Hubble is back in service after the gyro scare, and NASA is still considering reboot options

The Hubble Space Telescope resumed science observations on Friday after ground-based teams spent most of the past three weeks evaluating the performance of a precise gyroscope, NASA said.

The noisy gyroscope is an important part of the observatory’s signaling system. Hubble’s gyroscopes measure how fast the spacecraft is rotating, helping the telescope point its aperture toward distant cosmic wonders.

Hubble continues to provide valuable scientific data to astronomers nearly 34 years since its launch aboard NASA’s Space Shuttle Discovery in 1990. Five other shuttle servicing missions have repaired Hubble, updated its science instruments, and replaced hardware deteriorated due to long-term use in outer space. Among other tasks, astronauts on the last shuttle repair mission in 2009 installed six new gyroscopes on Hubble.

Moving parts sometimes break

Gyroscopes have always been one of the parts of Hubble that requires the most maintenance. A wheel inside each gyroscope rotates at a constant rate of 19,200 revolutions per minute, and the wheel, in turn, is enclosed within a cylinder suspended in a thick liquid, according to NASA. Electronics inside each gyroscope detect very small movements of the wheel axis, providing Hubble’s central computer with information about the spacecraft’s rotation rate. Very thin wires route the signals from the gyroscopes, and these wires can degrade over time.

Three of the six gyroscopes installed on Hubble failed in 2009, and three more are still in operation. The three gyros that are still working rely on a newer design for longer life, but one of these units has shown signs of wear in the past few months. This gyroscope, called Gyro 3, has always shown “consistent noisy behavior,” said Pat Cross, Hubble project manager at NASA’s Goddard Space Flight Center.

Hubble typically needs three gyroscopes to function normally, so ground controllers parked Gyro 3 for about seven years until Hubble needed it in 2018, when another gyroscope failed, leaving only three still operational.

“In August, we saw problems,” Cross told Ars this week. “It would intermittently produce some rate information that didn’t match the observed spacecraft body rates, but it was short-lived, and we were determining what that performance was like and how much longer we could tolerate it.”

The gyroscope’s performance worsened in November when it provided incorrect data to Hubble’s control system. The gyroscope sensed that the spacecraft was changing direction when it was not actually moving. “That then contributed to an error in the situation that led to some kind of aberration,” Cross said.

An automated program on Hubble detected the errors and put the spacecraft into “safe mode” twice last month. Hubble resumed scientific observations quickly each time, but then went into safe mode again on November 23. Hubble managers took some extra time to collect data about the health of the gyroscope. Engineers ordered Hubble to move back and forth, and the suspected gyroscope seemed to work consistently well.