Nasa says its celebrated planet-hunting Kepler spacecraft, which broke down in May when a wheel that controls where it points failed, cannot be fixed and will never again search for planets around other stars.
The disappointing news brings to an end, for now, one phase of the most romantic of space dreams, the search for other earths among the exoplanets of the Milky Way. Nasa has already asked astronomers for ideas on how to use the hobbled spacecraft, whose telescope remains in perfect shape.
Even as they mourned the end of Kepler, astronomers said its legacy would continue as they worked their way through a trove of data the spacecraft has gathered.
At last count, Kepler had discovered 3,548 possible planets, and 135 of them - some smaller than the earth - have been validated by other observations, including earth-bound telescopes. But hundreds or thousands more are in the pipeline, said William Borucki of Nasa's Ames Research Laboratory in Mountain View, California, Kepler's originator and principal investigator.
"The most exciting discoveries are going to come in the next few years as we search through this data," Borucki said. "In the next few years we're going to be able to answer the questions that inspired Kepler: are earth-like planets common or rare in the galaxy?"
Kepler was launched into an orbit around the sun in March 2009. Its official mission was to determine the fraction of stars in the galaxy that harbour earth-like planets by carrying out a survey of some 150,000 stars in the constellations of Cygnus and Lyra, looking for the dips in starlight caused by planets passing, or transiting, in front of their suns.
Three dips, or transits, are considered the criteria for a planet candidate, which means the earth or a planet in a similar orbit that was habitable would take three years to show up. Accordingly, Kepler was designed to operate for four years, but other sun-like stars turned out to be more jittery in their output than expected, making the detection of earth analogues more difficult. Since astronomers could learn a lot more from Kepler if it went on collecting data, the decision was made last year to extend the spacecraft's mission for three more years, until 2016.
In order to do its job of precisely monitoring starlight, Kepler has to keep pointing accurately enough so that each star in the field of view stays on the same pixel in the detector, equivalent to pinpointing a soccer ball in New York's Central Park as seen from San Francisco.
Kepler was launched with four reaction wheels, essentially gyroscopes, of which three are needed to keep it pointed. Last summer, one wheel showed signs of too much friction and was shut down. A second wheel failed in May, putting the spacecraft into safe mode and jeopardising the exoplanet search.
Astronomers began to sing dirges. Geoffrey Marcy of the University of California, Berkeley, dashed off a poem, which said, in part, "Let jet airplanes circle at night overhead/ Sky-writing over Cygnus: Kepler is dead."
Nasa engineers spent several months trying to resurrect Kepler's pointing ability. "We had very little hope it was actually going to be recoverable," said Charles Sobeck, a Kepler deputy project manager.