From: Jet Propulsion Laboratory
Posted: Monday, August 12, 2002
As the Galileo spacecraft continues its long trek back in towards Jupiter for its final planned science pass in November, the pace of activity picks up. In addition to the routine maintenance activities that look after spacecraft health and safety, special tests are beginning in preparation for the Amalthea flyby.
On Thursday, August 15, the spacecraft will perform a test of an attitude maintenance strategy being considered for the upcoming flyby. Normally, when Galileo is in close to Jupiter, the high radiation environment creates enough noise in the star scanner electronics to mask the signals from all but the very brightest stars. For a typical flyby of the innermost of the four large satellites, Io, one single bright star can still be reliably detected, and the spacecraft maintains its attitude knowledge using an on-board software routine called One Star Attitude Determination. Frequently during such a flyby, however, a large body such as Io temporarily blocks that single star from view, and the one-star routine must be configured to go into "hibernation", expecting to see no stars at all for a period of perhaps 15 minutes. For the November flyby, Galileo is going to pass much closer to Jupiter than it has ever done before. The increase in radiation at this closer distance will cause even the brightest star in the sky to disappear into the electronic noise for a period of up to nine hours! The hibernation test we will perform this week will tell us how the spacecraft systems respond to being told to ignore the sky altogether for extended periods of time.
On Monday, August 19, Galileo closes the distance to Jupiter to a mere 300 Jupiter radii (21.4 million kilometers or 13.3 million miles). Though 3.5 million kilometers closer than its farthest reach during this orbit, we are still farther from Jupiter than we have been since before entering orbit in December of 1995. The spacecraft is still well outside the magnetosphere of Jupiter on the sunward side of the planet, and continuous data collection by the Magnetometer, the Dust Detector, and the Extreme Ultraviolet Spectrometer instruments provides scientists with information about the interplanetary medium.
On Tuesday, August 20, and again on Saturday, September 7, the spacecraft will turn in place approximately 4 degrees to keep the communications antenna pointed towards Earth. On Wednesday, August 21, routine maintenance of the propulsion system is performed.
On Sunday, September 1, the Near Infrared Mapping Spectrometer team collects some engineering data from the instrument to assist in the final calibration of the response of the instrument. These data will aid in the interpretation of the final set of science calibration data collected from the instrument in March of this year.
On Friday, August 16, the next series of tape recorder tests will begin. This is the last test that will exclusively use the recorder's slowest speed. This test moves back and forth the full length of the tape without stopping. This action repeats 10 times, for 20 full passes over the tape.
Another test begins on Saturday, August 24, when we switch gears to use a faster tape speed. This tape speed is about 13 times faster than the slowest speed, and is the fastest we plan to use the recorder during the final flyby. This first high-speed test travels back and forth the length of the tape in 6 hops per track. On Monday, September 2, the next high-speed test begins that travels the length of the tape in only 2 hops.
The data that we are receiving from the current set of tape tests indicates that the tape is still somewhat sticky. We are still able to reliably move the tape, however, and we believe that by exercising the recorder nearly continuously between now and November, we can reduce the stickiness and keep the tape moving freely. This will enable us to successfully record the magnetospheric data as planned as we plunge through the depths of the Jovian radiation field in November.
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: http://galileo.jpl.nasa.gov http://www.jpl.nasa.gov/galileo
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