Spaceflight Insider

Hubble enters safe mode after encountering gyroscope problem

hubble space telescope 2009

The Hubble Space Telescope as seen by the departing STS-125 crew after a week servicing the observatory in 2009. Photo Credit: NASA

NASA’s Hubble Space Telescope hasn’t had a tune up in almost 10 years and it has encountered a problem with one of the three gyroscopes actively being used by the spacecraft.

Hubble entered into safe mode almost a week ago on Friday, October 5 around 6:00 p.m. EDT (22:00 GMT). While the telescope’s instruments are still operational – Hubble’s science operations have been placed on hold for now.

The Hubble Space Telescope has to remain steady when it is directed toward its targets and the affected gyroscope is used for this purpose. Safe mode places Hubble in a configuration that should allow controllers on the ground to diagnose and correct the problem before bringing the spacecraft back into service. According to an overview of the gyroscopes appearing on the Space Telescope Science Institute’s website:

…only gas bearing gyros are capable of providing the combination of extremely low noise, excellent stability, and high sensitivity to motions that is required for HST observations. Each gyro has a wheel spinning at a constant rate of 19,200 rotations per minute on gas bearings. The wheel is mounted in a sealed cylinder, which floats in a thick fluid. Electricity is carried to the motor that spins this wheel by thin wires, or flex leads, approximately the width of a human hair. The wires are immersed in the fluid along with the wheel. Changes in the gyroscope rates induced by movement of HST are captured by onboard electronics. This information is then fed to Hubble’s central computer where it is analyzed. The HST pointing is changed through the use of several reaction wheel assemblies. Each assembly contains spinning wheels, which when spun at varying rates, create the appropriate torques required for the desired movement.

Hubble has been on orbit for about 28 and-a-half years ever since Space Shuttle Discovery deployed it during STS-31 in 1990. Shuttle Atlantis, on STS-125, conducted Servicing Mission-4 in 2009, the final re-servicing mission to the observatory. During STS-125, six new gyros were installed.

As with most of the expensive vehicles sent into the black, Hubble comes with numerous redundancies. Hubble needs three of these gyroscopes to perform its responsibilities as proficiently as possible. However, Hubble can get by with just one. 

The gyro in question had been showing signs of failing for about a year and the telescope is already down two. The remaining three gyros are considered to be “enhanced” and are expected to have “significantly longer operational lives.”

NASA will form an Anomaly Review Board while engineers located at the agency’s Goddard Space Flight Center and the Space Telescope Science Institute review telemetry from the spacecraft and work to develop methods to bring the gyro back to normal operating levels in a “nominal” mode with three gyros.

Experts within the industry who are familiar with the telescope as well as engineers on the Hubble Team will use their knowledge of this component to bring it back online.

If they are unable to do so, Hubble will enter into a “reduced-gyro” mode which should allow for scientific operations, in a slightly reduced capacity.

STS-125 was the last of the Hubble re-servicing missions to the lead spacecraft in NASA’s Great Observatories. It is thought that the Hubble Space Telescope could be operational into the 2030s.







Jason Rhian spent several years honing his skills with internships at NASA, the National Space Society and other organizations. He has provided content for outlets such as: Aviation Week & Space Technology,, The Mars Society and Universe Today.

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