Galileo on Course, But One Antenna Still Won't Unfurl
WITH a final boost from Earth's gravity, the Galileo planet-probing spacecraft is heading for its last pre-Jupiter target - the asteroid Ida. It should pass about 2,400 kilometers (1,500 miles) from that body Aug. 28.
Meanwhile, Galileo's handlers at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena are still trying to unfurl the craft's high-gain antenna. The stuck antenna is the one failure on what, so far, has been a highly successful mission. The antenna unfurls like an umbrella but acts as though three of its 18 ribs are stuck.
At JPL, which manages the Galileo program for the National Aeronautics and Space Administration, controllers have been "hammering" the deployment mechanism by turning the drive motor on and off rapidly. This has produced a little movement. But, at press time, it had not freed the antenna.
Project manager William O'Neil says the prospects for freeing the antenna "are not all that good at this point." The hammering will continue until Jan. 19. If it isn't successful by then, members of the project team will go to a backup plan that should enable them to fulfill most of their original mission.
Galileo is expected to gather massive amounts of data on Jupiter and its moons after it arrives at the planet Dec. 7, 1995. The high-gain antenna is designed to transmit these data quickly and efficiently. By beefing up the tracking system on Earth and "compressing" the data to streamline transmission, the project can use the spacecraft's low-gain antenna, which is working well.
Galileo's data transmission rate would still be only 0.75 percent of what was wanted. Data may have to be tape-recorded and transmitted slowly during quiet periods as the craft cruises through the Jovian system. "We know how to do that," Mr. O'Neil says. "And thank goodness we can get 70 percent of our mission."
That includes the data from a probe that will descend into Jupiter's atmosphere. It also includes some 4,000 images. This is far less than the 50,000 images originally desired. But O'Neil explains that they are the high-priority pictures with high resolution of detail. He says that what would be lost mainly would be lower-resolution images, motion pictures of the planet's atmosphere, and details of the magnetic field and associated particles.
"All in all, it's still an excellent mission," O'Neil says.
Galileo's ability to do its job using the low-gain antenna was shown in its survey of the asteroid Gaspra in October 1991. Mission scientists recovered the last of the Gaspra data from Galileo's tape recorder when the craft swung past Earth last month. They now have the first good close-up look at an asteroid ever taken.
Besides pictures and analysis of surface composition, the data include a strong hint that Gaspra has a magnetic field. Project scientist Torrence Johnson says the craft recorded changes in the interplanetary magnetic field consistent with Gaspra being magnetized like some of the iron meteorites. This isn't "proof" of magnetization. But it's a "very suspicious coincidence," Dr. Johnson says. He expects an equally successful survey of Ida if the craft continues to function well.
The accuracy of Galileo's navigation has turned out to be another mission plus. At its closest encounter, the craft zoomed past Earth at a distance of 304 kilometers over the southern Atlantic Ocean at 10:09 a.m. EST Dec. 8. That was within a kilometer of the aiming point and 0.1 second of the desired time.
Earth's gravity flicked the craft onto its course to Jupiter, adding 13,300 kilometers an hour (8,300 m.p.h.) to its speed in solar orbit. This guarantees its arrival at Jupiter Dec. 7, 1995. The craft has traveled once around Venus and twice around Earth to gain the energy to reach Jupiter.
The recent maneuver was so accurate that JPL navigators canceled a course correction planned for Dec. 21. O'Neil says this was the third navigational maneuver that was canceled as unneeded during Galileo's three-year odyssey, conserving rocket fuel and giving Galileo an extra reserve to use at Jupiter.