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...Or make that 1850 feet of quarter-inch Mylar tape.
Testy, troublesome, balky, stiff, sticky---these are words used to describe Galileo's tape recorder (see photo), which stuck just minutes after it took its first color picture of Jupiter. The incident occurred on October 11, just 8 weeks before Galileo's scheduled arrival at the giant planet. Scientists first learned of the problem when the recorder signaled its controllers that it had failed to stop spinning after a command to rewind. In a frightening coincidence that occurred within hours of the onboard recorder problem, a duplicate testbed recorder at JPL failed during an unrelated routine trial run. It was later determined that this recorder had torn the tape from its reel. Galileo's team knew that if this had happened in space, the Galileo recorder would be useless.
The shoe-box-sized reel-to-reel recorder works something like a VCR. It was designed and built by the California-based manufacturer, Odetics. Odetics is an active partner with JPL in the investigation of the anomalies. Similar mechanical recorders have been flown on Magellan, the Compton Gamma Ray Observatory, and other spacecraft without this particular problem. (Newer NASA spacecraft like Cassini, now under construction, will carry two modern solid-state recorders rather than a mechanical reel-to-reel recorder.)
Over the next 9 days, both spacecraft data and the JPL testbed lab recorder were investigated. A number of problems were identified that could possibly explain the anomaly. Finally, the onboard recorder was ready for testing on October 20. The news was good. The device played back data as requested, leaving investigators to conclude that the tape had remained at the end of the recorded image of Jupiter for the 15 hours that the capstans spun instead of rewinding. As a precaution, the tape was wound forward 25 extra turns, burying the possibly damaged section on the reel, but sacrificing the October 11 Jupiter-approach image.
Galileo Project Manager Bill O'Neil did his best to bolster confidence that the mission's primary objectives could still be met even if the recorder proved unusable at some future date. "There's a whole lot more we can do than we imagined before we looked at this in detail," said O'Neil, predicting that even without the recorder, at least half the mission's objectives could still be accomplished. Data from remote sensing instruments would be the most impacted, including photo images and spectral data. But much of that data could be rerouted directly to main flight computer memory storage areas, although the memory in the computer is nowhere as large as that of the recorder. "You might be surprised, but people are not real down in the mouth," said O'Neil at the height of the crisis. "Everybody is just doing everything they can to get through this."
In the interest of reducing risk to the probe's 75-minute atmospheric relay mission on arrival day (December 7), the planned arrival day high-resolution close-ups of two of Jupiter's most tantalizing moons, Io and Europa, had to be given up. Galileo came within 900 kilometers of Io as it brushed past on encounter day, and many scientists were saddened by the lost opportunity for images. However, Galileo did capture important fields and particles science data within the Io Plasma Torus. Recent Hubble photos have shown new volcanic activities on Io and a faint oxygen atmosphere on Europa. "I would be disappointed if I were a scientist," admitted Don Ketterer, Galileo's NASA Headquarters Program Manager. "But I would look to what I was going to get in the future."
Although the recorder performed flawlessly during the encounter and probe-relay mission, it stuck again on January 18 in transition from forward on track 1 motion at 100.8 kbits/s to reverse at 7.68 kbits/s on track 4. The tape was not recording data at the time but was performing the first of five planned conditioning tests. "This looks consistent with the theoretical model of the tape sticking to the guide head, except that we were very surprised the tape stuck when it was stopped for just a few seconds," said Bill O'Neil. This time Galileo engineers knew what to do because they had been working with a lab model of a sticky recorder. As expected, the tape pulled free when engineers commanded the recorder to advance the tape. The whole incident was resolved in two days, although Galileo project members are still evaluating the implications of this second incident.
The tape recorder was originally included on the spacecraft as the backup to the high-gain antenna for the probe mission. Before the high-gain antenna failed to deploy, the plan was for the spacecraft to transmit its data directly and more than 1000 times faster than the current rate. After the antenna failure, the tape recorder has become a crucial part of the primary data-transfer link, storing data until spacecraft computers can process and radio them back to Earth at a slower speed.
Many opportunities lie ahead. Galileo is scheduled to make 10 visits to Jupiter's three other Galilean moons over the next 2 years and return over 1000 images of the planet and its moons. There is even some discussion of paying another close-up visit to Io during an extended mission, but the December 7 encounter exposed the spacecraft to a significant amount of radiation, and to repeat this exposure may be too risky.
The suspense of dealing with the tape recorder may have been stressful, but Galileo personnel have had their share of close calls and workarounds during the mission's life and so have learned to handle crisis well. As Galileo imaging team member Clark Chapman said upon hearing of the initial tape failure, "I can think of 20 times during this mission when I've felt the way I do now, and most of them meant nothing. The good news is ... [our systems] are very resilient."