This Week on Galileo March 19 – 25, 2001

This week’s major scheduled activity is a routine maintenance of the
on-board tape recorder, which occurs on Friday. On Thursday, a new set of
commands will be loaded into the spacecraft computers. These commands will
govern the activities of the spacecraft between March 24 (Saturday) and May 22.

This week’s playback begins with the final set of observations taken by the
Solid State Imaging camera (SSI) of the ring system of Jupiter. These
pictures were taken on January 2, as Galileo was looking back at a receding
Jupiter. From that vantage point, the Sun was behind the spacecraft, and
the rings could be seen in the light that they reflected back towards the
camera. When the Voyager cameras first observed the Jupiter ring system
back in 1979, that spacecraft was in Jupiter’s shadow, looking back in the
direction of the Sun, and the rings were seen by the light that scattered
in the forward direction by the tiny ring particles. By viewing the rings
from these varying geometries, scientists learn about the size and other
properties of the small grains that make up Jupiter’s rings.

This week also sees the beginning of playback for another portion of the
14-week-long continuous magnetospheric survey conducted by Galileo as it
passed through the depths of the system in December. The survey began in
late Octover of 2000 and continued through early February of this year. The
Fields and Particles instruments which participated in this observation
were the Dust Detector, Energetic Particle Detector, Heavy Ion Counter,
Magnetometer, Plasma Detector, and Plasma Wave instrument. The intent of
the investigation was to provide continuous sampling as the spacecraft
passed from the solar wind, through the outer reaches of the magnetosphere,
into the inner portions near Jupiter, and then back out again. Since the
ground communications antennas used to receive Galileo data must be shared
among many space projects, the on-board tape recorder was used to store the
data collected by the instruments approximately once per day, while the
antennas were busy elsewhere. The data now being played back were recorded
on the outbound leg of this orbit, beginning on January 1.

This survey was conducted in cooperation with the Cassini spacecraft, which
was passing by Jupiter at the same time, on its way out to Saturn. This
portion of the observation carried Galileo out through the magnetosheath,
the bow shock, and into the solar wind, outside of the direct influence of
Jupiter’s powerful magnetic field. By studying this passage, scientists
hope to learn more about how the planet’s extensive and complex
magnetospheric system changes with time. This type of study cannot be done
from Earth, but must rely on the presence of instrumented spacecraft, such
as Galileo and Cassini, which can immerse themselves in the environments
that they measure.