NASA's Hubble space telescope just lost a vital part, and one of its backups is 'acting drunk.' Here's why astronomers haven't lost hope.

• A vital part inside the Hubble Space Telescope, which records stunning images of the universe, failed on Friday.
• The part is called a gyroscope and helps Hubble point at celestial objects.
• It's the third of six total gyros to fail since 2009, when astronauts last serviced Hubble. Another gyro is acting "drunk," one official said.
• NASA may start using a single gyro to extend the telescope's lifespan, but this would limit the observations Hubble can make.
• When Hubble dies, NASA will have a few ways to get rid of - or rescue - the bus-sized cylinder.

The Hubble Space Telescope is famous for the stunning images it records of the universe, from distant galaxies to dark storms on Neptune and even plumes of water shooting from alien moons.

But the Earth-orbiting spacecraft won't last forever. NASA, the European Space Agency, and other Hubble partners got a sobering reminder of this fact last week, when a vital part called a gyroscope failed.

Hubble's gyros help tell the telescope that it's aimed at the right spot in space. Friday's failure marked the third failure of six total gyros that astronauts installed in 2009. So Hubble is left with three such devices - the exact number NASA needs to keep it operating in standard fashion.

Following the failure, the 12-ton, school-bus-size observatory went into safe mode, NASA said on Monday. But it's not the end.

"Hubble's instruments still are fully operational and are expected to produce excellent science for years to come," NASA said.

However, the space agency is not yet out of the woods: One of Hubble's three remaining gyros is on the fritz and not functioning reliably.

"We've had some issues with this gyro in the past, and we've got some possible leads on the current problem," Ken Sembach, director of the Space Telescope Science Institute, which operates Hubble, told Business Insider. "But the thing that's been clear on Hubble is that these gyros all have a mind of their own. I don't think anybody really knows what's going on with it right now."

NASA plans to continue investigating the issue while the telescope is in safe mode, ideally until a fix is found.

How Hubble's gyroscopes work and why they're so important

Hubble orbits Earth at 17,000 mph and rotates constantly to fixate on a particular object or new region of space.

The spacecraft has no rocket thrusters to orient itself. Instead, internal motors called reaction wheels spin up to provide momentum. The rest of Hubble responds by "pushing" in the opposite direction, causing it to rotate.

If Hubble doesn't know exactly where it's pointing, it'd miss targets and send back blurry pictures.

That's where the gyros come in. Hubble's gyros - ideally, three working at a time - act as an ultra-accurate balancing system that helps the telescope move in the most precise way possible.

Each Hubble gyro is a small cylinder with an internal "float" that spins thousands of times per minute. A special fluid helps the float spin effortlessly for more than five years without fail. Two small wires - each finer than a human hair - measure any change in the spin and turn that into an electrical signal.

The signal feeds into computers, which tell the telescope's reaction wheels how and when to move.

"You need the gyros to know where you are in space. Once you know where you are, you can tell the spacecraft to turn this way or that way," a Hubble telescope team member told Business Insider. (He asked to remain anonymous because he is not permitted to speak to press about the program.)

Instruments called fine guidance sensors, which look for fixed patterns of stars in the sky, also help keep Hubble aware of its position. And the telescope has magnetic sensors that provide some orientation data as well. But none of these devices are as great as the gyros, which are among the most accurate and stable ever made, according to NASA.

Hubble's first set of six gyros failed somewhat rapidly - bromine in the liquid ate away at the sensing wires - so NASA astronauts installed six new-and-improved gyros in 2009, dramatically extending Hubble's life.

Still, Sembach said even these newer devices are finicky, fragile, and will eventually fail.

"Everything inside has to be working just right in order for these things to work," he said.

How a 'drunk' gyro might cut into Hubble's future observations

Since NASA retired its space shuttles in 2011, it has not had a spacecraft able to service Hubble and replace the gyros.

So Hubble's three gyros are what's left to work with for the foreseeable future.

"So far, they seem to be doing well, minus this one that's acting drunk," Sembach said, referring to the third and oddly behaving gyro.

If Hubble's engineers can't figure out what's wrong with the "drunk" gyro and fix it with new software, they'll shut it down.

"If the outcome indicates that the gyro is not usable, Hubble will resume science operations in an already defined 'reduced-gyro' mode that uses only one gyro," NASA said. In that case, the agency would also turn off one of the remaining two "good" gyros to prevent wear-and-tear and extend the telescope's lifespan.

That change would amount to a roughly 50% cut in the area of sky Hubble could observe at a given moment. So the telescope would have to wait longer to study certain objects. It'd also be unable to move quickly enough to target surprise objects, such as exploding stars.

"Another potential downside is that observations that require tracking of moving targets (think objects closer than Jupiter) may be difficult to observe since these are normally done under gyro-only control," Sembach said in an email to Business Insider. (A newly discovered comet or a rogue interstellar asteroid might be two examples.)

Running on one gyro instead of three could add up to a 10% hit to the observatory's efficiency, Sembach said, but he added: "overall the science will be very similar (i.e., great!)."

"While reduced-gyro mode offers less sky coverage at any particular time, there is relatively limited impact on the overall scientific capabilities," NASA said.

On the plus side, saving one gyro for later might help Hubble operate well into the era of the James Webb Space Telescope, or JWST, which is NASA's next-generation, infrared-light-sensing observatory. It's scheduled for launch in 2021.

"Hubble is the world's premier astronomical facility, and it will be until JWST is launched," Sembach said. "Even then, it will have capabilities that'd complement JWST."

What will happen to Hubble after it stops working?

Eventually, the telescope may stop working altogether.

"This weekend's events remind us the spacecraft is aging, and that it will eventually have an end-state," Sembach said. "NASA and the scientific community will have to address this at some point."

Without some kind of intervention, Hubble is expected to crash to Earth in the mid-2030s. Although it's unlikely Hubble would hit anything after falling from space, NASA and its partners have committed to make sure Hubble doesn't pose any threat.

To that end, astronauts who serviced the telescope in 2009 screwed on a grappling fixture. This allows for a spacecraft to dock with Hubble, giving NASA a variety of options to deal with its observatory.

"You could grab it and de-orbit in the Pacific Ocean, because you don't want it hitting an inhabited area," Sembach said.

If that's the case, NASA would likely target the most remote region of the Pacific, a zone sometimes called "Point Nemo" or the "spacecraft cemetery."

Another option is for an uncrewed spacecraft to boost Hubble to a very high and stable orbit above Earth, where it would likely remain for thousands of years.

"Or you could potentially go back and service the thing," Sembach said. "Right now that's not possible, but maybe in 10 years. Who knows?"

There is a fourth (more radical) option: A large, next-generation commercial vehicle, such as a modified spaceship of SpaceX's upcoming Big Falcon Rocket system, could capture Hubble and bring it back to Earth for display in a museum.

"I would not be opposed to that," Sembach said.