory before fueling the second stage.
On July 7, after several days of postponements, with forecasts still showing a high probability of inclement weather at the time of Dawn’s daily launch windows, the projected date for the OTTR being in position continuing to slip, and the Big Crow needing to depart soon for its previously scheduled commitment, the decision became clear. The exploration of the solar system (and taxpayers who fund it) would be best served by not attempting to launch Dawn in July. Dawn has the capability to conduct its mission with a lift off after Phoenix, whereas the Mars lander needs to leave Earth in August.
This complex story would not have been told (even in this version, simplified so that readers of all species may reach the end in less than a generation) had myriad conditions not conspired to prevent the launch during the June - July period. There are too many aspects of reaching space and undertaking ambitious missions there to describe them all. Many are rarely mentioned because, complicated though they may be, they usually work well enough that they blend into the background. Of course, such a rich background is a crucial part of the tapestry we weave in attempting to probe the universe, and without it, the beautiful highlights would not be possible.
Now Dawn is preparing to vacate Space Launch Complex 17 while Phoenix prepares for the opening of its 3-week launch period on August 3. After Phoenix has left Florida for the chillier north pole of Mars, Dawn will once again take its place of honor at the top of the rocket. In the next log, we will see how Dawn gets to spend its unplanned Florida vacation as well as how the change in its launch date affects its mission of exploration far from Earth.
Dr. Marc D. Rayman
July 15, 2007