Trouble with one of the Hubble Space Telescope’s three remaining gyroscopes, critical for aiming and locking onto targets, has prompted mission managers to switch to a backup control mode that will limit some observations, but keep the iconic observatory running well into the 2030s, officials said Tuesday .

“We still believe there’s very high reliability and likelihood that we can operate Hubble very successfully, doing groundbreaking science, through the rest of the ’20s and into the 2030s,” Patrick Crouse, the Hubble project manager, told reporters during an afternoon teleconference.

At the same time, Mark Clampin, director of astrophysics at NASA Headquarters, said the agency had ruled out, at least for now, a proposed commercial mission to boost Hubble to a higher altitude using a SpaceX Crew Dragon spacecraft. The flight was suggested by SpaceX and Crew Dragon veteran Jared Isaacman as a way to extend Hubble’s lifetime.

By boosting the telescope to a higher altitude, the subtle effects of “drag” in the extreme outer atmosphere, which act to slowly-but-surely pull a spacecraft back to Earth, could be reduced. Isaacman, a billionaire who chartered the first fully commercial flight to low-Earth orbit in 2021, is in training to lead three more SpaceX “Polaris” missions, including a flight this summer in which he plans to become the first private citizen to stand in an open hatch and float, if not walk, in space.

But project managers said Tuesday Hubble is in no danger of falling back to Earth any time soon. The latest calculations show the observatory will remain in orbit until at least 2035, allowing time to consider possible options, if warranted, down the road.

“After exploring the current commercial capabilities, we are not going to pursue a re-boost right now,” Clampin said. “We greatly appreciate the in-depth analysis conducted by the NASA and (the SpaceX-Isaacman) program, and our other potential partners, and it’s certainly given us better insight into the considerations for developing a future commercial reboost mission.

“But our assessment also raised a number of considerations, including potential risks such as premature loss of science and some technology challenges. So while the reboost is an option for the future, we believe we need to do some additional work to determine whether the long-term science return will outweigh the short-term science risk.”

Hubble’s decades of service in space

The Hubble Space Telescope was launched aboard the shuttle Discovery on April 24, 1990, with a famously flawed mirror, the opening chapter of an improbable tale in which spacewalking repair crews turned a national embarrassment into an international icon of science.

Hubble was initially hobbled by an error during the 94.5-inch primary mirror’s fabrication that resulted in an optical defect known as spherical aberration, preventing the telescope from bringing starlight to a sharp focus.

But engineers quickly figured out a way to correct Hubble’s blurry vision. They designed a new camera equipped with relay mirrors ground to prescriptions that would exactly counteract the primary mirror’s aberration. Another device, known as COSTAR, was designed to direct corrected light into Hubble’s other instruments.

During a make-or-break December 1993 shuttle servicing mission, the new Wide Field Planetary Camera 2 and COSTAR were installed by spacewalking astronauts. They also replaced Hubble’s solar panels and other critical components.

NASA would go on to launch four more servicing missions, installing new, state-of-the-art instruments and replacing aging components like critical fine guidance sensors and gyroscopes, which move the telescope from target to target and then lock-on with rock-solid stability for detailed observations.

The gyroscopes are critical to Hubble’s longevity. The telescope was launched with six ultra-stable gyroscopes, but only three at a time are needed for normal operation. During the final servicing mission in 2009, all six were replaced. Three of the new units included components susceptible to a form of corrosion, while the other three featured an improved design that greatly reduced or eliminated that risk.

In any case, by the time Hubble’s 30th anniversary rolled around in 2020, all three of the six older-model gyroscopes had failed.

One of the remaining three units, gyroscope No. 3, began acting erratically earlier, and its performance progressively worsened. On May 24, the gyroscope was taken offline, putting the observatory into protective “safe mode,” halting science operations while engineers discussed their options.

Knowing gyroscope failures were inevitable, engineers earlier developed software that would allow Hubble to operate with just two gyroscopes or even one. The downside was that the telescope could only reach targets in about half the sky at any given time instead of 85% or more with all three gyroscopes.

Even though the telescope could be operated more efficiently with two gyroscopes, engineers concluded it would make more sense to put one of the two remaining healthy units in stand-by mode and to operate Hubble with just one gyroscope, holding the other in reserve for use as needed.

“Our team first developed a plan for one-gyro operations over 20 years ago, and it is the best mode to go forward to prolong Hubble’s life,” Crouse said. “There are some limitations. It will take us more time to (move) from one target attitude to the next and to be able to lock on to that science target.

“That will lead to lower efficiency for scheduling science observations. We currently schedule about 85 orbits a week and we expect (to be) able to schedule about 74 hours a week, so about 12% reduction in scheduling efficiency.”

In addition, because the telescope’s movement in single-gyroscope mode is less precise and subject to error, “we won’t have quite as much flexibility as to where we can observe in the sky at any one time. But over the course of a year, we will have the full sky available to us.”

One other limitation: the telescope will not be able to lock onto and track targets closer than the orbit of Mars, though such observations were rare even in the three-gyroscope mode.

In the meantime, engineers plan to implement the one-gyroscope control mode in the coming days, and to return Hubble to science operations around the middle of the month.

“We updated reliability assessments for the gyros … and we still come to the conclusion that (we have a) greater than 70 percent probability of operating at least one gyro through 2035,” Crouse said.

The infrared-sensitive James Webb Space Telescope is building on Hubble’s legacy, pushing deeper into space and time and producing a steady stream of discoveries as it moves to the forefront of space-based astronomy. But Hubble is still making world-class observations, and astronomers want to keep it operating as long as possible.