Gyroscope Failures Force NASA to Change How It Points Hubble Space Telescope

The Hubble Space Telescope was launched by the space shuttle Discovery on April 24, 1990. Avoiding distortions of the atmosphere, Hubble has an unobstructed view peering to planets, stars, and galaxies, some more than 13.4 billion light-years away. Credit: NASA

NASA plans to transition Hubble to a single-gyro mode to address ongoing issues with one of its gyroscopes. This adjustment is aimed at prolonging Hubble’s operational life and maintaining its scientific output, despite expected minor limitations in tracking and slewing capabilities.

After completing a series of tests and carefully considering the options, NASA announced that work is underway to transition its Hubble Space Telescope to operate using only one gyroscope (gyro). While the telescope went into safe mode on May 24, where it now remains until work is complete, this change will enable Hubble to continue exploring the secrets of the universe through this decade and into the next, with the majority of its observations unaffected.

Gyroscope Challenges and Adjustments

Of the six gyros currently on the spacecraft, three remain active. They measure the telescope’s slew rates and are part of the system that determines and controls the direction the telescope is pointed. Over the past six months, one particular gyro has increasingly returned faulty readings, causing the spacecraft to enter safe mode multiple times and suspending science observations while the telescope awaits new instructions from the ground.

This one gyro is experiencing “saturation,” where it indicates the maximum slew rate value possible regardless of how quickly the spacecraft is slewing. Although the team has repeatedly been able to reset the gyro’s electronics to return normal readings, the results have only been temporary before the problem reappears as it did again in late May.

This is a cutaway diagram of the Hubble Space Telescope, with components labeled. The forward shell houses the telescope’s optical assembly. In the middle of the telescope are the reaction wheels and the bays that house the observatory’s control electronics. The aft shroud houses the scientific instruments, gyroscopes, and star trackers. Credit: NASA’s Goddard Space Flight Center, ESA

Transitioning to Single-Gyro Mode

To return to consistent science operations, NASA is transitioning the spacecraft to a new operational mode it had long considered: Hubble will operate with only one gyro, while keeping another gyro available for future use. The spacecraft had six new gyros installed during the fifth and final space shuttle servicing mission in 2009. To date, three of those gyros remain operational, including the gyro currently experiencing problems, which the team will continue to monitor.

Hubble uses three gyros to maximize efficiency but can continue to make science observations with only one gyro. NASA first developed this plan more than 20 years ago, as the best operational mode to prolong Hubble’s life and allow it to successfully provide consistent science with fewer than three working gyros. Hubble previously operated in two-gyro mode, which is negligibly different from one-gyro mode, from 2005-2009. One-gyro operations were demonstrated in 2008 for a short time with no impact to science observation quality.

Expected Limitations and Adjustments

While continuing to make science observations in one-gyro mode, there are some expected minor limitations. The observatory will need more time to slew and lock onto a science target and won’t have as much flexibility as to where it can observe at any given time. It also will not be able to track moving objects closer than Mars, though these are rare targets for Hubble.

Continuing Legacy and Future Operations

The transition involves reconfiguring the spacecraft and ground system as well as assessing the impact to future planned observations. The team expects to resume science operations again by mid-June. Once in one-gyro mode, NASA anticipates Hubble will continue making new cosmic discoveries alongside other observatories, such as the agency’s James Webb Space Telescope and future Nancy Grace Roman Space Telescope, for years to come.

Launched in 1990, Hubble has more than doubled its expected design lifetime, and has been observing the universe for more than three decades, recently celebrating its 34th anniversary. Read more about some of Hubble’s greatest scientific discoveries.