August 12 - September 8, 2002
The Next Four Weeks on Galileo
As the Galileo spacecraft continues its long trek back in towards Jupiter
for its final planned science pass in November, the pace of activity picks
up. In addition to the routine maintenance activities that look after
spacecraft health and safety, special tests are beginning in preparation
for the Amalthea flyby.
On Thursday, August 15, the spacecraft will perform a test of an attitude
maintenance strategy being considered for the upcoming flyby. Normally,
when Galileo is in close to Jupiter, the high radiation environment creates
enough noise in the star scanner electronics to mask the signals from all
but the very brightest stars. For a typical flyby of the innermost of the
four large satellites, Io, one single bright star can still be reliably
detected, and the spacecraft maintains its attitude knowledge using an
on-board software routine called One Star Attitude Determination.
Frequently during such a flyby, however, a large body such as Io
temporarily blocks that single star from view, and the one-star routine
must be configured to go into "hibernation", expecting to see no stars at
all for a period of perhaps 15 minutes. For the November flyby, Galileo is
going to pass much closer to Jupiter than it has ever done before. The
increase in radiation at this closer distance will cause even the brightest
star in the sky to disappear into the electronic noise for a period of up
to nine hours! The hibernation test we will perform this week will tell us
how the spacecraft systems respond to being told to ignore the sky
altogether for extended periods of time.
On Monday, August 19, Galileo closes the distance to Jupiter to a mere 300
Jupiter radii (21.4 million kilometers or 13.3 million miles). Though 3.5
million kilometers closer than its farthest reach during this orbit, we are
still farther from Jupiter than we have been since before entering orbit in
December of 1995. The spacecraft is still well outside the magnetosphere of
Jupiter on the sunward side of the planet, and continuous data collection
by the Magnetometer, the Dust Detector, and the Extreme Ultraviolet
Spectrometer instruments provides scientists with information about the
On Tuesday, August 20, and again on Saturday, September 7, the spacecraft
will turn in place approximately 4 degrees to keep the communications
antenna pointed towards Earth. On Wednesday, August 21, routine maintenance
of the propulsion system is performed.
On Sunday, September 1, the Near Infrared Mapping Spectrometer team
collects some engineering data from the instrument to assist in the final
calibration of the response of the instrument. These data will aid in the
interpretation of the final set of science calibration data collected from
the instrument in March of this year.
On Friday, August 16, the next series of tape recorder tests will begin.
This is the last test that will exclusively use the recorder's slowest
speed. This test moves back and forth the full length of the tape without
stopping. This action repeats 10 times, for 20 full passes over the tape.
Another test begins on Saturday, August 24, when we switch gears to use a
faster tape speed. This tape speed is about 13 times faster than the
slowest speed, and is the fastest we plan to use the recorder during the
final flyby. This first high-speed test travels back and forth the length
of the tape in 6 hops per track. On Monday, September 2, the next
high-speed test begins that travels the length of the tape in only 2 hops.
The data that we are receiving from the current set of tape tests indicates
that the tape is still somewhat sticky. We are still able to reliably move
the tape, however, and we believe that by exercising the recorder nearly
continuously between now and November, we can reduce the stickiness and
keep the tape moving freely. This will enable us to successfully record the
magnetospheric data as planned as we plunge through the depths of the
Jovian radiation field in November.