Space-watchers saw the handwriting on the wall months ago, but now it’s official: NASA’s Kepler spacecraft, by far the most successful planet-hunting telescope in history, isn’t coming back. Engineers realized back in May that one of the three reaction wheels that let the probe aim at its targets had gone bad—and despite every effort to fix the problem, said NASA astrophysics director Paul Hertz at a press conference yesterday, “we cannot recover three-wheel operations.”
But rather than gnash his teeth and tear out his thinning hair, Kepler’s founding father and principal investigator William Borucki, of the NASA Ames Research Center in California, declared the mission “spectacularly successful.”
He’s not in denial: after spending the 1990s banging on NASA’s door trying to get Kepler approved, he and his team got the go-ahead for a four-year mission in 2000, and the spacecraft was launched in 2009. Kepler successfully completed that phase last year. What’s been cut short is the so-called “extended mission,” a three-year renewal approved in early 2012.
Kepler’s first four years were mind-blowingly productive: the telescope found some 3,500 new worlds orbiting distant stars—or more accurately, 3,500 “planet candidates,” of which only 135 have been confirmed so far. Astronomers both inside and outside the Kepler team are convinced, however, that the vast majority will turn out to be bona fide planets. “When I conceived the mission,” said Borucki, “it was like we were standing in a desert. Now we’re submerged in an ocean of data.”
And those 3,500 only represent Kepler’s first two years of operations. Astronomers are still sifting thorough the second two years, and “we’re convinced the most exciting discoveries are still to come,” noted Borucki. That’s a powerful prediction given that Kepler has found planets orbiting double stars—something previously seen only in Star Wars—a six-planet solar system and a planet the size of Earth’s moon.
But the true quarry, as Borucki has stated openly from the beginning, was to find planets like Earth—about the same size as our home world, orbiting stars like the Sun, and in the “Goldilocks zone” in their solar system where temperatures are not too hot, not too cold, but just right for life to be possible. Kepler hasn’t found any of these yet, but it has come awfully close: it’s found Earth-size (and smaller) planets, but not in the Goldilocks Zone, and it’s found planets a bit bigger than Earth with just the right temperatures—but they’re orbiting stars dimmer than the Sun. Still, when scientists extrapolate from what the probe has already found, it’s clear that our galaxy conservatively holds at least 17 billion worlds about the size of Earth.
True twins of Earth, moreover, may well be lurking in the data still being analyzed— although, said Borucki, “we’ll have to dig down hard.” He figures it will be another three years before all of the numbers are finally crunched, and until then, Kepler lives on as a zombie mission: dead in space, but still going strong back home on Earth.
Or maybe not even dead. Engineers have given up on fixing the hobbled satellite, but Kepler’s light-gathering mirror and electronics are still working fine. So even before the telescope was declared incapable of doing its original job, NASA started soliciting proposals for what other sorts of science it might be able to do. “We’ve gotten a wide variety of suggestions,” said Borucki, “including searches for asteroids, comets and supernovas.”
Another idea: look for the telltale flicker as a distant planet passes in front of an even more distant star. If things line up just right, the planet’s gravity can act as a lens, focusing the starlight and making it brighten momentarily. Until they take a hard look at Kepler’s capabilities with just two working reaction wheels, notes Borucki, “we have no way of knowing what’s practical.” Beyond that, budget pressures may force NASA to stop pouring scarce money into a crippled Kepler and funnel it elsewhere.
No matter—the probe’s legacy is already firmly established. “Kepler has been a crucial step in our exploration of the galaxy,” said Borucki. “If we had found that there were very few Earths out there, we would be making very different choices about future missions. Instead, NASA is moving ahead with the Transiting Exoplanet Survey Satellite and looking ahead to the James Webb Space Telescope, which among other thing will study the atmospheres of nearby exoplanets. “Yes, it would have been better if the mission had gone on longer,” said Borucki. “But I’m very satisfied.”