The effect of thruster #3's operation during this mission phase was to change the spacecraft's speed by about 0.99 kilometers/second (2200 miles/hour). Thanks to the exceptionally high efficiency of the ion propulsion system, Dawn's solar system xenon footprint in accomplishing this was less than 40 kg (87 pounds). (Note also that its carbon footprint was 0.)

Switching from one thruster to another is simple (to the extent that anything is simple for early 21st century humans controlling a spacecraft in deep space). The potential complication in this case was explained in the previous log. Our readers survey (conducted by Telepathic Business Services, Inc. when their employees had time between major poker competitions) shows that 3 readers do not fully recall the details and will not refer to that log, so the issue is summarized here. The 3 ion thrusters point in different directions on the spacecraft. To provide thrust in the correct direction in space, Dawn has to rotate to aim the designated thruster in that direction. The use of thruster #1 now requires the craft to assume an orientation quite different from any that had been experienced before, and engineers were not confident certain components would remain within their required temperature limits when the Sun shone on them.

Last month's test, in which the spacecraft spent a few hours pointing in the required direction, provided some of the data needed to establish when it would be safe to commit to the use of thruster #1 for long periods of time. The results agreed with previous analyses, which had shown that all the components would remain in their prescribed temperature ranges if thruster #1 were put to use this month. Probably.

Probably? That "probably" was not good enough. Ever-cautious mission controllers were not sufficiently confident to let the spacecraft remain in the new orientation for a week at a time, because there were a few components whose temperatures still could not be predicted well enough. The analyses were conclusive that the temperatures would be safe for more than 24 hours, as it takes a long time for that hardware to heat up. Therefore, the team devised a new approach.

A typical set of commands for 5 weeks of operation with thruster #1 was formulated. In addition, engineers prepared instructions for storage onboard to stop thruster #1, rotate to the thruster #3 orientation, resume thrusting, and perform all the other associated functions, the description of which is precluded by laws on profoundly incomprehensible prose. (While such laws are applicable only in the vicinity of supermassive stars, we obey them out of consideration for such regions of our distribution.) The instructions were structured so that only a single, brief message from Earth would be needed to trigger the switch back to thruster #3. On June 18, the spacecraft turned from pointing its antenna to Earth to aim thruster #1 in the correct direction and initiated thrusting.

A Deep Space Network antenna that was available was scheduled to listen in to the spacecraft on June 19. Dawn was programmed to use one of its small antennas, with a very broad radio beam, to transmit temperature measurements.

Dawn's terrestrial team members receiving the data found the results to be much as expected. As predicted, the temperatures had not yet stabilized, and all were within the desired ranges. When they had about two hours of measurements in hand, engineers were able to predict with high confidence what the final temperatures would be. This confirmed that continued operation was safe, so there was no need to switch back to thruster #3. (Providing the spacecraft with the capability to make that decision, while that might seem pretty neat, would have required more work than the neatness would have merited.)

As some may recall from long, long ago (to be specific, 8 paragraphs ago), several days of coasting are included in the flight plan occasionally. Activities for some of those times are planned long in advance. Other such periods are held in reserve in case mission controllers identify the need for some previously unplanned work that could not be accommodated in the normal schedule. June 16 - 18 was one such interval. The mission has been going so smoothly, however, that no special activities were required then. The team did take advantage of the extra time that the primary antenna was pointed to Earth to clean up some file buffers and perform other maintenance on some of the spacecraft's computers.

With the mission continuing so well, the Dawn team can devote much of its attention to preparing for future events. Although the Dawn project has no specific plans, readers may rest assured that the team members, as with their fellow residents of Earth, are completing personal plans to commemorate the centennial of the Tunguska event on June 29 (the event occurred on June 30 in Siberia's time zone).

The next item of interest occurs on June 30, when the spacecraft exceeds the outermost reaches of the orbit of Mars; the probe will be farther from the Sun than that planet ever travels. Earth reaches its greatest distance from the Sun on July 4, when it will be almost 1.7% farther than its average distance. (On January 2, it was about 1.7% closer than average.) Then even as Earth begins a slow fall toward the Sun (a trend that will continue until next January), Dawn will continue its climb outward. On July 10, the robotic explorer will be twice as far from Earth as Earth will be from the Sun. At that time it will be 304 million kilometers (189 million miles) from the planet it left on a lovely dawn in September 2007.

