30 Jan 2011
Dear Providawntial Readers,
Dawn continues its flight through the asteroid belt, steadily heading toward its July rendezvous with Vesta, where it will take up residence for a year. The spacecraft has devoted most of the time since the previous log to its familiar routine of thrusting patiently with its ion propulsion. But both the distant craft and the operations team each took a little time away from this month, spending it instead in the future. Although the project has not yet perfected time travel, it has achieved enough capability to conduct two successful ventures into the near future.
On Jan. 10, Dawn performed some of the activities that it will execute in its low altitude mapping orbit (LAMO) at Vesta. From the probe's vantage point in LAMO, much of the sky will be filled by the ancient protoplanet, only about 180 kilometers (110 miles) away. Vesta will appear as large as a soccer ball from a mere 18 centimeters (7 inches). Completing one revolution every four hours in LAMO, the spacecraft will devote most of its time to training its scientific instruments on the rocky world beneath it, teasing out the secrets Vesta holds about the dawn of the solar system. Other times, the robotic explorer will loop around the body while aiming its main antenna at distant Earth, transmitting its findings and receiving new instructions.
As we have seen in previous logs, to control its orientation in the zero-gravity of spaceflight, Dawn usually relies on one or both of its star trackers. Despite the utterly perplexing etymology, these devices track stars in order to help the attitude control system establish the spacecraft's orientation (or "attitude"). The tracker's camera takes pictures of stars five times per second. Its internal computer recognizes patterns of stars, much as you might recognize some of the beautiful constellations visible from your planet and use them to orient yourself at night.
The spacecraft's two star trackers are mounted so each will have a different but clear view of the star-studded sky when the scientific instruments are pointed at Vesta. But during some turns and while the main antenna is pointed to Earth, there will be times when the nearby world will obstruct their views. You too might have difficulty seeing stars if a soccer ball were positioned immediately in front of your face.
For those times when both star trackers end up pointing at Vesta, and thus are unable to provide the attitude control system with the orientation, the probe relies on gyroscopes. Using these spinning masses, attitude control can sense turns and keep track of how the attitude changes even when the star trackers are unable to yield useful information.
While Dawn is in the great void of interplanetary space, there is nothing to interfere with the star trackers. (They are aligned so that neither one points near the sun in any of the spacecraft's normal orientations.) When the ship sailed by Mars for a gravitational boost on its way to the asteroid belt, it used the gyros when the planet blocked the view of the stars. That worked well enough for that brief event, but engineers wanted to confirm that their strategy of swapping from star trackers to gyros would be effective during the much longer events that will occur in orbit around Vesta.
To verify their plans, the spacecraft was configured to operate as it will in LAMO. It pointed its instruments as if Vesta were nearby and rotated to keep them aimed at the surface, just as it will when it circles the colossal body. (The instruments remained unpowered, because this was a test of the attitude control system and, of course, there was nothing for them to observe.) After a time following that pointing profile, Dawn turned to direct its antenna to Earth and held the planet in its radio sights. Following those activities, it conducted a nearly identical pattern. The second time, however, the onboard sequence of commands included a crucial difference. Engineers had incorporated extra instructions to prevent data from the star tracker from being used by the attitude control system when it stopped pointing at the surface of Vesta and started its next turn. This trick in software perfectly mimicked the effect of about 530 kilometers (330 miles) of giant protoplanet blocking starlight. Dawn continued through the sequence without missing a beat. It pirouetted toward Earth and held its orientation there with its gyros, moving with grace and accuracy just as a blindfolded dancer might display on a stage, relying on her vestibular system to accomplish her masterful performance. After two and a half hours, the sequence restored star tracker data.
For the entirety of the test, the spacecraft recorded detailed data on its attitude and on other parameters so engineers subsequently could assess how their plans worked. The analysis showed that the robot's performance was even better than anticipated.
After the successful test, the spacecraft reconfigured for normal interplanetary thrusting and set course again for the real Vesta. Meanwhile, mission controllers were preparing for their own simulation of life in the vicinity of Vesta.
Dawn's exploration of that rocky protoplanet will require much more than a sophisticated probe carrying out its assignments in the forbidding depths of the asteroid belt. The operations team will need to keep it healthy and furnish it with up-to-date plans. In many cases, team members will need to analyze or process data quickly and deliver their results to the next person in line, and each step has to proceed on schedule to keep the mission advancing smoothly and productively. In an undertaking as complex as orbiting a remote, massive, and previously unexplored world, surprises are sure to occur, and some of them likely will be unwelcome. As part of formulating intricate plans for the year Dawn will spend in orbit, the team has developed strategies to account for the unexpected. Some of these plans were exercised this month in an "operational readiness test," or ORT, conducted not in the harsh, alien setting of the asteroid belt but rather in the mysterious, unique environment of JPL.
