The End of Kepler?: With its field of view now drifting because of a failure of two of its four reaction wheels, NASA has asked astronomers to submit ideas on how the hobbled spacecraft might still perform good science. Image: Carter Roberts/NASA
NASA just can’t quit Kepler.
On 15 August, the agency announced that it would stop trying to revive the failed reaction wheels that gave the planet-hunting telescope its precise pointing ability. That essentially brings an end to the main goal of the 4-year-old mission, which has found 3,548 candidate planets by looking for tiny dips in starlight that indicate a planet’s passage, or transit, across that star.
But the agency left room for hope: two weeks earlier, it had asked astronomers to submit ideas by 3 September on how the hobbled spacecraft might still perform good science. Nature has learned about some of the options in the running, out of the dozens of proposals expected.
Ideas range from a survey of potentially hazardous near-Earth objects to a study of Jupiter-sized exoplanets in large orbits. Kepler scientists will sort through the proposals and decide by 1 November which ones, if any, to recommend to NASA headquarters for further review.
To secure funding from the space agency, the Kepler team will have to show that the studies could not be done by other telescopes. This will be no easy task — especially given that engineers are not sure how well Kepler can perform with just two of its four spinning reaction wheels, which act as stabilizing gyroscopes.
“We’re in a real quandary,” says Kepler principal investigator Bill Borucki at NASA’s Ames Research Center in Moffett Field, California. “We just don’t know what Kepler can do.”
With three working wheels (a fourth was a spare), Kepler was able to exactly counterbalance the persistent push of sunlight, locking on to targets with such precision that light from a particular star always fell on the same tiny fraction of an individual pixel. But the wheels have a history of poor performance, and in July 2012 one failed — followed by another in May (see go.nature.com/4w1ufr). Although the craft’s thrusters can still act as a crude version of a third wheel, they cannot replicate the pointing accuracy that three wheels provided, and the telescope’s focus will drift. Over time, starlight will start to fall on different pixels with slightly different sensitivities. “Every single day, it’s as if you’re going to use a different detector, a different telescope,” says Kepler scientist William Welsh of San Diego State University in California.
Kepler’s drift could be minimized by keeping it pointed in the same plane in which the craft orbits the Sun. But that presents a complication. Some of the best science is expected to come from follow-up observations of the field of about 150,000 stars that Kepler has been focused on, and that star field does not lie in the plane.
In one proposal, offered up by Welsh and his colleagues, the craft would continue to stare at this original star field to search for Jupiter-sized planets. Such bodies are sufficiently large that when they pass in front of their parent star they produce a dip in light that can be detected by Kepler even in its compromised state.
Welsh’s group would target Jupiters for which Kepler has recorded only a few transits — those that take more than a year to orbit their star. It usually takes a minimum of three transits to confirm the existence of a planet. Catching the third transit could make the difference between a possible and a definitive discovery.
The craft is too shaky to discover an Earth analogue from scratch, but Welsh suggests that it might also be possible for Kepler to add statistical significance to Earth-sized candidates for which transits have already been captured. And David Hogg, an astronomer at New York University, believes that, over the course of many months, Kepler’s drift could be used to map out the different light responses of the pixels. That calibration, if detailed enough, could be enough for Kepler to resume its hunt for Earth analogues, says Hogg.