BOSTON — GALILEO, the wandering Jupiter probe, will at last set course for the giant planet this December. After nearly three years in space, it has opened a new kind of astronomical exploration - close-up study of asteroids.
In June, Galileo scientists released a composite picture of asteroid Gaspra. They constructed it from images the spacecraft took when it passed within 1,600 kilometers of that 19-kilometer-long body last October. Nearly two centuries after Italian astronomer Guiseppe Piazzi spotted the first known asteroid (Ceres) on Jan. 1, 1801, astronomers finally know what one looks like. Now they look forward to Galileo's second asteroid encounter next year when it passes an object named IDA on Aug. 28, 1993.
Meanwhile, Galileo project manager William O'Neil and his team of controllers are preparing for the spacecraft's Dec. 8 encounter with Earth. This is the critical maneuver that will flick it into an orbit that takes it past IDA and on to its Dec. 7, 1995, rendezvous with Jupiter. The craft's main antenna still refuses to deploy. Nevertheless, Mr. O'Neil says he feels cautiously confident about the success of the mission.
Galileo was originally designed to launch from a space shuttle using a booster that would put it on a direct course to Jupiter. But, after the Challenger accident, the launch was reassigned to an unmanned rocket that didn't have the thrust to achieve that orbit. Instead, Galileo was launched Oct. 18, 1989 on a wandering journey to pass once by Venus and twice by Earth. On each such pass, the planet's gravitational pull boosts the spacecraft's orbital energy and adjusts its course for the next encounter.
This "gravity-assist" navigation has worked well. But the main antenna opened only part way when controllers tried to deploy it after Galileo made its first Earth flyby on Dec. 8, 1990. The antenna opens like an umbrella. O'Neil says it acts as though three of the ribs are stuck.
Galileo's controllers at the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, Calif., which manages the mission for the National Aeronautics and Space Administration (NASA), have tried repeatedly to free the ribs by turning the craft. They hoped the stresses of heating and cooling as the antenna turned toward and away from the sun would loosen the mechanism. O'Neil says that they have given up on this after a final attempt last month.
Instead, he says, they anticipate a general warming of the antenna as Galileo returns to Earth. It reaches the perihelion of its orbit - the point closest to the sun - Dec. 13. O'Neil expects that, by then, the antenna should be warm enough for thermal adjustment to give it "the dimensions it had when manufactured." He explains: "Hangups are often a matter of dimensions. This dimensional adjustment, by itself, could free the ribs." If that doesn't work, controllers will try hammering the deployment mecha nism by turning the drive motor on and off rapidly.
In any event, Galileo team members now think they can carry out 70 percent of the Jupiter survey mission even without the main antenna. They had thought the craft's low-gain antenna, which they have been using, couldn't carry the required data flow. They now know how to squeeze more data into a signal that the antenna can carry, and how to beef up the ground receiving network.
O'Neil says that the team also "will be prepared to do the IDA flyby with or without the high-gain antenna." This should give astronomers more insight into the origin and evolution of these bodies. The number and type of craters on Gaspra suggest it is a fragment of a larger body that broke up about 200 million years ago.
Astronomers consider asteroids debris left over from formation of the solar system. Their makeup should reflect early solar system history. NASA hopes to continue asteroid exploration with other spacecraft. To this end, it has commissioned Johns Hopkins University's Applied Physics Laboratory to design a compact craft called NEAR (Near Earth Asteroid Rendezvous) that could meet and orbit asteroid Nereus by the end of the decade.