Today on Galileo
Tuesday, October 16, 2001
Day Two of the Io 32 Encounter
The hectic pace of the Io closest approach and its jam-packed observing
sequence is now fully five hours behind us. Galileo has sped on to a
distance of 160,000 kilometers (99,440 miles) from Io. But there are still
good science observing opportunities to be taken advantage of. And take
advantage of them we do!
At 12:10 a.m. PDT [See Note 1] the Photopolarimeter Radiometer instrument
(PPR) turns its focus on Jupiter. The observation scans the instrument back
and forth across the limb of the giant planet, mapping the temperatures of
the different layers of the atmosphere.
At 12:44 a.m. the Near Infrared Mapping Spectrometer (NIMS) collects a
thermal map of the entire visible disk of Io, looking for volcanos and
other hot spots.
At 1:41 a.m. PPR maps the temperatures on the dark side of Europa. This is
the only observation made of this icy moon on this orbit. Europa was the
central focus of a series of nine orbits earlier in the Galileo mission
(the orbits labelled E11 though E19). At that time, we steered the
spacecraft to within 200 kilometers (124 miles) of the surface of that
satellite. During this orbit, we only close to within 340,000 kilometers
(211,000 miles), at 2:27 a.m., but that is still close enough to detect
temperature variations across the surface features.
At this point we are finally able to take a breather, and the next
observation doesn't come along until 6:58 a.m., when the Solid State
Imaging camera (SSI) snaps a global color image of Io, which now fits
within a single SSI frame. This view includes the volcanos Pele, Zamama,
and Isum, with the Prometheus volcano appearing near the limb.
At 8:54 a.m. PPR again views Jupiter, scanning again the same patch of real
estate that it viewed just after midnight. By now, that portion of the
planet has rotated around so that it appears in the center of the visible
disk of the planet.
By 12:29 p.m. the spacecraft has receded far enough from Jupiter (out to 15
Jupiter radii, over 1 million kilometers or 666,000 miles) that the
radiation environment has cooled down considerably. At this point, the
electronic noise induced in the circuitry and sensors of the star scanner
detector has faded, and the control software is again instructed to look
for three stars to guide it, instead of the single bright star it has
relied upon for the past 47 hours.
At 12:58 p.m. SSI again images Io in color, this time with the satellite
filling only a quarter of the field of view. This vista will include the
volcano Loki near the terminator, or day-night boundary on the dynamic body.
At 2:31 p.m. SSI snaps another picture of the small inner satellite
Amalthea. This view will also include several background stars, and will be
used by the Navigation Team to help refine our knowledge of the orbit of
that body. This will help us fine-tune the trajectory of the spacecraft to
achieve the desired fly-by of Amalthea in November 2002.
At 2:53 p.m., and again at 8:27 p.m., NIMS acquires temperature maps of the
entire visible hemisphere of Jupiter. With these two views, the scientists
will be able to study the dynamics of the turbulent region in the wake of
the Great Red Spot, and will have a view of both the north and south
auroral regions of the planet.
Finally, at 10:21 p.m., a performance test of the spacecraft gyroscopes is
executed. The electronics in the gyroscope system have proven to be very
sensitive to the radiation environment sensed by the spacecraft as it flies
close to Jupiter. This test will determine the extent of the degradation in
the circuitry due to this most recent pass, in preparation for the planning
of a spacecraft maneuver coming up on Friday.
As you can see, the pace has slacked off quite a bit now, but we're still
going strong, and there's more to come!
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