Jupiter-bound Galileo starts with the sun.

Jupiter-bound Galileo starts with the sun

The Galileo spacecraft, launched Oct. 18 onto a complex course that will put it in orbit around Jupiter in 1995, wasn't scheduled to begin its scientific activities until next February. But the craft went to work early, analyzing a powerful solar flare only days after launch, in what space scientist Edward C. Stone calls "Galileo's first scientific result."

In addition to serving as project scientist of the Voyager 1 and 2 missions from the California Institute of Technology in Pasadena, Stone heads a team working with an instrument aboard Galileo. NASA added the device, called the Heavy Ion Counter (HIC), to Galileo's primary scientific payload to help track the craft's response to collisions with ionized sulfur and oxygen atoms trapped in the Jovian magnetic field.

Such ions are part of Jupiter's radiation belts, which are so intense that Galileo uses chips specially hardened against radiation to protect its microcircuits. The project's engineers hope such chips will prevent the ion bombardment from accidentally altering or damaging the settings of computer memories and other electronic components.

Most of the time, the HIC would be incapable of measuring the levels of charged particles coming from the sun, but lately the sun has been in the most active part of its 11-year cycle. The day after Galileo's launch, the craft encountered particles emitted from a major solar flare, which continued for several days and was strong enough for the HIC to detect a full range of particles. Though Galileo is dedicated primarily to planetary objectives, its initial scientific accomplishment gave scientists a detailed mass spectrum of the particles cast out from the sun's corona by the flare.

It was just luck that the flare occurred while Galileo was still close enough for its radio transmissions to be relatively strong, Stone says. Data from the HIC could be sent only during the flight's first 20 days. Keeping the device on duty longer would have required unfurling Galileo's umbrella-like high-gain antenna, a risky venture so close to the sun's heat. Instead, the high-gain antenna will remain closed until 1991 (making the HIC data available again), after the craft has swung around Venus and headed back out for the first of its two trips past Earth.

In addition to confirming that the device worked, the big flare provided an unscheduled but successful test for the hardened chips. Although the flare probably posed less hazard to Galileo than will Jupiter's radiation belts, both Stone and project scientist Torrence Johnson of the mission control center at Jet Propulsion Laboratory in Pasadena told SCIENCE NEWS they were pleased to find that the particle outburst neither permanently damaged the chips nor even once temporarily altered the settings that govern how they work. The researchers add that nonhardened chips might have suffered as many as a dozen permanent or temporary alterations, leading to potentially critical effects on Galileo's operations or scientific observations.