October 11, 1999
Galileo Continues Observation Campaign of Volcanic Moon
A few short hours after passing within a few hundred kilometers of Io's surface, Galileo
continues its observation campaign of the volcanic moon. Early on Sunday morning, the
spacecraft went into safe mode when a memory error was detected in one of the onboard flight
computers, while the spacecraft began the first of the planned recordings. The memory error
was apparently caused by the intense radiation that the spacecraft is exposed to during the
Io encounter. Through an intense and heroic effort, the flight team was able to quickly recover
the spacecraft and Galileo resumed its flight sequence about 2 hours prior to the Io flyby.
Today's activities include observations performed primarily by the Remote Sensing instruments,
but the Fields and Particles instruments also continue their low resolution survey of the
Jovian magnetosphere. Galileo's observations of Io are completed today, as is the magnetosphere
survey. Playback of the data stored on the spacecraft's onboard tape recorder during the last
couple of days is initiated at 11:00 pm PDT-SCET (11:33 pm PDT-ERT, see Note 1).
The Near-Infrared Mapping Spectrometer (NIMS) makes the first observation of the day. In it,
the NIMS looks at the plume of the Pele volcano. The geometry of the observation is such that
the plume, if present, will be on Io's limb, with Jupiter's disk in the background.
After NIMS completes its observation, the Solid-State Imaging camera (SSI) performs a regional
observation of Io. Images taken during this observation will be combined with images taken in
July to produce stereo views of the region. The NIMS also takes a regional look at Io, providing
data that will be used to study surface composition and thermal emissions. Next, the SSI captures
a color view of Io's full disk, including the best color coverage of the region of Io containing
the Loki and Pillan volcanoes, with the Acala region on Io's limb.
The Extreme Ultraviolet Spectrometer (EUV) enters the encounter scene with a distant look at
Io's torus. The Io torus is a doughnut-shaped region with its inner edge bounded by Io's orbit.
It is a region of particularly intense plasma flow and radiation activity inside the Jovian
magnetosphere. The Io torus is constantly replenished by volcanic activity on Io. Similar
observations have been performed during Galileo's previous encounters, and the aggregate data
set will allow scientists to examine long term variations in the torus' size and shape.
The SSI then looks at Io while it is eclipsed from the Sun by Jupiter. SSI captures a color
view of Io, with the volcanic regions Loki, Pele, Pillan and Marduk on the moon's limb. Being
on the limb will facilitate the identification of plumes that may be present. In addition,
comparison of clear filter and 1-micron images will enable measurement of hot spot temperatures.
The other color images will aid in identifying the nature and source of diffuse atmospheric emissions.
The encounter's observation schedule is closed out today with a final remote look at the Io
torus. Again, the EUV adds more data to the existing set that will allow scientists to perform
long term morphological studies.
Come back tomorrow and learn what data will be transmitted to Earth during the remainder of
the week. Look for the return of This Week on Galileo!
Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian 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 occured is known as Earth Received
Time (ERT). Currently, it takes Galileo's radio signals 33 minutes to travel between the spacecraft