After nine years of scanning the stars and discovering thousands of new planets, the Kepler space observatory has run out of fuel and this week was officially shut down by NASA.
The spacecraft, which was designed and managed out of Mountain View's Ames Research Center, is credited for vastly expanding astronomers' understanding of the universe, particularly by showing that planets are not only present but pervasive throughout the Milky Way galaxy.
Now located about 94 million miles from Earth, the spacecraft was launched in 2009 in a longshot mission to find planets by searching for faint dimming of starlight. It ended up wildly exceeding expectations: the telescope found more than 2,600 planets, in addition to 2,900 more promising "candidates" that require further analysis.
In sum, the Kepler mission revealed that distant star systems are much like our own -- with a multitude of orbiting planets, explained Paul Hertz, NASA astrophysics director.
"Kepler revolutionized our conception of our place in the cosmos," he said. "Because of Kepler, what we think about our place in the universe has changed."
Through its discovery of many potentially habitable planets, the Kepler spacecraft also kindled a surge of interest in the possibility of extraterrestrial life. The theory of distant life became much more plausible once it became clear that many other planets could have environments similar to Earth, said Seth Shostak, senior astronomer at the Mountain View-based SETI Institute.
"Until Kepler began scrutinizing the heavens, we honestly didn't know if such bio-friendly worlds were plentiful or exceedingly scarce," Shostak said. "Thanks to its work, we now know there are about a trillion planets just in our own galaxy. And even if only one in a hundred is blessed with liquid oceans and atmospheres, that still means that 10 billion cousins-of-Earth pepper the Milky Way."
Despite its astounding success, the Kepler program initially faced skepticism at NASA, and the mission might never have happened if not for its team's tenacity. The program's lead researcher, William Borucki, initially pitched the concept of a Kepler observatory to NASA in 1992. His idea was to launch into orbit a high-powered photometer to simultaneously search an array of stars. The data would be transmitted to Earth for computers to interpret and single out any signs of potential planets.
The idea was rejected by NASA administrators four times. Kepler wasn't accepted for a formal mission until 2000, after Borucki successfully performed a proof-of-concept at Mt. Hamilton's Lick Observatory. The full Kepler spacecraft, which was about the size of a wide automobile, was launched from Cape Canaveral in March 2009.
By the standards of astronomy, the results were immediate. Within six weeks of collecting data, Kepler revealed five previously unknown planets, and soon discoveries of hundreds more came in. Now retired, Borucki said Monday that he was delighted that Kepler could "open a new vista in astronomy."
The spacecraft was originally intended to last just three and a half years, but researchers decided to keep the equipment going for as long as possible to keep collecting data. By 2013, solar pressure caused two of Kepler's four reaction wheels to fail, meaning that it could no longer mechanically point its telescope.
The Kepler team was able to perform a clever workaround by using solar pressure as a makeshift "third wheel" to point its observation. The vessel was able to continue gathering data for four more years, even as its systems were showing further deterioration.
"It was the little spacecraft that could; it always did what we asked of it, and sometimes more," said Jessie Dotson, Kepler project scientist at Ames.
The final blow for Kepler came earlier this month, when the systems team discovered the spacecraft's ability to point had significantly degraded. It was later learned that its fuel reserves had plummeted, and the team began downloading the last of its data. On Monday, NASA officials announced they were shutting down radio contact and retiring the mission.
The loss of Kepler comes as equipment problems are also mounting for another iconic space observatory, the Hubble telescope. Earlier this month, the nearly 30-year-old orbital telescope was put into hiatus after two of its gyroscopes failed. The systems were later restored.
NASA officials are already looking ahead to future missions to follow up on Kepler's success. Earlier this year, the space agency launched the Transiting Exoplanet Survey Satellite (TESS), a satellite observatory partially funded by Google. TESS is equipped to search an area 400 times greater than Kepler's, and researchers expect to find about 20,000 additional exoplanets, possibly with greater insight as to the atmosphere.