Hubble plays spin the bottle with last few gyros

Time to dust off those contingency plans?

The Hubble Space Telescope (HST) has again suspended science operations due to an ongoing gyroscope problem.

NASA reported that the instruments were stable and described the spacecraft as being in "good health" after it dropped into safe mode following "faulty telemetry readings" from one of the gyroscopes.

The HST, a NASA and ESA project, was launched in 1990. NASA recently celebrated the 15th anniversary of the last time astronauts serviced the spacecraft. During that mission, various components, including the gyroscopes, were replaced, giving what engineers hoped would be another five years of life.

Fifteen years on – considerably longer than estimated – and Hubble is down to three gyroscopes, and one is clearly on the way out, considering the frequency of safe mode incidents. Hubble uses its gyros to point the observatory, and three of the six units need to function normally for standard operations. Three gyros have now failed, and another failure will mean engineers will have to invoke a contingency plan and switch to one-gyro mode.

Engineers devised the one-gyro mode after Hubble was put into two-gyro mode to prolong its life before the final Space Shuttle servicing mission. The difference between the two modes is negligible, meaning that engineers would be able to put a gyro in reserve.

Moving out of three-gyro mode does, however, reduce the observatory's efficiency. One-gyro mode uses magnetometers, sun sensors, and star trackers for the failed gyros. The instruments are used in a multi-step process to point Hubble in the right direction. According to NASA: "Once Hubble is on target, the steadiness of the telescope in one-gyro mode is almost comparable to that of a full three-gyro complement."

Since its launch, the HST has far exceeded expectations in terms of science and longevity. The US space agency said it "anticipates Hubble will continue making discoveries throughout this decade and possibly into the next."

However, without a servicing mission on the books to deal with the spacecraft's failing components, it is increasingly likely that engineers and scientists will need to start invoking contingency operations if Hubble is to reach the 40th anniversary of its launch and beyond. ®

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