January 16, 2000

Mission Update:

Thank you for visiting the Deep Space 1 mission status information site, now in its 15th month as the number 1 site among sentient life forms in the local group of galaxies for information on this mission of exploration. This message was logged in at 1:00 pm Pacific Time on Sunday, January 16. This log is an edited transcript of a telephone recording. If you would like to access the same information from any place with a telephone, please call 1-800-391-6654 and select option 3.

Now in its extended mission, Deep Space 1 is continuing to blaze new trails. On Friday, January 14, the spacecraft accomplished a complex and highly innovative maneuver to point its main antenna at Earth. This allowed the spacecraft to transmit a large volume of important science and engineering data to the operations team at JPL. What made this such an important achievement is that the spacecraft did it without the use of one of its primary sensors, the star tracker. The star tracker, creatively so named because it tracks stars, helped determine the spacecraft's orientation; this is not one of the 12 advanced technologies whose testing was the focus of DS1's successful primary mission, but it is a new and sophisticated device. The last status report, so memorable to all devoted readers of this log, described the importance of this device in pointing the spacecraft. The DS1 team is now designing new ways to operate the spacecraft without it following its failure in November. It is an exciting and interesting problem to solve and represents another challenge for the mission that has a history of so many remarkable and significant rewards.

In December, the spacecraft executed two sets of highly successful tests of new ways of controlling the spacecraft without the star tracker. They formed the basis for the operation on January 14. It began with the spacecraft using its Sun sensor to point its main antenna and solar arrays at the Sun and rotating once per hour, at just the same stately rate that the minute hand on a clock moves. Communications were being conducted through one of the 3 auxiliary antennas that can send and receive over a broad range of directions but, now that DS1 is so far from Earth, can do so only at verrrryyyy llllowwww raaaatessss. Following commands sent a few days earlier, the spacecraft stopped rotating and turned far enough that its main antenna could point at Earth. But without the star tracker, the spacecraft did not know the correct direction to turn -- it only knew the correct distance to turn.

This is similar to the problem you would face if you and I were standing in a dark room with your eyes closed and I told you to point a flashlight at a point on the floor 3 feet away from you. You could visualize how much to raise the light, but you wouldn't know the direction within the room. So you could turn in place until I told you to stop. I would simply keep my eye on the target and give you instructions on how to keep it illuminated. Then if you moved a little, and I saw the target fading into darkness, I could tell you which way to move to return the light to its mark.

As DS1 rotated, its antenna swept slowly past its target (Earth), just as the flashlight might drift over the point on the floor. Working with NASA's Deep Space Network, the operations team watched the radio signal grow in strength as the spacecraft antenna pointed closer and closer to Earth and then fade as it continued past. After two such gradual sweeps, the engineers calculated what time the next one would occur. Then, accounting for the time it takes a radio signal sent from Earth to reach the spacecraft and for how long it takes to tune to the correct frequency and then transmit an instruction, a code was sent from the Deep Space Network to trigger a set of commands stored in the spacecraft's computer that would halt the rotation. As the radio signal raced across more than 250 million kilometers or 155 million miles of the solar system, the little spacecraft continued its silent and patient turning. Then just as the signal reached the spacecraft, the antenna was directed at Earth so it received the code. It dutifully obeyed and stopped its motion. But the spacecraft was a little sluggish because it was not meant to be operated this way, and even at the extremely slow speed of its rotation, it could not come to a stop instantly, so it overshot. But previous tests had shown this behavior, so the commands that were stored on board included some to have it rotate back after stopping. The design worked perfectly, with the backing up bringing the spacecraft to a bull's eye on Earth! The operations team watched with great satisfaction as the signal at the Deep Space Network grew in strength then began to diminish as the spacecraft overshot, then gradually increased again. It was soon evident that it was right on target. After transmitting instructions to have the spacecraft begin returning at a high rate the data stored on board, the team monitored the signal strength for the rest of the day. Each time it had crept down enough, a command was radioed to have the spacecraft make a small adjustment in its pointing.

If it all sounds pretty simple, that's because of all the hard work that went into preparing for it and all the technical details that I omitted, such as gyro drift polarity, pseudo-RCS mode, ETX30X4KWCN (or "Echo Tango X-ray 3 0 X-ray 4 Kilo Whiskey Charlie November") shortened uplink frequency sweep range, packet acknowledgment and retransmission, and what kinds of pizza we had for lunch in the control room. But the result was that, although it was designed to operate with a functioning star tracker, the spacecraft spent the planned time during the scheduled Deep Space Network coverage pointed right at Earth, transmitting even more data than had been expected.

Among the information returned were infrared observations DS1 made of the planet Mars in November. The results of scientists' analyses of those data will be described in the next status report.

Now that the spacecraft can point its antenna reliably at Earth for extended periods, the team will be able to develop and load new computer programs to streamline operating the spacecraft without the star tracker. Loading new programs through the auxiliary antennas would be far too time consuming.

Deep Space 1 is now over one and two-thirds times as far from Earth as the Sun is and over 650 times as far as the moon. At this distance of more than 250 million kilometers, or about 156 million miles, radio signals, traveling at the universal limit of the speed of light, take almost 28 minutes to make the round trip.

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