Today on Galileo
Saturday, August 4, 2001
The Encounter Begins

This morning at 4:49 a.m. PDT [See Note 1] Galileo's 31st encounter begins. Once again, this hardy spacecraft plunges deeply into Jupiter's intense radiation field to fly by the fascinating moon Io, the innermost of the giant planet's four largest satellites. These four satellites, also called the Galilean satellites, share the namesake of the spacecraft, Galileo Galilei. This Italian astronomer spied these fascinating worlds in 1610 when he first gazed through his primitive telescope. How different things are today, when we can visit them with robotic spacecraft such as our Galileo, and unravel their secrets by getting up close and personal! From tiny points of light, the Voyager and Galileo spacecraft have transformed these bodies into full and complex worlds.

First up, at 9 a.m. PDT, the Photopolarimeter Radiometer instrument turns on its power and performs a calibration by looking at a target plate mounted on the spacecraft. This observation establishes a baseline reading for the operation of the instrument against which the science measurements taken later in the sequence can be compared. When this is completed, the instrument sets its sights on Callisto for a measurement of the polarization of the light from that body. By studying how the polarization of the reflected sunlight changes as we view the body from different angles, scientists can determine the small-scale structure of the surface. This observation takes place near the closest approach to Callisto by Galileo on this orbit, but that is still from a distance of 350,000 kilometers (220,000 miles), nearly three times the distance at which Voyager 1 passed the satellite in 1979.

At 1:10 p.m. PDT, the attitude control software on the spacecraft is configured to use a single star as its primary reference. Ordinarily, the software uses the signals from three or four stars to accurately determine the orientation of the spacecraft. When in the high radiation environment close to Jupiter, however, noise floods the star scanner detector and masks the signals from the fainter stars. By focusing on a single bright star, whose signal is above the noise, the software can safely maintain its knowledge of Galileo's attitude. The star being used is the same one we used on the last flyby in May, Achernar, or Alpha Eridani, the brightest star in the constellation of Eridanus, the River.

At 4:30 p.m. PDT, the Solid State Imaging camera (SSI) is turned off and then turned on again, just before its first observation. In recent orbits, the SSI instrument has experienced problems during our passage through the radiation belts which result in completely saturated, overexposed images. Experience has shown that cycling the power may help to clear this situation, at least temporarily. Eight times during this encounter the SSI instrument will cycle power and reload its internal software just before key blocks of observations, to make sure that the instrument is as healthy as we can make it. This first SSI observation is an attempt to view a plume of material which may still be erupting from the Tvashtar volcano on Io. This volcano has been extremely active lately, and is a prime target of observations during this flyby, since the path of the spacecraft will take it very nearly directly overhead of the feature just after closest approach. This observation will also serve as part of a survey to determine what other volcanic features may be currently active.

At 9:30 p.m. PDT, SSI is cycled once again, and a single picture is taken of Callisto, viewing that portion of the satellite that perpetually points towards Jupiter. This picture will capture the Lofn and Heimdall regions of Callisto near the terminator, or day-night boundary, of the satellite. This image will help to determine the relative geological ages of the regions, as well as examine the transition between two different terrain types.

At 11:00 p.m. PDT, the suite of Fields and Particles instruments (the Energetic Particle Detector, Magnetometer, Heavy Ion Counter, Plasma Wave, and Plasma instruments) complete their configurations in preparation for tomorrow's close pass by Jupiter and Io, and begin collecting and transmitting to Earth real-time science data. This continuous real-time data collection will span 59 hours during this sequence, concluding Tuesday morning about 10 a.m. PDT.

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 49 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.

For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page at one of the following URL's: