Astronomers Await Galileo Fly-By of Gaspra Asteroid

GALILEO, the Jupiter survey spacecraft, is headed for a unique rendezvous as it continues its round-about voyage to the giant planet. On Oct. 29, it is due to meet up with the asteroid Gaspra. This first-ever close-up scrutiny of an asteroid is also a daunting navigational challenge. Project scientist Torrence Johnson says this object, which, astronomically speaking, is a tiny speck in space, will "probably be the hardest planetary target yet."

As if this were not enough, mission scientists now face the possibility that Galileo may not be able to talk to them when it encounters Gaspra.

The craft's umbrella-like 16-foot-wide high-gain antenna has so far failed to open completely. Without that antenna, the spacecraft will have to store the Gaspra data on a tape recorder and play them back over its low-gain communications antenna when it returns near Earth in December 1992.

Thus the Galileo team at the California Institute of Technology's Jet Propulsion Laboratory (JPL) is heading into the most challenging summer and fall since the spacecraft left Earth in October 1988.

Engineers at JPL, which manages the Galileo project for the National Aeronautics and Space Administration, are studying how to overcome the antenna problem.

Gaspra is a main-line asteroid-belt object with a surface brightness twice that of our moon. Dr. Johnson says that most experts think it may look like the Martian moon Phobos, which is similar in size. However, he adds:

"We could be surprised, though. This thing may belong to a type of asteroid that may be fragments of stony-iron meteorites. There may be differences on the surface that are not evident, for instance, on something like Phobos."

This uncertainty adds spice to the encounter, Johnson says, because "first-time-ever type of pictures are always exciting just in the sense that you wonder whether there's some surprises."

Seeing Gaspra is a bonus the program gains from having to take the long road to Jupiter. Because of safety restrictions imposed on space-shuttle cargoes after the Challenger accident, Galileo's booster rocket lacked the thrust to send the craft on a direct course to the planet.

Instead, Galileo gains speed by flying once past Venus (February 1990) and twice past Earth. Gravitational interaction with these planets gives the craft a boost as it passes. Galileo made its first Earth pass last December. This put it on a course that takes it by Gaspra and back past Earth in Dec. 1992.

As a second bonus of these fly-bys, the mission team has opportunities to exercise the craft's planet-scanning capabilities before it reaches Jupiter in December 1995. Except for the sticky antenna deployment, Galileo has performed well in these exercises.

Looking at Venus and Earth and its moon gave controllers opportunities to practice with several of the Galileo's observation instruments. The Gaspra fly-by will also exercise the craft's ability to keep its instruments trained squarely on a target while it is whizzing past at several kilometers a second.

As Galileo flew past Earth, it collected data showing distribution of water vapor in the upper atmosphere. Such data are important in studying the fate of the ozone layer and aspects of the theorized greenhouse effect. Johnson says this "may be the best data set" for such information that will be available until special Earth-scanning satellite instruments are orbited late in the decade.

Galileo's performance in the Venus and Earth fly-bys has been so good Johnson says he feels confident "we're going to be able to achieve all the objectives we thought about" for Jupiter. These include sending a probe into Jupiter's atmosphere and fly-bys of many of the moons. Looking forward to pictures of Jovian satellite surfaces that are as detailed as satellite images of Earth, Johnson says "the idea of looking at Europa with that type of resolution ... just sends shivers up my spine."

In surveying the Jovian system, Galileo will have to endure what Johnson calls one of the toughest radiation environments one can imagine. The craft's electronic systems have been designed and tested to endure that irradiation. Johnson says he believes the craft can survive well for its nominal two-year mission and perhaps a few years beyond that. However, he adds that loss of electronic components to radiation may well be the limiting factor in the spacecraft's usefulness.

But the high-gain antenna is crucial. Galileo can transmit its voluminous data over its low-gain antenna, as it did in passing Earth. But it needs the high-gain dish to send those data from Jupiter. If engineers haven't managed to fully deploy the big antenna, a radio relay satellite might be launched on a fast track to meet Galileo at Jupiter. JPL planners are studying that option now.