After NASA's Hubble Space Telescope entered "safe" mode about two weeks ago, its operations team has been scrambling to bring a balky gyroscope back online. Now, the space agency says it believes it has fixed the problem.

"The Hubble operations team plans to execute a series of tests to evaluate the performance of the gyro under conditions similar to those encountered during routine science observations, including moving to targets, locking on to a target, and performing precision pointing," NASA said in a news release. "After these engineering tests have been completed, Hubble is expected to soon return to normal science operations."

Ground operators put the telescope into a stable configuration earlier this month after one of the three active gyros that help point the telescope failed. According to NASA, the gyro that failed last week had been exhibiting end-of-life behavior for about a year, and its failure was not unexpected.

Hubble has three pairs of two gyroscopes, with each pair consisting of a primary and back-up gyroscope. Moreover, in each pair, one of the gyroscopes is of an "old" design, while the other is an "enhanced" (or newer) design intended to last for a longer period of time.

After the failure this month, all three of the "old" design gyros have stopped working. This left NASA with two enhanced gyros that were functioning normally and one that had acted up more than seven years ago before being taken out of service at that time. The Hubble telescope can operate on just a single gyro, but three working ones are optimal for normal operations.

Back to science, soon

During the last two weeks, operators have been trying to bring this third, previously balky gyro back online. And they're now reporting some success.

Within the gyroscope is a wheel spinning rapidly inside a sealed cylinder, and some blockage in the fluid around this cylinder appeared to be causing erroneously high spin rates. A series of maneuvers—including turns in opposite directions—seems to have cleared any blockage.

"Following the October 18 maneuvers, the team noticed a significant reduction in the high rates, allowing rates to be measured in low mode for brief periods of time," NASA reports. "On October 19, the operations team commanded Hubble to perform additional maneuvers and gyro mode switches, which appear to have cleared the issue. Gyro rates now look normal in both high and low mode."

Now, the space agency plans to test the gyro under conditions like those during routine science activities. Once these tests are done, the telescope should resume normal science observations.

The Hubble Space Telescope has been in operation since its launch in 1990.