NASA's Galileo spacecraft is proceeding toward its December rendezvous with Jupiter, with spacecraft engineers greatly relieved at last weekend's test results showing that its onboard tape recorder remains functional.
On Tuesday, Oct. 24, a revised spacecraft command sequence radioed to Galileo began issuing instructions ordering the spacecraft to resume regular readouts of data from the memories of several science instruments. The spacecraft also returned to normal housekeeping duties, executing scheduled engineering operations such as flushing of rocket thrusters.
The new command sequence replaced the one ground controllers stopped after the October 11 tape recorder problem in which the data tape recorder failed to cease rewinding after recording an image of Jupiter.
The tape recorder had remained in a standby mode until Friday, Oct. 20, when it was tested and proved still operational. Detailed study of engineering data from the spacecraft indicates that the tape recorder can be unreliable under some operating conditions, project officials said. The problem appears to be manageable, however, and should not jeopardize return of the nearly 2,000 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.
Tuesday's work on the spacecraft included 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 won't be played back.
With just weeks to go before Galileo's Dec. 7 arrival at Jupiter, project engineers are busy analyzing the tape recorder's condition to fully understand its capabilities and weaknesses. "We need to be sure we fully understand the system that we have now," said Galileo Project Manager William J. O'Neil.
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.
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 Galileo's Jupiter atmospheric probe as the cone-shaped probe dives into the giant planet's swirling atmosphere over the course of the 75- minute descent mission. During that time, the probe will collect the first-ever direct measurements of the chemical makeup and weather of the solar system's largest planet.
"Our priorities are clear," said O'Neil. "We have to get all the probe data." Other flybys of the Jovian moons, including frequent "volcano watch" monitoring of Io, occur throughout the mission, giving ample opportunity to collect data on all the moons. Late in the mission, O'Neil said, a close Io flyby might be made to make up for the close Io flyby data that will be sacrificed December 7.
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