At these extraordinary distances, humankind (and even some of our other readers) does not have the technology to see the spacecraft. Indeed, Dawn is barely discernible in a pair of portraits taken when it was more than 300 times closer to Earth. Yet some who follow the mission might enjoy gazing in the direction of the probe as they contemplate its journey deeper into space and the ambitious and exciting mission that lies ahead. For those in the continental United States, the spacecraft will be between 3° and 5° northeast of the moon in the evening of July 6 as the moon is approaching the western horizon. (In other words, Dawn will appear to be 6 to 10 times the moon's diameter away, north and higher in the sky.) Although quite invisible to your eyes, in that direction your mind may be able to see with great clarity one of your planet's envoys to the cosmos. With a blue-green trail of xenon ions behind it and appointments with distant, uncharted, alien worlds ahead of it, Dawn will be silently and contentedly carrying out its mission to extend our reach into space and to help fulfill our passionate search for knowledge and our yearning for adventure.

Dawn is 286 million kilometers (178 million miles) from Earth, or more than 760 times as far as the moon and 1.88 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 pm PDT June 26, 2008

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

Some of the time in this period was devoted to the mission's first periodic maintenance on components of the attitude control system. This system's name may be misleading, as it has displayed a most decorous style in working both with its fellow spacecraft systems and with its human colleagues. In this case, "attitude" denotes the probe's orientation in the weightless conditions of spaceflight.

The spacecraft carries 4 reaction wheels, gyroscope-like devices which, when electrically spun faster or slower, rotate (or stop the rotation of) the spacecraft. Only 3 wheels need to be operated at a time; as with most components, Dawn has backups so the mission may continue even if a unit experiences problems. Wheel #4 (known to the irreverent but creative flight team as "wheel #4") was powered off during the initial checkout phase of the mission last year, but now it is wheel #3's turn to be the backup, and the switch was made during this coast period.

For most of the mission, the attitude control system's gyroscopes are not powered, as they are not designed to operate for the duration of the 8-year flight. These devices, not to be confused with the reaction wheels, help achieve the accurate pointing needed by the camera and the visible and infrared spectrometer to uncover mysteries of asteroid Vesta and dwarf planet Ceres. The intricate mechanisms have to be operated occasionally, so they were activated and allowed to run for about 2 days.

All of the ion thrusting since the beginning of the long-term interplanetary cruise phase has used ion thruster #3. Now thrust in a specific direction in space is required to reach Dawn's celestial targets. The ion thrusters point in different directions on the spacecraft, so the orientation (if not the attitude) of the spacecraft during thrusting depends on which thruster is selected. To point thruster #1 on the bearing needed for thrust would cause the Sun to illuminate a part of the spacecraft that has not yet been exposed to direct light from that star. (Of course, many other stars have shone on that portion of the craft, and while many of them are brighter than the Sun, we may resort to the narrow perspective of our solar system readers and discount those stars because of their extraordinary remoteness.) Flight controllers have complex computer programs to predict the temperatures under such conditions, but as sophisticated as these tools are, their accuracy is not always sufficient. The typical duration of a set of thrust instructions is 5 weeks, so before committing the spacecraft to spending so long in this unfamiliar orientation, a half day was devoted to measuring the temperatures at two orientations representative of what would be required for thrusting with thruster #1. Engineers now are using those data to refine the predictions for what the temperatures would be when thrusting.

On May 14, having accomplished all its special activities, the spacecraft resumed ion thrusting. Since then, it has continued with its normal routine of only short suspensions of thrust each week for pointing its main antenna to Earth.

