Galileo Status Report
PUBLIC INFORMATION OFFICE JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Now in the home stretch toward Jupiter, NASA's Galileo spacecraft is one month and 24 million kilometers (15 million miles) from the giant planet. On Dec. 7, Galileo's atmospheric probe will plunge into Jupiter's swirling clouds to conduct its 40- to 75-minute mission directly measuring the weather and makeup of Jupiter. On the same day, the Galileo orbiter will fire its main rocket engine to enter orbit around Jupiter, to conduct two years of detailed studies of the Galilean moons, the planet and the magnetosphere.
The balky spacecraft tape recorder which failed to rewind properly on Oct. 11 has been tested and found operable. Because of that problem, however, team members have decided to use the tape recorder in a very conservative fashion during the Jupiter approach and arrival. Detailed study of engineering data from the spacecraft indicates that the tape recorder may be unreliable under some operating conditions, project officials said. The problem appears to be manageable, however, and should not ultimately jeopardize return of the more than 1,000 images of Jupiter and its moons that are to be stored on the recorder for playback over the course of Galileo's two-year tour in orbit around the giant planet.
On Oct. 24, the spacecraft executed commands for the tape recorder to wind 25 extra times around a section of tape possibly weakened when the recorder was stuck in rewind mode with the tape immobilized for about 15 hours. Due to uncertainty about its condition, spacecraft engineers have declared that portion near the end of the tape reel is "off-limits" for future data recording. The extra tape wound over it secures that area of tape, eliminating any stresses that could tear the tape at this potential weak spot. Unfortunately, the approach image of Jupiter that Galileo took October 11 is stored on the portion of tape that is now off-limits, and will not be played back. Since the tape recorder incident, Galileo project officials have decided to not to take pictures of Io and Europa on the day the spacecraft arrives at Jupiter. Instead, they will devote the tape recorder that day to gathering data from the Galileo orbiter's unique passage through the Io plasma torus and the unique parachute descent of the Galileo atmospheric probe.
The tape recorder is a key link in techniques developed to compensate for the loss of use of Galileo's high-gain antenna, which is stuck in a partially open position. Data must now be sent at a much lower data rate through Galileo's low-gain antenna. The tape recorder is to be used to store information, particularly imaging data, until it can be compressed and edited by spacecraft computers and radioed back to Earth.
The great interplanetary dust storm Galileo observed in recent months has apparently abated. Galileo's magnetometer, extreme ultraviolet and dust instruments have continued active measurements of the environment near Jupiter, sending their data by way of computer memory readouts. In addition, the extreme ultraviolet instrument has begun mapping the plasma torus of sulfur and oxygen ions emitted by Jupiter's satellite Io. These activities will continue through the Jupiter approach, but images and other data requiring high-rate tape recording will be deferred until after the spacecraft is in orbit and the probe data have been played back.
The spacecraft is spinning at about 3 rpm, operating normally and transmitting science and engineering data at 10 bits per second. It is 901 million kilometers (560 million miles) from Earth, and its speed in orbit is 6.5 kilometers per second (about 14,500 miles per hour).