The Hubble Space Telescope is old, at least by technology standards. The telescope started orbiting Earth nearly 25 years ago, way before we had things like iPhones and smartwatches. And as Hubble stays up there in space, its equipment becomes outdated and it stops operating to capacity.

So what happens to Hubble when it reaches a point that it can no longer function?

The answer is a grim one: Hubble will probably get knocked out of Earth's orbit and then plummet back to the planet, burning up in its atmosphere on the way down.

During the space shuttle program, astronauts updated Hubble, traveling to and from it via space shuttle. Once there, they repaired its equipment and installed new technology. NASA scheduled the last such mission for 2005.

However, after space shuttle Challenger exploded returning to Earth, NASA's plans changed. There was worry that because Hubble-headed astronauts had no "plan B" if something went wrong with a shuttle, that the mission was too dangerous.

But Hubble desperately needed the work because its capacity was down to 20 percent. So in 2009, it finally got its maintenance and updates. That mission, dubbed STS-125, involved a lot of work, but eventually, Hubble had 100 percent full functionality.

That's the last time we've heard about a mission to work on Hubble. And now, as it's sitting out there in space, it's degrading again. Every shake and vibration on the telescope knocks it just a little more out of orbit. Eventually, it will fall out of orbit and head back to Earth.

"Its orbit will decay," says Mike Massimino, one of the astronauts on STS-125. "The telescope will be fine, but its orbit will be bringing it closer and closer to Earth. That's when it's game over."

Of course, NASA could send a mission to push it back into a stable orbit, but the problem is that the agency has not stated a desire to do so. A new telescope, the James Webb Space Telescope (JWST), launches in 2018 and will orbit Earth from 900,000 miles away, allowing it to see farther than Hubble. It probably doesn't make sense to schedule a potentially dangerous mission to repair Hubble and fix its orbit when the launch of the JWST is right around the corner.

So with a Hubble repair mission unlikely, what is its fate? More than likely, because of its size, NASA will have to help it in its fall to Earth, adjusting it so that some of its parts, particularly its large mirrors, which can sustain high temperatures, don't hit anyone.

There's still a chance NASA could decide to keep Hubble going longer, though, even after the JWST launch, and many are hoping for that.

"I think we should keep Hubble going. You learn a lot when you do these types of service missions, flying people up to do another repair mission," says Massimino. "Hubble can't do it all, but you can still get a lot of great astronomy done from it."

[Photo Credit: NASA]