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article imageNASA fixes Hubble telescope the old-fashioned way — flip a switch

By Karen Graham     Oct 24, 2018 in Science
After the Hubble Space Telescope was put into safe mode due to a gyro failure on October 5, the one remaining gyroscope kept as a spare didn't work as expected, so there were some tense moments as NASA engineers turned to some unusual maneuvers.
On Monday, October 8, NASA officials announced the Hubble Space Telescope had been put into protective "safe mode" while engineers attempted to troubleshoot problems with one of the spacecraft's three remaining gyroscopes.
As was explained by Ken Sembach, director of the Space Telescope Science Institute at Johns Hopkins University in Baltimore, the gyroscopes are used to measure the speed at which the spacecraft is turning, and are needed to help Hubble turn and lock on to new targets, and the Hubble team was optimistic the problem could be solved.
Hubble needs three active gyroscopes to detect movement and ensure it is locked in place in the right direction for its scientific work. With Hubble having only two working gyros, engineers turned to the telescope's last remaining gyro, held as a backup. It has been almost a decade ago that all six of Hubble's gyros were replaced, using three new models and three of the older gyros.
STS-60 mission to repair the Hubble Space Telescope.
STS-60 mission to repair the Hubble Space Telescope.
NASA
The older gyro models are known to have some problems after 50,000 hours of service. The other three newer gyros have had fewer problems with their operation. When Hubble was put into a safe mode in early October, it has been operating normally with two newer gyros and one older model. The third newer unit was being held in reserve.
Fixing the problem
The gyroscopes on the Hubble telescope are sealed in chambers containing a highly viscous fluid. A wheel inside the gyro spins at 19,200 revolutions per minute (rpm), allowing the telescope to detect even tiny movements via angular momentum, according to NASA.
Electricity is carried to the motor by thin wires, approximately the size of a human hair, that are immersed in the fluid. Electronics within the gyro detect very small movements of the axis of the wheel and communicate this information to Hubble’s central computer. These gyros have two modes — High and Low.
The High mode is a coarse mode that governs large rotation rates, such as turning the telescope from one target in space to another. The Low mode is a precision mode used for measuring finer rotations when the spacecraft locks onto a target and needs to stay very still, sort of like fine-tuning a pair of binoculars here on Earth.
The Hubble Space Telescope as seen from Space Shuttle Discovery during its second servicing mission ...
The Hubble Space Telescope as seen from Space Shuttle Discovery during its second servicing mission (STS-82)
NASA/ESO/STI
When NASA first turned the backup gyro on, it was spinning too fast, preventing the Hubble from locking on to a target and staying still. So NASA engineers tried an old trick used to fix most electronic products: they turned the gyro 0ff, then quickly turned it back on.
They were hoping this maneuver would clear anything blocking the wheel, but that didn't work. So from ground control, back here on Earth, NASA brought out the big guns: they initiated a maneuver that would shake the gyro a bit. Of course, NASA described the procedure in technical terms, but basically, they did what most of us would do when faced with something that just won't work. Here's how NASA describes the procedure:
"On Oct. 18th, the Hubble operations team commanded a series of spacecraft maneuvers, or turns, in opposite directions to attempt to clear any blockage that may have caused the float to be off-center and produce the exceedingly high rates. During each maneuver, the gyro was switched from high mode to low mode to dislodge any blockage that may have accumulated around the float.
Following the Oct. 18th maneuvers, the team noticed a significant reduction in the high rates, allowing rates to be measured in low mode for brief periods of time. On Oct. 19th, 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."
More about Hubble telescope, NASA, Gyroscope, flip the switch, Rotation
 
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