NASA's planet-hunting Kepler space telescope suffered a failure in one of the reaction wheels that keep it orbiting, the space agency said Wednesday.

Kepler is orbiting some 40 million miles from Earth, rendering it too far away to send astronauts on a repair mission. As a result, Kepler may be transferred to a less prestigious mission, directing its gaze much closer to home in a search for so-called "near-Earth objects", i.e., meteors and asteroids.

"I wouldn't call Kepler down and out yet," said John Grunsfeld, a former astronaut and Hubble repairman who is NASA's associate administrator for space science, at a news conference, according to the New York Times.

Kepler, launched in 2009, was designed to "monitor 100,000 stars in a single patch of sky. It looked for tiny eclipses of Earth-like planets."

Kepler has already been able to identify 115 planets and has a list of 2,740 other candidates. It has concentrated on stars in the constellations Cygnus and Lyra, "looking for dips in starlight caused by planets passing, or transiting, in front of their suns," according to The New York Times:

Just last month, astronomers announced Kepler's discovered two distant worlds that are the best candidates for habitable planets. The other planets found by Kepler haven't fit all the criteria that would make them right for life of any kind - from microbes to man.

Kepler's mission was supposed to be over by now, but last year, NASA agreed to keep it running through 2016 at a cost of about $20 million a year.

For the past four years, Kepler has focused its telescope on a patch of the Milky Way hosting more than 150,000 stars, recording slight dips in brightness - a sign of a planet passing in front of the star, according to the Associated Press