by Marc D. Rayman
Pasadena - July 4, 2000 - Following a rescue mission of cosmic proportions, Deep Space 1 is in powered flight again with its ion propulsion system gently but persistently pushing the spacecraft along towards an encounter with comet Borrelly in September 2001.
The whole story leading up to the resumption of ion thrusting deserves special mention in the history of space exploration. Indeed, in the admittedly highly biased but reasonably well informed opinion of your faithful correspondent, the entire undertaking has been one of the most challenging yet one of the most successful and impressive robotic space rescues ever accomplished.
A great deal of difficult work lies ahead, but providing DS1 with a new lease on life represents an outstanding achievement in NASA's efforts to extend humankind's reach into the cosmos.
First, a quick recap for the seven species new to this popular source of information: Two months after the end of its extremely successful primary mission, Deep Space 1's star tracker, which was responsible for determining the probe's orientation in the zero-gravity of space, ceased operating.
By that time, DS1 had already achieved more than it set out to do, so it could have been retired to rest on its many laurels. But engineers devised a way to restore the craft's sense of direction by writing new computer programs to use the camera instead of the star tracker.
Previous mission logs, still the focus of admiration in two of the spiral arms of the Milky Way Galaxy, have described the extraordinary difficulty of this job.
The challenge was made still greater when the team elected to aim for the grand prize of being ready to resume thrusting in July in time to give DS1 a chance to encounter a comet in September 2001.
The new system that has allowed Deep Space 1 to regain full three-dimensional control and knowledge of its orientation works in part by taking pictures of a reference star.
As the spacecraft rotates, the reference star appears to drift away from the center of the camera's view, so the computer programs analyze the pictures and determine how to reorient the craft to bring the star back to its intended location.
This provides the means to keep the spacecraft stable. You can imagine a similar situation in a more terrestrial setting. Suppose you were in a boat in the middle of the ocean, with no references such as land or the Sun.
Because of Earth's gravity you can distinguish up from down, but you cannot find any of the points of the compass. In the same way, once the star tracker failed, DS1's Sun sensor allowed it to know the direction toward and away from the Sun, but it could not sense other directions in the cosmic ocean. Now suppose you wanted to travel a certain direction: you would not know how to steer your craft. But if you had a telescope mounted on your boat and you could locate a star in the direction you wanted to go, you could keep looking through the telescope as you journeyed along to make sure you maintained your desired heading.
If you cruised for a while without checking it, you might find that you had begun drifting in a slightly different direction, but you could correct your course when you checked on the star. DS1 uses its camera in the same way, by holding a star in its view.
Following the rapid pace of ambitious and successful testing reported in the June 18 mission log, the team continued by evaluating how well the new system could work to provide a stable platform for thrusting with the ion propulsion system.
On June 21, with the spacecraft locked on a star in the view of the camera, after more than 7 months of dormancy the ion propulsion system obediently began thrusting.
The attitude control system, whose on-board task of controlling the spacecraft's orientation was made so difficult when the star tracker failed, not only has to keep the spacecraft steady while the ion propulsion system is firing, but it actually uses the ion drive to stabilize the spacecraft's rotation whenever it is thrusting.
After testing this in the Deep Space 1 simulators located on Earth, controllers were eager to see how well it would work on the real spacecraft.
They were rewarded when data transmitted from the tremendously distant probe revealed that it was operating exactly as it was supposed to.
Engineers have devised a new way to chart a course from where DS1 is now to where it will reach the comet by thrusting with just a few selected stars in the camera's view during the next year. I like to call these celestial references "thrustars".
When we want it to aim the main antenna at Earth, we choose a reference star that is in just the right position so that when the camera is pointed at it, the antenna faces Earth. This is known as an Earthstar.
The Earthstar that is used changes as the spacecraft and Earth gracefully perform their separately choreographed orbital ballets around the Sun.
On Friday June 23, DS1 turned through a large angle to point to the first thrustar. During the few minutes that the spacecraft is turning, it can't hold a star in the camera's view, so it relies on its gyros. (Instead of old-fashioned rotating mechanical devices, DS1's gyros use laser light traveling through loops of optical fiber.)
When it turned away from the Earthstar, the antenna was not pointing at Earth, so it could not communicate with the home planet of its controllers. But following instructions stored on board, it found the thrustar, locked on, and remained there for about an hour.
Then it turned back to the Earthstar and returned the data it had accumulated while it was away. This allowed engineers to verify that it could indeed find the thrustar, and yielded an early opportunity to determine whether any adjustments were needed in the myriad settings used to operate the new system before it would be on its own for a longer duration.
Following further tests over the next few days, on Wednesday, June 28 DS1 turned back to the thrustar, brought its ion engine to the maximum throttle level it could support at this distance from the Sun, and began a week of thrusting.
It is scheduled to turn back to a new Earthstar on July 5 and report on its week of thrusting. If the data look good, a few days after that it will resume thrusting on the same thrustar.
The extremely ambitious schedule the operations team had set early in June did not have DS1 thrusting toward the comet until July. Given the difficult and complex work that had to be completed in such a short time, it is remarkable that everything went so smoothly that the team could get ahead of schedule, accomplishing such a great deal in the face of such a great challenge. Truly, this rescue is one for the history books.
Deep Space 1 is now 2.1 times as far from Earth as the Sun is and more than 820 times as far as the moon. At this distance of nearly 315 million kilometers, or over 195 million miles, radio signals, traveling at the universal limit of the speed of light, take more than 35 minutes to make the round trip.
DEEP SPACE ONE
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