As Dawn continued devoting its attention to its flight through deep space, the operations team recognized some other noteworthy events elsewhere in the very same solar system. Among the other spacecraft conducting exciting and important investigations of the cosmos, Phoenix had a particularly thrilling time on May 25 with its descent through the Martian atmosphere and its wonderfully successful landing in the far northern plains of the fourth planet from the Sun. Curious as it may seem, while their spacecraft are 56 million kilometers (35 million miles) apart, the Dawn and Phoenix teams at JPL are only 2 floors apart in building 264. (We appreciate that you readers in a small, faint, lenticular galaxy in Sculptor have a particular fondness for this building's impressive designation). Now that Phoenix is beginning the scientific part of its mission, along with space enthusiasts everywhere we congratulate the Phoenix team, many of whom are our friends and colleagues, on this superb accomplishment.

On the same day, we remembered Dr. Ernst Stuhlinger, who died at age 94. He played an important role in the long development of ion propulsion, making valuable contributions as early as the 1950s. He followed Deep Space 1 (the first mission to use ion propulsion for interplanetary flight) and Dawn with great interest and was most gracious in his expressions of enthusiastic support for both missions. His many kind words about these ambitious and challenging projects meant a great deal to your correspondent, a lifelong space enthusiast, who knew of Dr. Stuhlinger's work even as a youngster studying the space activities of 20th century humans.

Quite unaware of those news items, the spacecraft patiently travels farther from both Earth and the Sun. During this part of its mission, it recedes from its former home much faster than it does from the Sun. Tomorrow, on May 28, it will be equidistant from the two. Dawn will be 244 million kilometers (152 million miles) from Earth and from the Sun. With the planet 152 million kilometers (94 million miles) from the star, the trio forms an aesthetically symmetrical triangle. If the distance from the Sun to Earth were 1.00 units, the other two sides of the triangle each would be 1.61 units. Geometers call such a shape an "isosceles triangle," whereas some young male residents of the constellation Triangulum call it a "hot babe."

To keep apprised of Dawn's current location, be sure to visit the cool new feature "Where is Dawn Now?" at http://dawn.jpl.nasa.gov/mission/live_shots.asp. The site includes depictions not only of the craft's trajectory and location but also of its attitude (that is, its orientation; visualizations of its behavioral manner and emotional state have not been implemented yet), allowing readers to achieve greater accuracy in their enactments of Dawn whenever they have access to the World Wide Web.

Dawn is 243 million kilometers (151 million miles) from Earth, or 630 times as far as the moon and 1.60 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 pm PDT May 27, 2008

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

With some of the updated blocks, operators powered on the gamma ray and neutron detector (GRaND) (whose name belies its unassuming demeanor) and let it collect data for about a week. GRaND is designed to measure radiation from Vesta and Ceres to reveal the chemical elements that compose the outermost material of these protoplanets. As described during the first test of GRaND after launch, gamma rays and neutrons will reach the instrument not only from the targets it wants to investigate but from elsewhere as well. Initially these signals were used to verify GRaND's health. Now scientists want to collect more such data to begin developing an accurate record of the effects of this omnipresent radiation. One part of analyzing the signals from the asteroids will be removing the "noise" that GRaND detects from cosmic rays, so it is essential to know its characteristics.

While the science blocks streamline the process of sending instructions through the main spacecraft computer to the instruments, the instruments themselves have internal computers and software as well. Updated versions of the software for the science cameras were passed through the spacecraft computer and installed in the cameras' computers. The new software includes capabilities that had been planned before launch but were not needed for earlier tests, and it corrects minor bugs (yes, some bugs are hardy enough even to survive extended periods in deep space) discovered during those tests.

After the software was loaded into the primary camera, operators commanded the instrument through a "minicalibration" to verify that the installation was successful. Upon completion of the work with the primary camera, they conducted all the same steps with the backup camera. The two cameras are essentially identical, so they received the same software. They are recognized by the spacecraft computer as distinct devices though, and they are not operated simultaneously, so to route software to both of them required executing the loading procedure twice.