The Dawn project performed ORTs in 2007 (as described here, there, and elsewhere) as launch grew near and twice last year (one of which was described in July) to prepare for Vesta. Some ORTs focus on the team's ability to conduct the mission as planned, and in others, including the one this month, the team faces problems. Organized and overseen by test conductors (also known as simulation supervisors, sim sups, or more inspired names when they dream up more creative challenges for the operations team), the ORTs are nearly as elaborate as real operations, both in their planning and their execution, and they are very valuable experiences for the participants.
In this ORT, the Dawn team spent a week in the summer, when the spacecraft will be in the approach phase, only a few weeks from its first science orbit, known as "survey orbit." Over the course of the first few days of the exercise, they received more and more bad news. A record-breaking outpouring of radiation from the sun had damaged some of the memory components in Dawn's central computer and degraded its solar arrays. Dawn's unique mission to orbit two solar system targets is enabled by its ion propulsion system, and the near-constant thrusting depends on the uniquely powerful solar arrays. Just as the boost in predicted power in 2009 provided an increase in the time Dawn could spend at Vesta, a decrease now would translate to a shorter residence there before having to depart for Ceres. In addition, the simulated damage to the computer meant that many of the images already acquired of Vesta for navigation could not be recovered and transmitted, and the plans for subsequent storage of engineering and science data would have to be curtailed.
It also turned out that analysis of the earlier images in this rehearsal revealed that the tilt of Vesta's rotational axis was different from what astronomers had calculated from telescopic observations. Vesta, just as all large bodies in the solar system, rotates in a regular fashion around an axis. As we have seen, Dawn will take up a polar orbit around Vesta, the perfect choice for observing all of the illuminated surface. To do so, the orientation of the pole in space needs to be known. In other words, navigators need to know exactly where Vesta's axis points. To understand this, consider the globe of Earth. It is obvious it spins on its axis, but that axis just happens to point near the familiar star Polaris; it equally well could point elsewhere in space (as it has in the past and will again). Measurements from observatories, including the Hubble Space Telescope in 2007 and 2010, have been used to estimate the direction of Vesta's axis, and Dawn will improve upon those during the approach phase so the probe can be targeted to an orbit that takes it over the poles. In the manufactured future of the ORT, it was revealed that the actual orientation of the axis was farther from the prediction than many scientists had expected it might be.
That was not enough for the ever-thoughtful, endlessly creative sim sups. They also informed the team that the faux solar radiation inflicted even greater damage on other interplanetary spacecraft, so Dawn's scheduled use of the Deep Space Network would have to be cut back to allow those other missions to engage in recovery operations. To add to the difficulties, we were astonished to be informed that one member of the team had participated in some quite interesting activities that landed him in prison. Another, after involvement in an incident that merited headlines in some famous tabloids (although, curiously, not the intergalactically read Dawn Journal), also was unavailable to solve technical problems. Other engineers on the team had to fill in to make sure all the work was accomplished.
Still more misfortunes beset the beleaguered operators (who also managed some good laughs over the details provided by the test conductors), but they worked through all the problems, using a combination of the plans they had already developed and creative solutions devised during the course of the ORT. The challenges of these simulated operations in the future were compounded by their being faced when the team also was continuing with real operations in the present.
The actual surprises at Vesta surely will be different from those in the rehearsal (still, your correspondent is going to keep his eyes on those two adventurous team members), but the ORT gave the team an excellent sense of operations in difficult conditions. Although this was the last of the ORTs, more preparatory work remains before Dawn reaches its first intriguing destination.
As the probe continues its journey, less and less of what departed Earth atop a powerful Delta rocket more than three years ago is still onboard. With its famously frugal use of xenon propellant, it was only this month that the fuel gauge dipped to half. Dawn's tank carried 425 kilograms (937 pounds) at launch. It took more than 2.2 years of powered flight to exhaust half of that supply, during which the ion propulsion system imparted the equivalent of 5.7 kilometers per second (nearly 13,000 mph), far more than any spacecraft has been able to change its own velocity.
And yet the adventurer continues to propel itself, gradually maneuvering so its orbit around the sun will match that of its target, an uncharted world that beckons. Paying no attention to the blue-green wake of xenon ions behind it, the explorer's sights remain set ahead, on a destination growing ever closer, on the opportunity to uncloak the mysteries of Vesta, on the new knowledge that it will gain, and on the new questions that it will raise. It seeks not to satisfy but rather to sustain the powerful drive for exploration that compelled curious creatures, humble yet bold, confined to the vicinity of almost incomprehensibly distant Earth, to reach within themselves that they might then reach out into the cosmos.
Dawn is 6.1 million kilometers (3.8 million miles) from Vesta, or 16 times the average distance between Earth and the moon. It is also 2.78 AU (416 million kilometers or 259 million miles) from Earth, or 1,065 times as far as the moon and 2.83 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.
Dr. Marc D. Rayman
11:30 pm PST January 30, 2011