Galileo Status Report
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With less than a week to go before entering orbit around Jupiter, the Galileo spacecraft radioed confirmation late this week that it has penetrated Jupiter's environment, crossing over the boundary from interplanetary space into the giant magnetic cocoon around Jupiter called the magnetosphere.
Data from Galileo's magnetometer confirmed that the spacecraft passed the milestone November 26 at a distance of about 9 million kilometers (about 6 million miles) from Jupiter's cloud tops.
Jupiter's magnetosphere is like a giant bubble around the planet. A shock wave -- called "bowshock" after the wave that builds before the bow of a ship -- exists where the magnetosphere faces the solar wind, a stream of charged particles flowing outward from the Sun. As the solar wind flows around Jupiter, the magnetosphere tapers off like a wind sock, with the whole invisible structure moving in response to buffeting by the solar wind.
Galileo scientists said they first saw signs of the bowshock on November 16, but the bowshock apparently moved back and forth in response to alternate gusts and waning of the solar wind. "As the solar wind velocity increased, the shock moved inside the position of the spacecraft leaving Galileo again in the solar wind," said Dr. Margaret Galland Kivelson of University of California at Los Angeles and principal investigator on Galileo's magnetometer experiment. She said this crossing and recrossing of the shock wave happened several times between the first shock encounter on November 16, when the spacecraft was about 15 million kilometers (about 9 million miles) from Jupiter, and November 26 when Galileo finally crossed the main bowshock at 1800 UTC (10 a.m. PST) at about 9 million kilometers (about 6 million miles) out from Jupiter's cloud tops.
The magnetometer science team also found the first direct evidence that the jovian magnetosphere was either unaffected or had recovered in the aftermath of last year's impact of Comet Shoemaker-Levy with Jupiter. Some scientists had theorized that the magnetosphere might have been modified signficantly by the violent impact, but that appears not to be the case according to data from Galileo.
Meanwhile, Galileo engineers report that work has been completed on the spacecraft's tape recorder to assure its readiness for recording data during Thursday's atmospheric probe descent.
Galileo's extreme-ultraviolet spectrometer has concluded a month-long observation of the Io plasma torus, a doughnut-shaped cloud of ionized sulfur and oxygen surrounding Jupiter and originating from the volcanic moon Io. This cloud was observed close-up by Voyager spacecraft instruments in 1979. Galileo scientists are continuing to analyze the new data.
The Galileo dust detector has also continued monitoring the tenuous streams of dust emanating from the jovian system. Although the spacecraft is apparently out of the great dust storm it flew through last August and September, the number of particles detected never returned to the very low interplanetary count and is rising again as Galileo approaches Jupiter. The instrument will provide daily reports until Jupiter arrival.
Spacecraft engineers have finished testing the tape recorder in readiness for recording the probe's atmospheric data and the orbiter spacecraft's measurements of the Io plasma torus. They expect to further prepare the recorder for its playback and recording operations once in orbit around Jupiter.
The radio relay system on the orbiter, designed to receive signals from the probe, is being prepared for its critical mission December 7. The orbiter has been correctly oriented in space for the relay operation, and the relay antenna has been deployed. On November 28, Galileo was placed in its dual-spin mode, with the lower part of the spacecraft in fixed orientation (allowing the antenna to point at the probe) while the upper part continues to spin at about 3 rpm.
The navigation team had determined that Galileo's flight path did not require trimming by small maneuvers November 17 and 27, as had been allowed for in mission plans. A final maneuver to fine-tune the trajectory, scheduled for Saturday, December 2, has also been eliminated because the flight path was so near its intended target that the correction opportunity was not required.
The Galileo spacecraft continues to operate normally under the control of computer sequences transmitted by the flight team. Galileo is transmitting science and engineering telemetry at 10 bits per second via the stations of NASA's global Deep Space Network.
Today the spacecraft is 931 million kilometers (578 million miles) from Earth, so that its radio signals take almost 52 minutes to reach the Earth. It is about 5.9 million kilometers (4.6 million miles) from Jupiter, and its speed in orbit is about 10,000 mph and slowing.