Today on Galileo
Wednesday, January 16, 2002
The Pace Picks Up!
The day begins with Galileo still over 25 Jupiter radii from the giant
planet (1.8 million kilometers, or 1.1 million miles). Before 24 hours have
passed, that distance will have closed to less than 15 Jupiter radii (1
million kilometers or 670,000 miles). During that time, the spacecraft is
preparing for the close encounter that is lying in wait tomorrow.
At 5:00 a.m. PST [See Note 1] the attitude control software for the
spacecraft is configured to rely on sighting only a single star during the
Io flyby. The star scanner sensor ordinarily uses three or more stars as
reference points for determining the spacecraft's orientation. However, as
the spacecraft moves closer to Jupiter during encounter periods, the
radiation environment heats up and the sensor gets bombarded by energetic
particles, which cause noise in the electronic circuits. This noise tends
to drown out the signals from fainter stars or can even be mis-identified
as a star that is not really there. By selecting a single bright star,
whose signal is expected to be much higher than the noise level, Galileo
can reliably keep track of its orientation. During the 48 hours surrounding
the closest approach to Jupiter and Io, we will be viewing the star
Achernar (Alpha Eridani), which is the sixth brightest star in the catalog
we maintain for use by Galileo. This same star has been used successfully
in this same manner for the past 3 orbits.
Beginning at 9:30 a.m. PST, the tape recorder is moved to the correct
location on the tape to begin the recording of science activities.
Curiously, we don't always "begin at the beginning" when we record. When
the entire science strategy for the orbit is laid out, we usually choose a
particular block of high-priority, high-speed recording to occupy one
continuous track of the four tracks we have available to us. By adopting
this strategy, we don't have to stop our observation sequence to wait for
the tape to change directions. When we are near the closest approach to a
satellite, a few seconds can mean lost opportunities!
At 10:25 a.m. PST, Galileo reaches its closest point to the outermost of
the four large satellites of Jupiter, Callisto. But at a distance of 1.7
million kilometers (1 million miles), it is too far away to be worthy of
even a glance. Likewise, at 9:28 p.m. PST, our closest approach to
Jupiter's largest satellite, Ganymede, is a distant 1 million kilometers
(670,000 miles), and this body is also passed by for observations.
At 3:17 p.m. PST, the Photopolarimeter Radiometer (PPR) instrument is
turned on and records a brief calibration sequence. Following this
activity, at 4:00 p.m. PST, the instrument turns its gaze on Io for the
first observation of that satellite during this encounter. This distant
observation examines the thermal emissions from the dark side of the
satellite for 13 minutes.
At 8:45 p.m. PST, the Radio Science Team begins a 20-hour-long study of the
gravity field of Io, centered around the closest approach to the satellite.
This study consists of closely watching the radio frequency of the signal
transmitted by Galileo. As the spacecraft gets closer to Io, the gravity of
that body tugs on Galileo, and the frequency of the radio signal changes.
This is the familiar Doppler shift, usually described with the analogy of a
train whistle changing pitch as the train approaches and recedes from the
listener. With Galileo, the change in pitch of the radio signal corresponds
to the change in speed of the spacecraft. That corresponds to how hard Io
is pulling on the spacecraft, and that corresponds to the total mass of Io,
and, at a finer level of detail, to how that mass is distributed within the
satellite. An amazing amount of good science can be collected just from
listening to the radio!
Note 1. Pacific Standard Time (PST) is 8 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 35 minutes to travel
between the spacecraft and Earth. All times quoted above are in Earth