Gyroscope flaws: Hubble spins its wheels.

New equipment problems on the Hubble Space Telescope - this time electrical rather than optical - suggest the troubled craft may soon require more than corrective lenses to function properly. The failure of two Hubble gyroscopes in the past eight months and a recent electrical glitch in a third have prompted NASA to consider an emergency mission to replace these devices, which help orient the craft in space.

If two more of Hubble's six gyroscopes fail, the craft will lose its keen sense of direction - a feature vital to pointing the telescope accurately, explains John Campbell at NASA's Goddard Space Flight Center in Greenbelt, Md.

Hubble's latest woes began Dec. 3, when a feedback circuit on one of the gyroscopes ceased working. Researchers could no longer measure the gyroscope's rate of rotation - a key to gauging changes in the craft's direction. On June 29, just days after two brief malfunctions, an electronic component in a second gyroscope died. NASA scientists believe charged particles in Earth's atmosphere may have damaged one of this instrument's transistors, since the first sign of trouble occurred immediately after Hubble exited the South Atlantic Anomaly, a region of intense radiation. Researchers weren't unduly alarmed by this failure, Campbell notes, since a spacecraft needs only three gyroscopes for orienting itself and Hubble still had four working units.

Then, on July 26, electric current running through a third gyroscope suddenly increased slightly. This minor glitch has forced scientists to seriously consider sending a shuttle mission - which must be scheduled one year in advance - to replace the gyroscopes before any other problems arise. Campbell says NASA scientists will decide in September whether to request such a mission, after a panel of experts estimates the likelihood of additional failures.

These malfunctions puzzle NASA, particularly since the agency's calculations, based in part on experience with the International Ultraviolet Explorer (IUE) satellite, had indicated that the gyroscopes would last 14 years, Campbell observes.

"Obviously, those calculations ain't worth a damn," asserts Goddard telescope engineer Henry Hoffman. "History demonstrates they don't last that long." The Hubble gyroscopes were spares, Hoffman explains, built in 1975 for IUE and given electronic upgrades in the late 1970s and the 1980s. Of six nearly identical gyroscopes in the IUE, two failed within five years of the craft's 1978 launch. Only two of IUE's devices still operate; a highly accurate sun sensor on the craft takes the place of the third gyroscope normally needed to control motion, Hoffman says, and a star tracker could likely substitute if another IUE gyro falters.

Hubble's sun sensor, by contrast, lacks the accuracy to stand in for a gyroscope, he notes. He contends NASA should have learned a lesson from IUE and equipped Hubble with instruments that could pinch hit for a failed gyro. While no one expected the devices to falter after just 15 months in orbit, he says, "I'm never surprised when one fails. Gyroscopes are always the weak link."

Pierre Bely, an engineer with the Space Telescope Science Institute in Baltimore, recalls that when scientists designed Hubble in the early 1970s, they expected frequent shuttle flights to repair parts. "Gyroscopes were meant to be replaceable," he says.

Campbell notes that the 15-year storage of the gyroscopes' mechanical parts and the wear-and-tear on circuitry during testing may have contributed to their short life in space: The two devices that failed on Hubble experienced the most electrical testing.

NASA already plans a shuttle mission late in 1993 to make changes that will compensate for the telescope's flawed primary mirror. But the potential severity of Hubble's gyroscope situation, as well as another problem - vibrations created by the craft's solar panels - may warrant a separate repair mission one year earlier, Campbell says.