by Marc D. Rayman
Pasadena - June 21, 2000 - Deep Space 1 has a new lease on life, as the mission has had remarkable success with one of its greatest challenges.
For the first time since the spacecraft's critical star tracker stopped operating in November, DS1 has a complete and accurate knowledge of its orientation in space and is firmly controlling the direction it points.
Through wonderfully creative engineering, modern technological magic, and a great deal of hard work, this little solar system tourist is being restored by controllers hundreds of millions of kilometers away!
As all Deep Space 1 enthusiasts know, the craft's star tracker, which was responsible for determining DS1's orientation in the zero-gravity of space, failed two months after the end of the enormously successful primary mission.
The natural step to have taken then would have been to terminate the extended mission, with the recognition that DS1 had already accomplished more than it set out to do when it departed from Earth.
But JPL engineers devised a plan that included using the camera to replace the star tracker. The previous mission log, already legendary in dozens of galaxies, illustrated some of the difficulty of this job as well as the motivation for trying to do it so quickly.
After a blindingly fast schedule of designing the new system, working out myriad details, writing the computer programs, testing them with the ground simulation of the spacecraft, and planning techniques of flying the spacecraft and conducting the mission with the new system in place, controllers began radioing the software to DS1 on May 30.
The software was broken into 90 separate files, each of which was transmitted from one of the 34-meter (112-foot) diameter antennas of the Deep Space Network to the tiny craft farther away than the Sun.
Receiving these faint signals was the spacecraft's main antenna, which is only 30 centimeters (just one foot) in diameter; without the star tracker, that antenna could only be pointed to Earth using the innovative method the DS1 team developed earlier this year.
On June 3, when 81 of the files were on board and thousands of people were at JPL for its annual Open House, the software already running on the spacecraft had a brief problem that caused it to resort to its standard instructions of taking action to make sure the ship remains safe in the event of any unexpected occurrences.
The software reoriented the probe, pointing the main antenna back at the Sun, and rebooted the computer, as it has under other circumstances in the mission.
But by doing so, it deleted all the new files that had been sent to it. During the course of the day, the operations team quickly assessed the situation, determined that the spacecraft was healthy and could return to work, and restored it to its normal operating configuration.
The drama of the rapid and skillful work of the team, conducted in the DS1 mission control room, was unknown to the visitors at JPL.
Facing now an even tighter schedule after losing all the files, DS1 was granted some extra tracking coverage by agreement of the Deep Space Network and some of the other missions using that unique system.
The dedicated team redoubled its efforts and managed to transmit all 90 files and still be ready on schedule for the big step of running the new software on June 8.
Building upon the experience gained during 3 software replacements during the primary mission, the team prepared the spacecraft, commanded it to reboot, verified that the new software was indeed running smoothly, and reconfigured it for further operations.
But the hard work was far from over. Now the sophisticated new functions have to undergo complex testing in a very very short time. Our goal is to try to ready the spacecraft to resume long-duration thrusting with its ion propulsion system in July.
Controlling a spacecraft from across the solar system always requires great care. Keeping that in mind while following an unusually aggressive approach, the operations team quickly began systematically activating and evaluating the new capabilities.
Each major step requires designing and executing one or more tests in the ground simulator, analyzing the results and accounting for the differences between the simulated and the real spacecraft, translating the ground test inputs into commands to be sent to the spacecraft, preparing plans to protect it in the event that the new software does not behave as expected, transmitting the commands, observing the data as they stream back to mission control, making a prompt determination of whether the spacecraft is still healthy, and then analyzing the data in more detail in preparation for the next test.
After exercising a number of esoteric but important modes and functions, with names like Sun_standby_SSA level 3 and gyro bias estimation, perhaps the biggest step was taken on June 12, when the spacecraft was instructed for the first time to lock on to a star.
This is so important because the spacecraft needs two points of reference to both know and control its orientation. It has a sensor to allow it locate the Sun, which is the only easily recognized target for this solar system traveler, but that is not enough.
In the same way, if you are in a dark room, you know from the feel of gravity which way is up and down. But without some other reference, such as a compass or a view of familiar landmarks, you don't know which way north is.
The operations team calculated approximately where the camera would be pointed when the spacecraft's antenna was directed at Earth, so we knew which stars would be near the camera's view.
Using its new functions, the spacecraft started taking pictures, analyzing them to pick out any stars (and correcting for such distractions as cosmic rays and scattered light), and moving through a preset pattern until it found a star that met certain criteria.
This involved a great deal of complexity, and relied on communications among various systems on board including the attitude control system, the camera, and the autonomous navigation system, whose expertise in analyzing pictures from the camera was so aptly demonstrated during the primary mission.
When it found a star, the spacecraft stopped its search and used that star to remain fixed. The team was ecstatic that on the first try, the spacecraft performed so many functions so well.
It is still too soon to know how well the system will perform under a wide variety of conditions and over a long time, but the preliminary results are encouraging.
After locking to the star early Monday afternoon, the spacecraft remained locked all week, as the team conducted further technical tests and exercised more functions, including commanding DS1 to turn to and lock on to a different star.
Preparations are now underway for testing to determine whether the new system can provide a stable platform for thrusting with the ion propulsion system.
Even if the new software were not working so smoothly, the accomplishments of the past few months would be admirable indeed. But now, in addition to the great reward of seeing the intensive work beginning to prove so successful, this enterprise is, quite frankly, very exciting. There is nothing quite like being in control of a tiny probe so far from our home planet.
Deep Space 1 is now over twice as far from Earth as the Sun is and 800 times as far as the moon. At this distance of nearly 307 million kilometers, or over 190 million miles, radio signals, traveling at the universal limit of the speed of light, take more than 34 minutes to make the round trip.
DEEP SPACE ONE
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