Astronauts have visited the Hubble Space Telescope for repairs and maintenance on five occasions.

NASA's iconic Hubble Space Telescope is experiencing another mysterious glitch. Already, it's been offline and in "safe mode" for almost a week.

Hubble's scientific instruments safely shut down after several failures to communicate timing information between them. NASA engineers are analysing data from the spacecraft to study the issue, but they've shared little information so far.

A NASA spokesperson told Insider that it's "too early in the investigation" to say what might be causing the issue, how engineers might fix it, or when Hubble might come back online.

This isn't the first time Hubble has gone offline this year. The observatory spent nearly five weeks in safe mode after its payload computer suddenly stopped working on June 13. NASA engineers finally brought Hubble back online in July by activating some of its backup hardware - a risky, complex manoeuver that could have triggered new issues if done incorrectly. Now, four months later, it's offline again.

The Crab Nebula - a 6-light-year-wide expanding remnant of a star's supernova explosion - as imaged by Hubble.

Hubble is the world's most powerful space telescope. It has photographed the births and deaths of stars, spotted new moons circling Pluto, and tracked two interstellar objects zipping through our solar system. Its observations have allowed astronomers to calculate the age and expansion of the universe and to peer at galaxies formed shortly after the Big Bang. But Hubble has been orbiting Earth since 1990, and it's getting old.

"This is an older machine, and it's kind of telling us: Look, I'm getting a little bit old here, right? It's talking to us," NASA Associate Administrator Thomas Zurbuchen said in a video after engineers fixed the July issue.

"Could the cause of the problem have something to do with Hubble's age? The answer is almost certainly yes," Paul Hertz, director of NASA's astrophysics division, told Insider in July. "Someday, a component will randomly fail that we won't have a backup for. That's the most likely way the Hubble mission will end."

The glitch is harder to trace this time

The Hubble Space Telescope was deployed on April 25, 1990 from the space shuttle Discovery.

The first sign of trouble came on October 23, when Hubble's science instruments issued error codes. They had lost a synchronisation message, which provides the instruments with timing information so that they can respond to commands and collect data correctly.

The Hubble team reset those instruments and the telescope resumed its science operations the following morning. But on October 25, Hubble's science instruments sent back even more error codes, indicating the loss of several synchronisation messages. The instruments automatically entered safe mode, as they're programmed to do.

"The team is still working to isolate the problem," the NASA spokesperson told Insider. "They are looking at hardware design documents, software, and are collecting more data from the spacecraft to help diagnose it."

In June, after the payload computer failed, engineers were able to quickly narrow down the cause of the issue, since it could only be traced to a few systems. This time, they must cast a much wider net. NASA's Hubble team is investigating all the systems that interface with the science instruments and all systems associated with the synchronisation messages.

The Hubble Space Telescope in orbit.

"Hubble has really changed the way we look at the universe, and is still the most in-demand telescope in the world right now," Hertz said in July. "So scientists have not run out of things to do with it, which is why it's important to us that we keep working hard to bring it back into science operations, so that we can continue making great discoveries."

NASA is preparing to launch a new Earth-orbiting observatory, the James Webb Space Telescope, in December. That telescope is poised to revolutionise astronomy, peering into the depths of the universe with unprecedented precision in infrared light. But it's not a replacement for Hubble. In fact, they're supposed to work together.

Some of the Webb telescope's first tasks will be studying objects that Hubble has discovered. Then once Webb starts making its own discoveries, NASA hopes to follow up on them with Hubble.

"What astronomers are really looking forward to is the one-two punch of the Hubble-Webb combination," Hertz said.

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