Galileo Sprints Toward a Dramatic Finish
27 Feb 2003
(Source: Jet Propulsion Laboratory)
This Year on Galileo
November 5, 2002 - September 20, 2003
The Story So Far...
As usual for the combination of an aging spacecraft and an intensely energetic environment, Jupiter dealt Galileo a temporarily crippling blow during our flyby of Amalthea on Tuesday, November 5, 2002. Approximately 17 minutes after zipping by the tiny satellite at over 18 kilometers per second (41,000 miles per hour), as the spacecraft neared its closest approach to the giant planet, the intensity of the radiation caused a failure in computer circuitry that handles timing of the events on the spacecraft. This caused the computer to switch to a set of backup circuitry, which is a serious enough change to warrant the computer to declare an emergency, shut down operations, and phone home for help. Even in this relatively quiet state for the spacecraft, the radiation environment was still raging, and several additional faults triggered repeated software requests to place (or in this case, keep) the spacecraft computers in safe mode.
Once Galileo had cleared the depths of Jupiter's radiation field, engineers could start wading through the flood of error messages received to determine which ones represented temporary conditions, now passed, and which might represent permanent failures in spacecraft systems. Surprisingly, with the exception of the switched timing circuit, there appeared to be no hard failures! Indications were that we had successfully captured two full tracks of recorded science data, including the orbiter instruments' first taste of the environment well inside Io's orbit. It appeared that the spacecraft systems, though showing expected additional wear and tear due to the radiation exposure, were all still in operating condition! Computer processing, telecommunications, and attitude control were still providing the heartbeats of a working spacecraft.
Then it was time to begin to retrieve the digital gold stored on the tape recorder, and that was when the headaches returned. On Friday, November 8, 2002, test commands were sent to the recorder to attempt a short movement of the tape. Engineering measurements indicated that the tape did not move, and the signs pointed to a different problem than the sticky tape that has affected operations in the past. Over the next month diagnostic tests were run both on the spacecraft and on the ground that convinced engineers that the problem was most likely a radiation-induced failure in light-emitting-diode circuitry in the motor control of the recorder. Radiation experts suggested that time spent away from the radiation environment, as well as running current through the circuits without moving the tape, might help anneal the circuitry and allow a return to operations. More tests were performed on the spacecraft, during which over 111 hours of current was applied to the circuits, and eventually limited motion was restored! (Imagine a doctor trying to diagnose an illness when the patient is a half a billion miles away, and he has to wait at least an hour and a half between asking a question and receiving an answer! This is the nature of deep-space operations.) Though not fully operational, enough tape motion was possible to allow the playback process to return data. Finally, on Thursday, December 12, 2002, with careful control of the amount of motion allowed, playback of the recorded science data began.
Throughout this tape diagnostic period, standard maintenance of other on-board systems continued. On November 15, December 6 and December 27, the propulsion system was exercised to keep the lines cleared. On November 14 and January 7, tests of the gyroscopes were performed. These activities were in support of the final attitude change in the mission, on January 15, 2003. This turn was also the largest that Galileo has performed in three years, changing the pointing of the spacecraft by 18 degrees. At this final attitude, the communications antenna is now pointed in a direction that will be only 2.5 degrees away from Earth in mid-September 2003, when Galileo next and finally encounters Jupiter.
As part of final cleanup from the safing activities of early November, on December 16 and 18 the Plasma Subsystem and Energetic Particle Detector were turned on and configured for science data acquisition. These two instruments were turned off in response to the problems encountered in the intense radiation bombardment.
Since very few communications passes are scheduled with the giant antennas of NASA's Deep Space Network between now and September, on January 15, the spacecraft was instructed not to worry if it doesn't hear from ground controllers.
The Story Yet To Come...
Playback of the recorded Amalthea and Jupiter radiation environment data will continue until Friday, February 28. At that time, the playback process is stopped, and the tape recorder, that workhorse of data collection and return for Galileo over the past seven years, is consigned to a well-deserved retirement.
With playback completed, the Fields and Particles instruments (Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Subsystem, and Plasma Wave Subsystem) are configured to send their data directly to Earth in real time, and the spacecraft continues on its long, slow loop away from Jupiter before returning for our terminal encounter with the giant planet in September.
At this time, the high level of spacecraft monitoring via the Deep Space Network antennas that has characterized the past thirteen years drops to one contact per week, just enough to verify the health and status of the craft, and to verify that it is still on the correct trajectory. With the exception of a few flight controllers, the flight team, which once numbered in the hundreds, has moved on to other projects, other jobs, other lives.
The distant orbital loop takes the spacecraft farther from Jupiter than it has been since before entering orbit in December 1995. On April 14, Galileo reaches 370 Jupiter radii (26.4 million kilometers or 16.4 million miles) from the planet. This is about 1/6 the distance from Earth to the Sun, and light takes nearly a minute and a half to travel from Jupiter to the spacecraft!
During the summer months, as the Earth proceeds in its own orbit about the Sun, Jupiter, with Galileo in tow, appears to pass behind the Sun, an event known as Solar Conjunction. This limits our ability to hear from the spacecraft, due to interference from the Sun's turbulent atmosphere. Between Monday, July 28 and Monday, September 15, the radio signal from Galileo changes to put more power into the carrier signal, giving the ground antennas a better chance to receive the signal.
Between Tuesday, August 11, and Monday, September 1, the spacecraft is within 7 degrees of the Sun as seen from Earth, and communications of any sort are not expected. The spacecraft appears to be closest to the Sun on Friday, August 22, when the separation between the two is only 0.83 degrees.
On Thursday, September 18, Galileo is again streaking in towards Jupiter, and reaches 50 Jupiter radii (3.6 million kilometers or 2.2 million miles) from the planet. Finally on Saturday, September 20, just before 6 p.m. Pacific Daylight Time, Galileo is just over 18 Jupiter radii out, and a scant 19 hours before impact with the clouds.
For the conclusion of Galileo's trek through the solar system, tune in again in early September!
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page: http://galileo.jpl.nasa.gov