Both technology and cost were factors.
"No one had ever built a photometer with that kind of precision," Borucki said of the key component of the telescope.
But Borucki kept at it. When Kepler was proposed in 1992 and again over the coming years, "they said there's no such detectors, it would cost too much, that it would cost billions of dollars. Once the cost was under control, they said nobody has done automated photometry."
Kepler was finally approved and launched in 2009. Since then it has discovered 167 planets outside our solar system, while over 3,000 more are waiting to be confirmed.
"What we've discovered is really very startling," Borucki said. "Most stars have planets. If you look in old astronomy textbooks, they said it was very unlikely old stars had planets."
Borucki has worked at NASA Ames Research Center in Mountain View for over 50 years. In 1984 he published "The photometric method of detecting other planetary systems," the paper that was the start of his three decades of work for Kepler, a mission he conceived and implemented as principle investigator. For his work on Kepler last month Borucki was awarded the Samuel J. Heyman Service of America Medal by President Barack Obama, an annual award known as the "Oscars for public service."
NASA says that independent analysis of Kepler data finds that one in five stars like Earth's sun has a planet orbiting it that is up to twice the size of Earth, with a temperate environment.
Borucki says the next mission may be to develop sensors that could look into space and sense the sort of unnatural gases that only a technological society could emit into their atmospheres. "We need to to design the next missions to look at the atmospheres of those planets," Borucki said.
The end result of all this research is right out of Star Trek, the ability to zero in on a habitable planet to visit over 1,000 light years away. "If you invented a faster-than-light space drive, where would you go?" Borucki asked. "You are much younger than me, your job is to build that light drive."
On the wall of Borucki's office — amid early prototypes of Kepler's sensors, calculations on a white board and two flat screen monitors he uses with his computer — is a letter from the late Sagan, dated a month before his death in 1996. In it, Sagan tells Borucki that he was proud to be associated with the Kepler mission.
"During all these proposals, Carl Sagan was a member of our team," Borucki said. "Kepler has a great history of trying and failing and trying again," he said.
Borucki says his work on Kepler is what he's most proud of in his career.
"It was designed to be a stepping stone for the exploration of life in our galaxy," Borucki said. "That first step is: 'Are their any other earths in the habitable zone around stars we could live around?' It has accomplished that."
In May, the second of Kepler's four "thruster wheels" failed, and NASA decided not to fix them. Borucki and others are looking for ways to use Kepler without the thruster wheel "tracking" needed for the high definition photometry of planets outside the solar system. It is still "one of the world's great telescopes" that can be pointed anywhere NASA wants, with enough fuel for four more years, Borucki said. One possibility is to look at comets that could strike earth. Borucki says a proposal for "Kepler 2" will compete for NASA funds in January. So far Kepler has cost over $600 million.
With Kepler's data available to the public, scientists from around the world are paying close attention. Nearly 400 scientists from over 30 countries were expected to gather at Ames from Nov. 4 to Nov. 8 for the Kepler Science Conference. The number of attendees illustrates the impact of Kepler, Borucki said.
"The world is very, very interested in what the Kepler mission has been able to do," Borucki said. "Not only have we found planets of all sorts, we found out things about stars no one knew before. There's just a huge amount of info that has come from this mission."
And there's still a year's worth of data to be analyzed. Borucki hopes to find an "earth-sized in the habitable zone around a star that's just like the sun. We're getting close but we're not there yet." In a few months people may say, 'We have found that,'" he said.