This Week on Galileo
Wednesday-Sunday, October 17-21, 2001
The Last of Io 32 Observations
The pace is much slower now, but a handful of observations still remain on
Galileo's plate for the remainder of the week.
Wednesday morning at 2:40 a.m. PDT [See Note 1] the Energetic Particle
Detector instrument cycles its power off and on and reloads its computer
memory. The high radiation environment near Jupiter has caused upsets in
the instrument's computer memory in past orbits, and this pre-emptive
reload is to guarantee that the instrument is in the correct configuration
for the long haul.
At 10 a.m. the Photopolarimeter Radiometer performs a second calibration
sequence, the same as was done early Monday morning when the instrument was
first powered on for the encounter. By measuring the same calibration
target before and after immersion in the harsh environment near Jupiter,
any changes that appear in the response of the instrument can be factored
into the understanding of the science observations acquired.
At 3 p.m. the Near Infrared Mapping Spectrometer shuts down operations for
this flyby, resting peacefully until it awakens briefly in early December
for a calibration activity.
On Thursday, starting at 10 a.m. the Solid State Imaging camera (SSI)
spends 2.5 hours viewing Jupiter. This set of observations looks at a hot
spot in the atmosphere of the gas giant. Such regions are similar to one
into which the Galileo atmospheric probe was dropped in December 1995, when
Galileo first arrived at the Jupiter system. These images will help
scientists measure wind speeds in great detail near the hot spot, and help
determine whether the low water abundance seen by the atmospheric probe is
the result of simply local weather conditions, or whether it represents a
planet-wide dehydration. Such a dehydration would send the atmospheric
theoreticians back to the drawing boards to change their models of why
Jupiter is the way it is.
At 8:40 p.m. on Thursday, SSI snaps a picture of a portion of the Jupiter
ring system known as the Gossamer Ring. This ring extends out beyond the
orbit of the small inner satellite Thebe, which circles Jupiter at a
distance of 150,400 kilometers (93,500 miles) above the cloud tops. This
span puzzles scientists, since we believe that Thebe is the source of the
material in the ring, and that the particles should pass inwards towards
the planet as their orbits evolve.
Starting at 12:50 p.m. on Friday a spacecraft maneuver is planned. This
pulsing of the rocket thrusters alter the spacecraft trajectory, correcting
the flight path for small differences between the planned and actual
position of the spacecraft as it swung by Io and targeting for our next Io
flyby in January. Prior to this maneuver, the spacecraft performs an
on-board test of the gyroscopes that are used to maintain the attitude of
the spacecraft during the burn. This test will automatically update
software parameters that describe the current performance of the gyroscopes
after their recent bath in the radiation at Jupiter.
During this entire time period, the suite of instruments that measure the
energetic particles and the electromagnetic fields and plasmas surrounding
Jupiter have been quietly and continuously collecting real-time data about
the local environment of the spacecraft. This data collection is
independent of the recorded data collection talked about over the past few
days. The continuous survey of the Jupiter system began on October 15 and
will continue for about another week, until our attention turns to playing
back the recorded data.
Note 1. Pacific Daylight Time (PDT) is 7 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 41 minutes to travel
between the spacecraft and Earth. All times quoted above are in Earth