( CNN ) -- On Wednesday , NASA officials announced a serious problem with the Kepler satellite , the world 's most successful planet-finding machine . Since its launch four years ago , Kepler has found more than 2,700 possible planets orbiting stars other than our Sun , of which more than 100 have been confirmed . A few of these exoplanets resemble the Earth in size or mass . Recently , three Earth-like planets were even reported to be in the habitable zone : close enough to the star they orbit that water is liquid , yet not so close that it is boiling . Planets with liquid water may well harbor life . Now , the second of four of the Kepler spacecraft 's reaction wheels , which aim the vessel 's instruments , appears to have failed . It remains to be seen whether full repairs are possible . For the spacecraft to point accurately , at least three reaction wheels are needed , corresponding to the three dimensions ( up-down , north-south , east-west ) . The fourth wheel serves as a backup and provides cross-comparisons of data among the wheels . The first wheel failed last summer ; now Kepler has too few reaction wheels to keep pointing with sufficient stability . When a major component like a thruster fails on any spacecraft , operations software points the craft 's solar panels toward the sun to ensure a continuous power supply . Power is a satellite 's lifeblood : Lose it and you lose communications , so the satellite ca n't be oriented properly or take data . Backup batteries allow the solar panels to be misdirected for a few hours or so , before all power is lost . But batteries drain quickly , so engineers design software to make sure that when something goes wrong , the spacecraft points in a direction that preserves power . Urry : Three more homes for life in the universe ? Reaction wheels are spinning flywheels that carry `` angular momentum , '' a term roughly analogous to the force that keeps a car coasting even when the driver 's foot is off the gas . Spinning objects keep spinning unless they transfer angular momentum to another object . For example , if a flywheel is commanded to spin more slowly ( through an electric motor ) , the spacecraft will pick up spin to compensate . If the flywheel spins faster , the spacecraft will spin in the opposite direction . Increasing or decreasing the spin of a reaction wheel is therefore a way of pointing the spacecraft . This may sound like a complicated way to make a telescope move , but the problem is , there is nothing in empty space to push on . To close a door , you push on it . This works because gravity holds you firmly on the ground , and your feet stay put because of friction with the floor . If you pushed on an open door in space , it would push you in the opposite direction . In space , there is no standing still . So Kepler 's reaction wheels are essential for pointing the spacecraft accurately and steadily . Unfortunately , less stable pointing means less accurate photometry ( the measurement of light from the star ) . Since Kepler finds planets by measuring the tiny dips in a star 's brightness when a dark planet moves across the face of that star , less accurate photometry means Earth-like planets will be too hard to find . Urry : A meteor and asteroid -- 1 in 100 million odds . NASA is trying to figure how to fix or work around the broken reaction wheels . It has done amazing things before ; you do n't have to be an optimist to think there is still a chance to turn Kepler around . But the loss of a fully functional Kepler would be terrible . It has found more potential planets than any other facility or method . Kepler data have yielded an estimate of the total number of Earth-like planets in the Milky Way galaxy : at least 17 billion . That 's an Earth-like planet around one in every six stars . Fortunately , there are other ways to find planets than by detecting transits ( the passage of a planetary body across a sun ) , as Kepler does . In fact , the first few hundred exoplanets were found by the `` radial velocity '' technique , which detects tiny motions of a star as it and its planets orbit one another . A Yale astronomy professor , Debra Fischer , has pioneered clever improvements to this technique so that she can find 100 Earth-size planets , perhaps 10 % of which might harbor life . ( Hear the full story in Fischer 's TEDx talk , `` Why We Need to Find 100 Earths . '' ) Fischer is going after Earth-like planets in the habitable zone . After all , the discovery of life on another planet would cause a profound shift in our world view , akin to the Copernican shift from an Earth-centric to a Sun-centric world . So when Fischer says we should be `` the alien civilizations that explore other worlds , '' I say : With Kepler or without , it 's only a matter of time until we find signs of life on other worlds . Read more space and science news at CNN Light Years . The opinions expressed in this commentary are solely those of Meg Urry .
NASA says planet-finding Kepler satellite directional equipment failing . The spacecraft is designed to look for planets in the habitable zone of their stars, where liquid water can exist on a planet's surface and life could develop. She says NASA 's resourceful scientists working to fix it ; loss of its photometry would be huge . Urry : Yale prof has other method to find exoplanets ; we 're bound to eventually find signs of life .