NASA’s Hubble Space Telescope, famous for its brilliant pictures of the cosmos, is having some trouble in orbit around Earth. On Friday, an important mechanism on the spacecraft failed — one that is needed to orient the observatory in space and keep it pointed at parts of the sky for long periods of time. Now, Hubble is in a safe mode while engineers try to address the problem.

The broken mechanism on the Hubble is a gyroscope, or gyro. It’s a device that many spacecraft use to maintain their orientation in space. Gyros are essentially wheels inside the vehicle that spin, providing stability and maintaining the position of the spacecraft. They’re a key tool that Hubble uses to point and look at different spots in the Universe in order to observe stars, exoplanets, supernovae, and more.

Hubble is in a safe mode while engineers try to address the problem

Hubble, which launched in 1990, was originally built with six gyros, and the spacecraft needs at least three of them to work in order to operate efficiently. Over the years, Hubble’s gyros have run into problems and many have failed. At one point in 1999, so many were inoperable that the telescope couldn’t perform science observations for a few weeks. But during the Space Shuttle program, astronaut crews were able to visit Hubble periodically and replace the gyros. The last Hubble servicing mission occurred in May 2009, during which all six of the gyros were replaced.

Before the Friday failure, Hubble still had four gyros functioning, NASA confirmed to The Verge. But after this most recent one quit, the Hubble team has been trying to bring one of the four back online, which had been off for a while, according to a series of tweets from Rachel Osten, the deputy mission head for Hubble at the Space Telescope Science Institute. However, this other gyro is proving “problematic,” she says. If it can’t come back online, that means Hubble is down to just two functioning gyros. It’s not ideal.

It’s not a difficult decision, @astrogrant: the plan has always been to drop to 1-gyro mode when two remain. There isn’t much difference between 2- and 1, and it buys lots of extra observing time. Which the Astro community wants desperately. — Dr. Rachel Osten (@rachelosten) October 8, 2018

The situation isn’t a cause for despair just yet. Previously, Hubble engineers figured out a way to operate the spacecraft with only two gyros in an effort to extend Hubble’s lifetime. Additionally, NASA claims that the Hubble team had expected this particular gyro to fail for about a year. If the problematic gyro doesn’t come back online, the engineers have a plan that will allow the Hubble to operate with as few as one gyro. She argues there isn’t much difference between operating with two gyros and with one gyro; both modes can still allow astronomers to observe the sky and do valuable science work.

Ultimately, getting that gyro up and running would be the best-case scenario here. And time is crucial because Hubble can’t do any science while in safe mode. During this time, all of the spacecraft’s instruments are shut down, while only the most essential systems needed to keep the vehicle running remain on.

To repair Hubble you need a trained repair crew in a mid high LEO with a spacecraft that has two weeks + flight duration, lots of tool stowage, an airlock, a robotic arm. We used to have some of those but none now or on the horizon. #saveHST — Wayne Hale (@waynehale) October 8, 2018

Still, it’s not quite the end for Hubble just yet. This spacecraft has overcome worse situations in the past; when the observatory first launched, engineers soon realized that the spacecraft’s primary mirror had a flaw that prevented the telescope from getting the sharp images it was designed to capture. Although the mirror was ultimately replaced, for three years Hubble was unable to capture the sharp, amazing pictures that it’s now known for.

it’s not quite the end for Hubble just yet

Today, there aren’t any options to replace failed hardware on Hubble. The telescope was designed to be serviced by crews on the Space Shuttle, and there aren’t any vehicles currently available that could bring astronauts to the observatory and fix it in orbit. Some have considered sending robotic spacecraft to service Hubble, but none have panned out, with some plans determined unfeasible. Last year, The Wall Street Journal reported that the Trump administration was considering a plan to send a crew to Hubble on a commercial spacecraft called Dream Chaser being developed by the Sierra Nevada Corporation. However, such a flight has not been officially acknowledged and there hasn’t been any update since the original WSJ report.

Unless a concrete plan is made to send a new spacecraft to Hubble to fix it, the telescope will eventually become inoperable if all of its gyroscopes fail. And even if they remain active, Hubble is coming down eventually. The telescope orbits high up, around 350 miles up, but its path decays over time thanks to small particles from the Earth’s upper atmosphere that bombard the spacecraft and drag it down toward our planet. Additionally, Hubble doesn’t have any onboard thrusters either, so it can’t boost itself to a higher orbit. Most models show Hubble coming back to Earth naturally in the 2030s, according to Space.com. It’s a tad concerning since its primary mirror could survive re-entry, though most of it will likely burn up harmlessly.

But rather than let it fall on its own, NASA does have the option to guide Hubble’s descent to a safe area over Earth — perhaps over the ocean. During the last Hubble servicing mission, astronauts installed a mechanism on the spacecraft that could allow future vehicles — crewed or uncrewed — to meet up with the telescope and dock with it. Those vehicles could then potentially drag Hubble out of orbit, sending it on a fiery dive through Earth’s atmosphere where it doesn’t have the possibility of harming anyone. No plans have been made for this de-orbit, however.

As Hubble nears the end of its life, NASA is working to get the telescope’s successor into space. In 2021, the space agency plans to launch a new giant observatory called the James Webb Space Telescope. While the spacecraft has run into numerous delays and cost overruns, the JWST will be even more powerful than Hubble when it launches, capable of observing galaxies and stars that formed shortly after the Big Bang. However, JWST will sit about 1 million miles from Earth and there are no plans to service the telescope once it’s in space. If something breaks on JWST, engineers will need to fix it from Earth (if possible).

In the meantime, Hubble still should have some valuable observational time left. The telescope is known for many groundbreaking discoveries. It’s helped astronomers figure out just how fast our Universe is expanding, and it’s captured images of planets outside our Solar System. Hopefully, there is still quite a bit more science to come from Hubble, once it exits safe mode.

Update October 9th, 8:50AM ET: This post was updated to clarify the purpose of a gyroscope.