Mission to save NASA telescope that's falling to Earth

A three-armed spacecraft has rocketed into orbit to rescue a NASA telescope that's in danger of crashing back to Earth.

Northrop Grumman launched Katalyst Space Technologies' Link spacecraft on Friday from the Marshall Islands in the Pacific.

The Pegasus rocket blasted off from the belly of a modified aeroplane, putting Link on course to reach and capture NASA's Swift Observatory in about a month.

Launched in 2004, Swift is sinking faster than ever because of recent solar storms.

NASA is paying $US30 million ($A43 million) for Katalyst to capture the telescope and boost its orbit so it can continue tracking some of the biggest explosions in the universe, such as gamma ray bursts and exploding stars.

If all goes well, Swift could be back scanning the cosmos by September.

Observations are on hold to preserve the telescope's orbit as long as possible.

NASA's Hubble Space Telescope could be a candidate for a similar salvage operation in a few years.

It's also slipping in altitude because of increased atmospheric drag caused by the sun's outbursts.

The 1.4 tonne Swift is circling 360km above Earth.

Katalyst aims to raise the telescope's altitude by 240km, back to where it all began.

Link's thrusters will fire to boost Swift slowly, so there's no heavy jostling.

Katalyst threw the mission together in just nine months.

NASA insisted on a rush job because the telescope will be too low to recover by the fall.

Without a boost, it's predicted to plunge to its demise in October.

Bad weather and technical issues caused a series of last-minute launch delays.

"This is a high-risk, high-reward mission," Katalyst Space CEO Ghonhee Lee said before lift-off.

"The biggest danger was always we don't launch anything and we let Swift burn up in the atmosphere. So we were always trying to avoid that risk, and our team has done that."