NASA’s Kepler spacecraft has been through a lot during its nine years surveying the skies. It’s spotted thousands of exoplanets in far away solar systems, but it’s also suffered mechanical breakdowns that threatened to end the mission early. NASA kept Kepler running with some clever mission tweaks, but nothing will save it from the inevitable end when it runs out of fuel. NASA says that’s not far off, so it’s placed the spacecraft into sleep mode.
Kepler entered service in 2009, observing scores of stars for telltale signs of planets. Kepler uses the transit method of exoplanet detection. As exoplanets orbit their host stars, there’s a chance they’ll pass in front of the star from our perspective in the Sol system. Kepler watches for these tiny dips in luminance, which can reveal an exoplanet’s size, orbit, and other properties.
The original Kepler observation campaign ran into trouble in 2013 when a second reaction wheel failed. Those devices keep the telescope pointed at its observational targets, which is essential for planet hunting. NASA was able to devise an alternative operating mode for Kepler that uses the solar wind to stabilize the telescope for shorter periods of time. The so-called K2 phase started in late 2013.
NASA reports that original Kepler mission identified 2,244 candidate exoplanets with 2,327 of them confirmed. Kepler’s K2 phase resulted in 479 candidates exoplanets and 323 confirmed. So, most of the exoplanets we know of are courtesy of this one spacecraft (we know of around 4,000 total). When K2 began, NASA only expected it to complete 10 observational campaigns, but it’s managed 17 so far.
The agency now believes Kepler’s fuel reserves are running out. There’s no fuel gauge on the spacecraft, but engineers can calculate how much it has used with a high degree of accuracy. Therefore, the craft is in hibernation mode to conserve what little is left in the tanks. NASA plans to bring Kepler back online in August to collect data via the Deep Space Network. After that, Kepler will begin its 18th observational campaign. NASA will keep Kepler running for as long as possible, but it may not be able to move on to a 19th campaign.
As Kepler reached the end of its mission, NASA already has another planet-hunting telescope in space. The Transiting Exoplanet Survey Satellite (TESS) launched in April and will begin returning data soon. Even as Kepler winds down, its deep-space replacement is winding up to take over and expand on its mission.
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