Galileo Millennium Mission Status 17 Dec 2002
NASA’s Galileo spacecraft has begun transmitting high-priority
scientific information that was collected and stored on its tape
recorder during the orbiter’s early-November dash by Jupiter, which
brought it closer to the planet than ever before.
Damage from naturally strong radiation near Jupiter had left the tape
recorder inoperable for weeks. Galileo’s flight team traced the
problem to a light-emitting diode in the electronics controlling the
motor drive, and then gradually and carefully completed a successful
long-distance repair job.
“We’re delighted playback has begun. There was no guarantee we could
get to this point,” said Dr. Eilene Theilig, Galileo project manager
at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “It’s a real
tribute to the dedication and creativity of the team, and it continues
the tradition set by the larger Galileo teams earlier in the mission
who overcame challenging setbacks to keep the mission a success.”
Playback and transmission began late last week, after tape movements
earlier in the week positioned the tape to start the download with the
data of greatest scientific interest. That information is from a
period starting on Nov. 4, when Galileo was beginning to get closer to
Jupiter than the moon Io’s orbit. It ends when the spacecraft shut
down its instruments on Nov. 5 as part of a precautionary standby
reaction to other radiation-induced problems with the spacecraft’s
computer shortly after passing the inner moon Amalthea.
“We hope this will be the best data set that’s ever been collected
about the inner region of Jupiter’s magnetic environment,” said JPL’s
Dr. Torrence Johnson, Galileo project scientist. The tape playback
may also provide recorded information about dust particles that form
Jupiter’s faint “gossamer” ring, which Galileo flew through on Nov. 5.
The recovery was achieved by running a current through the damaged
diode to anneal, or repair, radiation-caused damage. The first
annealing attempt of six hours produced barely discernible
improvement. Three additional treatments, for a total of 83 more hours
of annealing treatment, produced progressive improvements, to the
point that the tape recorder can run for about an hour at a time. A
fifth treatment produced no additional gain. However, normal playback
runs the tape recorder for only a few minutes at a time, so the
improvement appears sufficient, said JPL engineer Greg Levanas, who
helped plot the recovery strategy.
The flight team plans to continue playback until mid-January, the
scheduled end of Galileo’s mission operations.
Galileo’s tape recorder became critical to the mission’s strategy for
handling science data after the spacecraft’s main antenna failed to
open fully during the journey from launch in 1989 to arrival at
Jupiter in 1995. Most information from onboard science instruments has
been recorded onto tape during the busy days when the spacecraft has
flown near one of Jupiter’s moons. There have been over 30 such
flybys since 1995. The recorded data has been played back slowly for
transmission to Earth via Galileo’s smaller secondary antenna during
the weeks and months before each subsequent flyby.
Galileo has operated five years longer than its original prime mission
and has received more than four times the cumulative dose of harmful
radiation it was designed to withstand. Last month’s encounter took
the orbiter nearly twice as close to Jupiter as it had ever been
before, and exposed it to the strongest radiation it has yet
experienced. In particular, the spacecraft entered the most intense
region of a trapped proton belt and was exposed to 40 times the proton
radiation of any single previous passage close to Jupiter, probably
more than the rest of the mission combined.
The diode that radiation apparently damaged in the tape recorder is a
gallium-arsenide semiconductor component that emits light. The
motor-drive control has three of them. Light from them shines through
windows in a rotating wheel onto detectors on the other side of the
wheel. That setup senses the turning of the wheel and feeds digital
logic that controls drive signals for the motor.
The damage apparently came from high-energy protons from Jupiter’s
radiation belt displacing atoms in the semiconductor’s crystalline
molecular lattice. Passing a current through the diode for hours
serves as a way for electron flow to cause some of the displaced atoms
to shift back to their original lattice positions.
Galileo has nearly depleted its supply of the propellant needed for
pointing its antenna toward Earth and controlling its flight path.
While still controllable, it has been put on a course for impact into
Jupiter next September. The maneuver prevents the risk of Galileo
drifting to an unwanted impact with the moon Europa, where it has
discovered evidence of a subsurface ocean that is of interest as a
possible habitat for extraterrestrial life.
Additional information about Galileo and the discoveries is available
at
JPL, a division of the California Institute of Technology in Pasadena,
manages the Galileo mission for NASA’s Office of Space Science,
Washington, D.C.
