By Dr. Marc Rayman
JPL - August 1, 1999 - Deep Space 1 successfully accomplished an extraordinarily challenging encounter with asteroid Braille late Wednesday evening.
As many readers will know, reaching this tiny asteroid was a bonus. DS1 has already been remarkably successful in its primary mission of testing 12 advanced technologies that will allow many future missions to explore the solar system.
Prior to this encounter, very very little was know about asteroid Braille. In fact, it is so small and distant that astronomers had had great difficulty even in pinning down its location.
So beginning on Sunday July 25, after flying for 9 months toward the asteroid, and less than 4 days before getting there, AutoNav began taking pictures to try to find it.
As expected, the little asteroid did not show up in the first pictures. On Monday, a faint image began to appear, so vague that AutoNav was not able to detect it, but more sophisticated processing of the images on Earth revealed the asteroid.
It turned out to be over 400 kilometers, or more than 250 miles, from where it was expected it to be. Because time was getting short and the small asteroid still did not show up in AutoNav's analysis of the images, the DS1 navigation team estimated from the pictures where the asteroid was.
Then they used the same computer programs that AutoNav has on board to design a course correction that was radioed to DS1. Then with less than a day and a half to go, using that information just as if it had designed the course correction itself, AutoNav turned the spacecraft and ordered the thrusters to be fired. The activity went well and put the spacecraft on course for the new estimated location of Braille.
As DS1 continued to speed toward Braille, it continued to take navigation pictures. But the dim asteroid refused to show up in AutoNav's analysis of the images.
Finally, early Wednesday morning, only 17 hours before arriving, AutoNav was able to lock on to Braille. But before it could design its own course correction, a problem arose on the spacecraft.
A small bug in the extremely complex software manifested itself. AutoNav has a record of its locations at different times, and it uses this whenever it computes its orbit.
As it updates the information, it drops the oldest information. Because it has been trying to zero in on Braille and has been making course corrections more frequently than usual, it recently accumulated a larger set of information than normal.
In essence, it tried to process too much data on Wednesday morning. Anyone who has used a computer knows that sometimes the programs run into trouble, and the computer needs to be restarted to clear it. Well a more sophisticated response occurred when DS1's computer detected this problem and allowed protective software to stop all spacecraft activities, reboot the computer, turn off nonessential devices, and place DS1 in a predefined safe configuration known to the operations team as Sun standby SSA.
The spacecraft turned to point at the Sun (the only easily identifiable landmark from anywhere in the solar system), used a back-up antenna, and awaited help from Earth. A shocked operations team discovered around 5:30 Wednesday morning that the spacecraft had entered standby, and the encounter with Braille was going to occur Wednesday evening.
Working astonishingly quickly, the team sent commands to the spacecraft to return it to its normal operational state. The spacecraft was instructed to turn to point its main antenna at Earth. The combination camera and spectrometer was turned back on.
The advanced ion and electron spectrometer, which measures charged particles in space, has high voltage power supplies that use up to 8000 volts. It has to be turned on slowly and delicately, but it was brought back to its correct settings.
The suite of sensors designed to monitor the electric and magnetic fields of the ion propulsion system had been reprogrammed to try to detect the asteroid. But when they were turned off, they reverted to their old program, so the team reloaded the newer software.
But what about further course corrections? When the computer rebooted, it deleted the results of AutoNav's detection of the asteroid. So now with only a few hours to go, it was not on an accurate enough course to reach the asteroid, and while the operations team was restoring DS1 to its normal configuration, AutoNav could not design a new course correction.
But from 116 million miles away, the ever-resourceful team was able to recover from the spacecraft's memory 3 of the pictures AutoNav had taken and analyzed before the problem arose. They were transmitted to Earth, analyzed, and used to compute the final course correction.
The information was transmitted back to the spacecraft in time for AutoNav to take over. Working against the clock the entire time, the operations team sent the last command to the spacecraft with only 4 minutes to spare before it had to turn its antenna away from Earth to execute the course correction.
In fact, while all this was going on, DS1 continued rushing toward Braille at a speed of over 9.6 miles every second. From the time the spacecraft entered standby until the last command was sent by the operations team, the spacecraft moved well over half a million kilometers or more than 300,000 miles closer to Braille. That's more than the distance between Earth and the moon.
Once DS1 turned, the operations team could only watch anxiously. As planned, the spacecraft returned only very limited data during the flyby, as pointing its camera at the asteroid meant that it could not point its antenna at Earth. After such a complex and challenging day, everyone wondered whether DS1 was really ready to encounter the asteroid.
AutoNav carried out its final course correction flawlessly just 6 hours before reaching Braille. Following that it resumed taking distant images to assure that it could point the camera as it got in still closer.
It tried and succeeded four times, completing this process 1 hour and 10 minutes before its closest approach to the asteroid. Still at that time, it was 65,000 kilometers or over 40,000 miles away.
Then 28 minutes before encounter, AutoNav switched to a different mode and used a different part of the camera that was designed for use closer to celestial bodies. But the asteroid did not show up as expected, so AutoNav received no new information to analyze.
The mystery of why Braille could not be detected is still being studied. But it is likely because the asteroid was much much darker than astronomers had predicted or because of an unknown behavior of this part of the camera with a dim object.
In any case, when DS1 got close to the asteroid, although AutoNav knew approximately where the asteroid was, its information was not sufficient to point the camera accurately enough to capture close-in images. It was adequate however to acquire infrared data, and DS1 also collected all the data desired with the ion and electron spectrometer and with the other sensors.
Images at the moment of closest approach were never planned, so these other data could be collected. After closest approach to the asteroid, the spacecraft turned over to look back, as AutoNav switched back to its normal mode. It correctly pointed the camera and captured images about 15 minutes after its closest approach to Braille.
This was a remarkable day for the team that has been overseeing a little spacecraft that has performed such a big job. DS1 accomplished the closest flyby ever of an asteroid, obtained important new information on the performance of AutoNav and the camera, acquired all the data desired from the instruments to measure charged particles and magnetic and electric fields, and collected infrared and black and white images of the asteroid.
Of course it would have been fun to have a detailed picture of asteroid Braille, but all in all it was a spectacular finale to an incredible mission!
It took the spacecraft about a day to transmit all of its data back to Earth. Scientists and engineers are analyzing the data now, and the results will be announced this week. This recording will be updated next weekend with the findings.
DS1 exceeded expectations in its testing of technologies, and now has an exciting encounter with an asteroid to its credit. The primary mission officially ends on September 18. NASA is considering extending the mission. There are many wonderful missions to carry out, but NASA only has limited funds, so it has not yet been decided whether DS1 will be allowed to continue operating.
But in case NASA does chose to continue the mission, DS1 is ready for quite an encore. On Friday, the reliable ion propulsion system, under control of AutoNav, fired up again. The sturdy ship is now on course for an encounter with two comets in 2001.
Using its advanced technologies and the experience from flying by Braille, DS1 will be well suited for this exciting new mission. Next week's recording will describe why these two comets are unusually interesting.
Deep Space 1 is now about 28% farther away from Earth than the Sun is and 498 times as far as the moon. At this distance of almost 191 million kilometers, or nearly 119 million miles, radio signals, traveling at the universal limit of the speed of light, take over 21 minutes to make the round trip.
Deep Space 1 Reports From Spacer.Com
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