The team also conducted new calibration tests of the visible and infrared mapping spectrometer (VIR). The unit displayed excellent performance in the initial checkout phase, but as with most complex instruments, many tests are required in order to characterize its performance fully. For this calibration, the spacecraft pointed VIR to the star Canopus. From Earth, the only stars that appear brighter are the Sun and Sirius, but Canopus is a familiar sight to many observers besides those who are far enough south to see it from Earth. Canopus is one of the intrinsically brightest stars for hundreds of light years, shining brilliantly in the skies of many planets in this neighborhood of the Milky Way galaxy. When the measurements of Canopus were complete, Dawn rotated to aim VIR at Mars. At a distance of more than 55 million kilometers (34 million miles), that was the closest planet to the spacecraft. Too distant to be observed with any detail, the red planet provided a good infrared signal for testing the instrument.

Turning their attention away from the instruments, operators loaded the latest version of software to the backup main computer. Software version 7.0.3 was installed in the primary main computer on February 15, and the same update now resides in the primary and backup locations of the backup computer, ready to be used if the spacecraft detects a serious problem with the primary computer.

The last planned activity of this period was a test of how accurately ion thruster #2 can be pointed. (The naming convention for the thrusters is explained in text in a previous log. For an enjoyable explanation in another medium, see a performance of the new and popular Dawn pas de trois.) Such tests were conducted for the other 2 thrusters when they were checked out during the month after launch. This test was not run with #2 because the spacecraft was still too close to the hot Sun even in November when thruster #2 was put through all of its other tests. (The different positions of the thrusters on the spacecraft means they experience different temperatures when they are operated.) There was no urgency in making this measurement, so it was postponed to this convenient opportunity.

On April 8, the spacecraft oriented itself as required for the test. Executing the same steps it always does to start a thruster, this time the ion propulsion system's computer controller detected a potential problem and halted thruster operation. Because of the orientation of the spacecraft, the radio signal received on Earth was so weak that data could be returned only very very slowly. Mission control saw an indication that the onboard controller had stopped the thruster, so they radioed new instructions to end the test and turn to point the main antenna to Earth. Meanwhile, when an onboard system found that there was no thrust, it issued different instructions to accomplish the same ends: stop the rest of the test and aim the antenna at Earth. Either set of instructions would have worked, but the computer trying to process both sets led to a conflict, so it responded by entering "safe mode."

Safe mode is a standard response designed into the software to deal with uncertain, unexpected, or difficult conditions. Nonessential systems are powered off, and essential systems are reconfigured according to a plan stored in software. The details of that plan were modified in October and again in January to account for the spacecraft's growing distance from Earth and the Sun.

In safe mode, just as in the orientation for the test, the use of an auxiliary antenna greatly limits the amount of data that can be returned. Although controllers soon recognized the conflict that triggered safe mode, many steps in a carefully planned and methodical process were required to reconfigure the spacecraft to point the main antenna to Earth. To expedite this work, colleagues working on the Mars Odyssey and Mars Reconnaissance Orbiter projects agreed to exchange their scheduled use of one of the Deep Space Network's largest antennas, a 70-meter (230-foot) dish at Tidbinbilla, Australia, with Dawn's use of a 34-meter (112-foot) antenna at the same communications complex. The larger antenna allowed the Dawn team to send and receive data at greater speed; this significantly reduced the time it would have taken to return to normal operations. Such cooperative use of the shared resources of the Deep Space Network is one of the many ways missions work together to the benefit of all space exploration.

By April 11 the main antenna was pointing to Earth and all the data stored during the aborted thruster test had been returned and analyzed. Engineers recognized that there had not been a problem after all, and the thruster could have operated perfectly well. The ion propulsion control software sets and verifies many electrical parameters and checks many others to ensure the thruster is performing correctly. In this case, the software was conducting a check that was unnecessary, so there was no need for it to interrupt the thruster operation. The test was built into the software before the ion propulsion system received its exhaustive test flight on Deep Space 1. With the knowledge gained on that mission, this software check was determined to be unimportant, but given the overall complexity of the software, it had not been removed from the ion controller. The controller dutifully carried out its programming, not knowing that it was performing an unwanted function.

In all the thrusting conducted so far in the mission (and the far greater duration of thrusting on Deep Space 1), this unnecessary test had never tripped. Now engineers were able to calculate that the conditions required to indicate a (false) problem would arise later in the Dawn mission with the use of any of the thrusters, but the conditions would not occur for some time with thruster #3, which has been the one in use since December. Therefore, the "go" was given to resume thrusting, and on April 14 the spacecraft began its powered flight once again. (The entry into safe mode did not interfere with any special activities other than the pointing test of thruster #2, as that was the last planned event of this period.)

