This Week on Galileo: The Pace Picks Up
16 Jan 2002
(Source: Jet Propulsion Laboratory)
Wednesday, January 16, 2002
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 Received Time.