NASA’s planet-hunting Kepler Space Telescope is almost out of fuel (19 Photos)

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After just over nine years in flight, looking deep into the Milky Way Galaxy, NASA’s Kepler spacecraft is nearing its end. Having survived numerous potential knockouts, from cosmic ray blasts to mechanical failures, Kepler is almost out of fuel and is expected to only have a few months worth left, marking the end of an extremely successful mission.The Kepler Space Telescope was launched on March 7, 2009, trailing Earth’s orbit at 94 million miles away so that our planet’s gravity and reflected light wouldn’t interfere with the telescope’s precise measurements. Designed to discover planets outside of our solar system, the hopes were that we could find Earth-sized exoplanets in the habitable zones of their parent stars.It’s actually incredible that the mission has lasted this long already because in 2013 all was almost lost. The spacecraft was designed with four reaction wheels used to keep it pointed in the right direction, needing at least three of these to continue functioning properly. Having lost one already, a second wheel broke leaving it unable to hold a steady gaze.Pictured here is the originally intended field of view.However, NASA managed to salvage the telescope by using the pressure of sunlight to temporarily stabilize it. This second phase, known as K2, required Kepler to shift its view to new portions of space roughly every three months, which are referred to as campaigns. Having estimated that the K2 mission could conduct 10 of these campaigns with the remaining fuel, it surprisingly completed 16 and this month it entered its 17th.During its nearly decade-long operation, the Kepler Space Telescope has found over 2,500 confirmed planets orbiting distant stars, with over another 2,500 more still waiting to be confirmed. 30 of these confirmed exoplanets are within habitable zones, where liquid water could exist like on Earth.When the telescope spots these distant planets, it’s not actually capturing an image of it, they’re way too far away for that. Kepler instead monitors stars for dips in brightness as a planet occasionally travels across the front of it. Based on how much light is temporarily blocked and the amount of time it takes to orbit around the star, scientists can estimate the size and possible composition of these exoplanets.Without a gas gauge, NASA will continue to monitor for warning signs of low fuel. They need enough left to power the thrusters so that they can aim the spacecraft back to Earth for its last data transfer.Although Kepler will soon be put to rest, left to its lonely orbit through the cosmos, another planet-hunting space telescope, NASA’s Transiting Exoplanet Survey Satellite (TESS), is set to launch on April 16, 2018.Let’s take a look back at some of the amazing discoveries made using the Kepler Space Telescope over the past 9 years.An artist’s concept of one of the first five exoplanets to be found on the mission, just a few months after launching. Named Kepler 4b, 5b, 6b, 7b, and 8b, they range in sizes similar to Neptune and up to larger than Jupiter.Kepler-16b, discovered in 2011, is the first planet known to orbit two stars, what’s called a circumbinary planet.Here are illustrations of the first Earth-size planets found orbiting a sun-like star outside of our solar system, called Kepler-20e and Kepler-20f. Kepler-20e, pictured here, is believed to be geologically active, represented by the active volcanoes seen in the image.Kepler-20f.Kepler-22b was the first planet that the mission confirmed to orbit a star’s habitable zone. It is 2.4 times the size of Earth.Like our solar system, Kepler-62 is home to two habitable zone worlds. Located about 1,200 light-years from Earth in the constellation Lyra, Kepler-62f is seen in the foreground orbiting its star, while the small shining object seen to the right is Kepler-62e.Signalling a significant step closer to finding a world similar to Earth, the 2014 discovery of Kepler-186f was the first validated Earth-size planet found to orbit a star in the habitable zone.In 2015, an ancient system was found. Kepler-444 formed when the Milky Way Galaxy was only two billion years old and consists of five tightly packed planets.This artist’s concept of K2-138 is the first multi-planet system discovered by citizen scientists in 2017 using data from the Kepler Space Telescope. First thought to be a four-planet system, a fifth planet has been confirmed with the possibility of a sixth still waiting to be analyzed.