NASA said the Hubble Space Telescope resumed scientific observations on Friday after ground teams spent much of the past three weeks assessing the performance of a troublesome gyroscope. This troublesome gyroscope is a key part of the observatory's pointing system. Hubble's gyroscopes measure how fast the spacecraft is turning, helping the telescope focus its aperture on distant cosmic wonders.
The Hubble Telescope has been operating for nearly 34 years since its launch in 1990 aboard NASA's Space Shuttle Discovery, but it still provides astronomers with valuable scientific data. Five additional shuttle servicing missions repaired Hubble, upgraded its science instruments, and replaced hardware that had deteriorated from long periods in space. Among other tasks, astronauts on the last shuttle servicing flight in 2009 installed six new gyroscopes on Hubble.
The gyroscope has long been one of Hubble's most maintenance-requiring components. According to NASA, the wheel inside each gyroscope rotates at a constant speed of 19,200 revolutions per minute, and the wheel is sealed in a cylinder suspended in a thick liquid. Electronics within each gyroscope detect tiny movements of the wheel's axis, providing information to Hubble's central computer about how fast the spacecraft is spinning. Hair-thin wires carry gyroscope signals, and these wires deteriorate over time.
Of the six gyroscopes installed on Hubble in 2009, three have failed and the other three are still working. The three still working gyroscopes have a newer design and last longer, but one of them has shown signs of wear over the past few months. Pat Crouse, Hubble project manager at NASA's Goddard Space Flight Center, said the gyroscope, named Gyro 3, has been exhibiting "constant noise."
"We discovered the problem as early as August," Kraus said. "It will sporadically output some velocity information that is inconsistent with the observed spacecraft body velocity, but this is short-lived and we are determining the characteristics of this performance and the extent to which we can tolerate it."
In November, the gyroscope's performance deteriorated because it fed Hubble's control system erroneous data. The gyroscope senses that the spacecraft is changing direction when in fact it is not moving. This creates an error in attitude, resulting in a bit of drift.
Automated software on Hubble detected the errors and put the spacecraft into "safe mode" twice last month. Hubble quickly resumed science observations each time, but entered safe mode again on November 23. Hubble managers spent some extra time collecting data on the health of the gyroscope. Engineers directed Hubble to move back and forth, and the suspected gyroscope seemed to be working fine all the time.
Engineers recognized that gyroscopes could easily malfunction, so they devised a new way to allow Hubble to continue providing scientific data even if only one gyroscope was operating. In this single-gyro mode, Hubble's control system will receive input from the single gyroscope as well as the magnetometer, solar sensor, and star tracker. In single-gyro mode, most of Hubble's science portfolio is unaffected, but the observatory may be limited in tracking some faster-moving targets, such as planets, asteroids or comets in the inner solar system.
The Hubble telescope itself does not have any thrusters to control the direction or adjust the orbit. It can only use rotating reaction wheels to twist the spacecraft from one direction to another. There are three fine guidance sensors to maintain the lock on the guide star, allowing Hubble to remain stable during scientific observations. The ground team is also paying close attention to these sensors. One of the precision guidance sensors has shown signs of performance degradation over the past two years. Debris or oil buildup on the bearings appears to be causing sensor drag, sometimes causing the sensor to stall.
Another maintenance mission?
While Hubble's imaging capabilities and resolution don't match those of the newer James Webb Space Telescope, the older observatory still plays a unique role in NASA's fleet of science missions. Hubble is sensitive to visible and ultraviolet light, while Webb's sensor detects infrared light.
NASA wants to extend Hubble's life as long as possible. Even if Hubble's sensors and instruments are still working, air resistance will eventually bring the telescope back into the atmosphere because the spacecraft has no rocket engines. Each shuttle servicing mission raises Hubble to a higher orbit to offset drag.
Hubble is currently flying at an altitude of about 320 miles (520 kilometers). According to current predictions, Hubble will return to the atmosphere in the mid-2030s, but this depends on conditions in the upper atmosphere, which changes with fluctuations in solar activity.
Last year, SpaceX and NASA announced a feasibility study to determine whether it would be possible to connect the SpaceX Dragon spacecraft to Hubble and re-raise the observatory to extend its orbital life. The six-month study, which ended earlier this year, is also expected to examine how Crew Dragon astronauts could potentially serve Hubble. If a repair mission is considered possible, replacing the gyroscope will definitely be a priority for NASA.
NASA and SpaceX have yet to announce the results of the feasibility study. In December last year, NASA also issued a request for information to other U.S. companies, hoping that they could come up with their own commercial solutions to enhance Hubble's orbit. The agency received eight responses to the solicitation, which did not include SpaceX's separate feasibility study. Ultimately, NASA may decide not to move forward with Hubble's commercial service mission.
NASA spokesperson Alise Fisher said the agency does not have an estimated timetable for completing its review of restarting Hubble research. "We will continue our internal review early next year."