It seems like NASA is announcing the discovery of a new Earth-like planet every few weeks—like so, and so, and so—but those days might be over for a long while. Not that there aren't more discoveries to make, it's just that the spacecraft responsible for them has a faulty "reaction wheel" that might mean the end of the mission, reports Space.com. Kepler needs three such wheels functioning properly to zero in on targets, but one is kaput. NASA sent up the craft with a spare when it launched the $600 million mission in 2009, but that had to be deployed last summer. Now it's out of spares.
Engineers aren't giving up hope of getting one of the two troublesome wheels working again, but if that proves impossible, Kepler will have to shift to a less ambitious sky-gazing mission. (It's 40 million miles away, so a manual fix is out of the question.) Kepler has been busy in its four-plus years, identifying 115 planets and another 2,740 likely candidates, reports the New York Times. "Before we flew Kepler, we didn't know that Earth-sized planets in habitable zones were common throughout our galaxy," says a NASA scientist. "We didn't know that virtually every star in the sky had planets around them. Now we know that."