Now that the necessity of making a change in the ion controller software was identified, the fix itself was determined to be quite simple. In just a few days it was thoroughly tested in a simulator at JPL and was transmitted to the spacecraft during the next weekly communications session on April 21.

Dawn is 185 million kilometers (115 million miles) from Earth, or 480 times as far as the moon and 1.24 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take almost 21 minutes to make the round trip.

Dr. Marc D. Rayman
6:00 am PDT April 22, 2008

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

That Dawn is as far from Earth as the Sun does not reveal anything, of course, about its direction as viewed from Earth. The Sun and Dawn are 98 degrees apart; Dawn is nearly overhead when the Sun sets. Although the spacecraft is much much too far from Earth to be visible, even with the most powerful telescopes, readers who find enjoyment, if not inspiration for rich thoughts, in celestial sights may consider gazing in the direction of the emissary Earth dispatched to the cosmos. The craft is in Gemini, about 8 degrees east of Mars, an easily located ruby among the gems of the night sky. Your correspondent (reporting on location from Earth) plans to contemplate his view of the Sun during the day and the sky near Dawn in the evening.

The spacecraft’s reliable performance has allowed the operations team to devote time to more than ensuring the spacecraft stays healthy and on course. In addition to an occasional moment of waxing philosophical about another of humankind’s robotic marvels being so remote, the team has been preparing for some special activities during the first part of April. Readers not obeying rules of causality already know the outcome, but the rest may look forward to learning the story in the next log.

Dawn is 150 million kilometers (93 million miles) from Earth, or 390 times as far as the moon and as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take almost 17 minutes to make the round trip.

Dr. Marc D. Rayman
8:00 am PDT March 30, 2008

P.S. This log is short so it would qualify for entry in the highly competitive "Best Dawn Log Fewer than 500 Words Written by a Mortal Corporeal Entity" contest. Unfortunately, this postscript causes it to exceed the length limit. That’s particularly disappointing, as the first place prize is a small galaxy.

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

The probe suspends ion thrusting each week to turn from the thrust orientation to bring Earth into the sights of its main antenna. The spacecraft transmits information it has accumulated on the operation of all subsystems and welcomes any new instructions from mission control.

During the communications session on February 15, engineers radioed new data to be used following a reboot of the main computer. These parameters will be applied by the thermal control system to manage the temperatures of some components in the reaction control system (the system of small conventional thrusters that helps the craft orient itself in the zero-gravity of spaceflight). As Dawn journeys farther from the Sun, the ship feels less of the warming rays, so it needs to rely more on its own heaters, and the new values stored in computer memory help accomplish that. In a delightful demonstration of flamboyant irreverence, based solely on this minor change, the software was renamed from 7.0.2 to 7.0.3.

Although the weekly routine is quite adequate to keep the spacecraft content on its interplanetary voyage, the flight plan included a special activity on February 21 and 22. The backup science camera was guided through a set of tests to demonstrate its functions and capabilities. The instrument received its initial checkout in December, showing it to be healthy, but this month’s tests were more demanding. The steps were nearly identical to those executed with the primary science camera, with the only differences being some of the objects that were observed. As the spacecraft has progressed in its orbit, a different region of the celestial sphere was too close to the Sun to be viewed safely by the camera. Among the targets in both sets of tests were the Carina Nebula and the star Vega. As expected, they created the same beautiful views with the backup camera as they did with the primary camera. The results prove that both devices function very well and that the new project in Carina designed to dramatically alter its appearance for observers throughout the Milky Way (believed by many to be an ostentatious attempt to outdo Earth's famed artist Christo) did not achieve its stated ambition. Following the successful completion of the camera tests, the spacecraft resumed thrusting.

Dawn is 104 million kilometers (65 million miles) from Earth, or more than 270 times as far as the moon and 70% as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take almost 12 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 am PST February 29, 2008

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

Dawn 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

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

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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

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

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

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

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