How ‘safe mode’ protects space telescopes like Hubble and TESS

When things go wrong, it's time to hunker down.
This image was taken from the Space Shuttle Atlantis just after the Hubble Space Telescope was intercepted by the Shuttle’s robotic arm during Servicing Mission 4. NASA

What could bears and space telescopes possibly have in common? Last month, both the Hubble Space Telescope and the Transiting Exoplanet Survey Satellite (known as TESS) ended up in a sort of hibernation. When faced with some kind of threat, these telescopes shut down all non-essential systems, doing only the bare minimum to keep themselves alive and prevent further damage if possible—sort of like how a bear makes it through a long winter.

For telescopes, their hibernation is known as “safe mode.” Instead of being endowed with a natural instinct for self-preservation, though, a telescope’s routines for entering safe mode are written into their code by the engineers who design them. 

Safe mode is “pretty much exactly what it sounds like,” explains Sam Factor, an astronomer at The University of Texas at Austin. “When the telescope encounters a problem that it can’t automatically recover from it shuts down all non-essential systems to keep the delicate science instruments safe.” This includes pointing its solar panels at the sun to ensure a steady supply of energy and pointing its antenna right at Earth to ensure it can receive instructions on what to do next.

Safe mode “is a state that will keep the telescope alive and hopefully not make anything worse while it waits for ground controllers to assess and fix the problem,” adds Factor. “Think of it like how you leave your house when you go on vacation in the winter. All the lights are off, your computer is shut down, but your heat is still on at a minimal level to keep your pipes from freezing and the refrigerator is still running. When you get home, everything should be ready to go.”

A number of situations could lead to a telescope entering safe mode. Sometimes a sensor is broken, producing bizarre readings that make it look like the telescope is malfunctioning, but actually everything is fine besides that one sensor. Other times, a hardware component actually does wear out and break or the software encounters an unexpected bug. 

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Some issues are particularly unique to space. For example, “in the harsh environment of space, a high-energy particle known as a cosmic ray could hit and flip a bit on a memory chip, requiring a reset to restore functionality,” says Claire Andreoli, a communications officer at NASA Goddard. If intense space weather is forecasted, a telescope’s operations team may even purposefully put it in safe mode to protect the delicate electronics while it rides out the solar storm.

“Every spacecraft is programmed to handle such scenarios,” adds Andreoli.

Every NASA spacecraft also has a dedicated team on the ground responsible for its operations. When a telescope enters safe mode, its team must collect all the information it can from the telescope’s onboard sensors and computers to determine the real issue. As for fixing the telescope, all repairs must be done remotely—some telescopes, like JWST, are simply too far away for astronauts to service, and others like Hubble haven’t been serviced in years after the sunsetting of the Space Shuttle program. This means scientists must get creative with software solutions, whether that’s squashing bugs in the code or introducing new pieces of code to work around failing hardware.

Often, safe mode incidents are just a part of a telescope’s life—who doesn’t have to go to the doctor every once and a while to make sure things are working properly after a health scare?—but these incidents still often leave astronomers on edge. Plus, any time spent in safe mode is time lost for doing science. Observations for space telescopes are scheduled months in advance, so a disruption to operations switches up that carefully planned timeline.

“Anything that was meant to be observed just gets pushed back to a later time,” explains Factor. Usually this means planned observations will still happen as planned, just with a delay, but “if it takes a long time to fix the telescope, that can mean that observations are lost if, for example, your target is time sensitive or in a region of the sky that can no longer be observed because of where the sun is,” he adds.

[ Related: Where do all those colors in space telescope images come from? ]

Both Hubble and TESS entered safe mode on April 23rd, and have now already recovered from their health scares. 

Hubble’s incident was due to an issue with one of the spacecraft’s gyroscopes, which are needed to properly point the telescope at its astronomical targets. This isn’t the first spacecraft to deal with an aging gyroscope; the planet-hunting Kepler Space Telescope famously suffered a gyroscope failure in 2013, and the Hubble team has been aware of the possibility of such a failure for years—so aware, in fact, that six new gyroscopes were installed on Hubble by astronauts in its final servicing mission back in 2009 to stave off problems. As of April 30, three of those gyroscopes are currently functional, including the one that caused the recent safe mode event (and a similar incident in November 2023). Thankfully, if this problematic gyro strikes again, NASA has a plan to make Hubble work with only one of the three parts operational.

TESS was also a victim of this finicky hardware. One of its reaction wheels (a type of gyroscope) malfunctioned after a botched restart from an earlier incident on April 8th. That issue has been fixed, with operations resuming on May 3rd, but the team is still trying to figure out what caused the first safe mode event on April 8th.

Keeping a spacecraft alive from afar is a major challenge, but safe mode provides the mission teams some time to figure out what’s wrong before more damage can happen—like a bear staying inside a nice warm cave instead of running out into a blizzard, or going to the doctor as soon as you notice a new twinge of pain instead of waiting until you need the emergency room. Of course, it’s always a bit stressful when something goes wrong, but at least for now both Hubble and TESS are safe, healthy, and ready to do more science.