Today on Galileo
Friday January 18, 2002
Farewell to Io!
The spacecraft is now fully recovered from yesterday's anomaly in which the
onboard fault protection routines detected a despun bus reset about 28
minutes before the closest approach to Io. Because this could be a
potentially harmful event, the spacecraft put itself into a safe mode and
canceled the science sequence. The flight team worked throughout the day
and evening to re-establish nominal spacecraft operations and to acquire
the final track of recorded data. Unfortunately, three tracks of data all
planned for recording within hours of closest approach to Io were lost
because of the spacecraft problem. At this time we think the problem
resulted from the radiation environment near Jupiter.
Galileo has now receded to 552,300 km (343,000 miles) from Jupiter and is
increasing that distance by 12 kilometers every second. At its closest
approach point, the spacecraft was 324,800 km (201,800 miles) above
Jupiter's cloud tops.
Though our observations of Io are now complete, other science opportunities
still await us over the next few days. At 4:30 a.m. PST [See Note 1] the
radiation levels experienced by Galileo have dropped to the point where the
attitude control software can again use three stars to determine the
spacecraft orientation. While close to Jupiter, a single bright star is
used to avoid confusion by radiation-induced noise in the sensor circuits.
At 5:41 a.m. PST the Solid State Imaging camera (SSI) snaps another picture
of the small inner moon Amalthea. This image will be used to help the
Navigation Team steer the spacecraft to a close encounter with that tiny
moon in November. At 7:52 a.m. SSI acquires a color image of the
Jupiter-facing hemisphere of the icy satellite Europa. This will be our
last glimpse of that fascinating moon for the remainder of the Galileo mission.
At 10:30 a.m. a standard test of the spacecraft gyroscopes is performed.
Over Galileo's 6 years in orbit, our old nemesis, radiation, has damaged
the circuitry that measures the signals sent by the gyroscopes. On each
orbit, after we have passed through the worst that Jupiter can dish out, we
perform a test to calibrate the gyros for use during maneuvers and turns.
At 4:20 p.m. Saturday the Near Infrared Mapping Spectrometer (NIMS) begins
the first of a set of three full-disk multi-color maps of Jupiter. This
final global mosaic of the planet for the Galileo mission ends six hours
later. When this observation is complete, the NIMS instrument is powered
off until a final calibration in early February.
At 11:45 a.m. Sunday SSI begins a 26-hour observation of the turbulent
region just west of the Great Red Spot in Jupiter's atmosphere. During this
time, SSI shutters 18 pictures, which will enable scientists to study how
the cloud forms change and evolve. The observation continues until 2 a.m.
Monday morning, at which time the camera closes its eye for the final time,
and the high-gear operations for another encounter wind down.
Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean Time
(GMT). The time when an event occurs at the spacecraft is known as
Spacecraft Event Time (SCET). The time
at which radio signals reach Earth indicating that an event has occurred is
known as Earth Received Time (ERT). Currently, it takes Galileo's radio
signals 35 minutes to travel between the spacecraft and Earth. All times
quoted above are in Earth Received Time.