Best Is Yet To Come For Galileo
From the "JPL Universe"
January 12, 1996
Project Galileo spent much of the year 1995 preparing for the meteoric dual-craft arrival at Jupiter Dec. 7. The early part of 1996 will be spent gathering the scientific harvest of that arrival day, followed by final preparations for the orbiter's scientific operations. The Galileo spacecraft will carry out four close satellite encounters in the latter part of the year.
At the start of 1995 the spacecraft was finishing the playback of its tape-recorded observations of the Shoemaker-Levy comet impacts with Jupiter the previous July. Then it was time for the flight team to send up new flight software, redesigning Galileo to do the first part of its Jupiter mission with the low-gain antenna.
In midyear the addition of new equipment at the Canberra, Australia, Deep Space Network site increased the station's sensitivity to Galileo's signals. In July the Galileo probe and orbiter spacecraft separated to fly their independent missions to Jupiter. First the combined craft was carefully aimed and oriented for atmospheric entry. Then the two craft separated. Finally, the orbiter's main engine was fired to aim that spacecraft for going into orbit around Jupiter.
In August, Galileo scientists reported that the spacecraft was immersed in the most intense interplanetary dust storm ever measured. It was the latest in a year-long series of dust events. Emanating from Jupiter's system, possibly from volcanoes on Jupiter's moon Io, the microscopic dust particles reached counts as high as 20,000 per day, compared with normal rates of less than one per day. After about a month, the intensity gradually declined, but scientists kept watch and hoped to learn more once Galileo was in the Jovian system.
During the latter part of 1995, Galileo's engineers observed and worked around a small number of spacecraft problems. After the first use of the main rocket engine in July, a check valve in the propulsion system gave evidence of leaking, but careful temperature management prevented damage.
Just after Galileo's camera took its first color approach picture of Jupiter, the spacecraft's tape recorder had a problem in rewinding. Several days of analysis and testing revealed that neither tape nor recorder was broken, but the tape had slipped instead of rewinding.
It was decided not to use that end of the tape, and not to operate at high tape speeds until after the high-priority probe data had been recorded and played back in December and early in 1996. This ruled out images of Io and Jupiter during the initial pass, but assured receipt of the data from inside Jupiter's clouds. Image recording was expected to resume after further testing in 1996.
On Dec. 7, the two Galileo craft completed their Jupiter arrival missions with complete success. The probe entered the atmosphere, decelerating from about 170,000 kilometers per hour (100,000 mph) in about two minutes and then proceeded down through the wind-torn clouds beneath its Dacron parachute, sending up its measurements for almost an hour. The orbiter, after measuring the Jovian environment and getting a gravity assist at Io, received and recorded the probe data and then fired its main engine to become the first artificial satellite of Jupiter.
Galileo began relaying the probe's atmospheric observations to Earth on Dec. 9. These data had been stored in on-board computer memory in condensed form as well as full-length on tape to assure their safe return to the scientists on Earth. The readout was completed just as Galileo and Jupiter entered superior conjunction, a position "behind" the Sun in our sky, where the sun's radio noise outshouts Galileo's radio for up to two weeks.
After Christmas, the spacecraft emerged from this confinement, and probe data readout resumed in January. The team plans to play back the tape-recorded data, including some orbiter instrument readings on the Io plasma torus and Jupiter's magnetosphere, in February, March and April.
The orbit insertion maneuver left Galileo in a slightly shorter first orbit than originally planned, with the first Ganymede flyby scheduled for June 27 instead of July 4, as originally planned.
A large maneuver is scheduled for March 13 or 14 to raise the inner end of Galileo's orbits away from the hazardous environment of Jupiter's charged-particle or "radiation" belts. In May and June, the flight team will augment the spacecraft operating software again, to provide data compression that will increase the number of pictures, spectra and other scientific measurements that can be recorded and transmitted over the low-gain antenna.
Galileo's first encounter with Ganymede and Jupiter, involving some observation of Io as well, begins June 22 and lasts about a week. The second Ganymede encounter sequence runs from Sept. 1-8.
The third satellite encounter is with Callisto Nov. 2-11, and the fourth with Europa Dec. 15-22. Each involves about a week of high-rate observation of Jupiter's atmosphere and magnetosphere and at least one satellite, gathering enough data to fill Galileo's tape recorder with the equivalent of more than 100 pictures. The close satellite flyby, at a distance of a few hundred kilometers, gives Galileo a gravity assist into the next orbit. In January 1997, Galileo and Jupiter will again go into solar conjunction before resuming the satellite close flyby encounters for about another year.
