NASA's storied Kepler Space Telescope — the craft which has discovered thousands of exoplanets since its launch in 2009 — is entering the retirement phase of its lifespan. NASA announced on Friday that Kepler staff had "received an indication that the spacecraft fuel tank is running very low" and "placed the spacecraft in a hibernation-like state in preparation to download the science data collected in its latest observation campaign."
Kepler suffered a mechanical malfunction with its steering system in 2013, forcing scientists to develop a clever alternate plan in which they used pressure from the sun's rays to act as a stand-in for one of its failed reaction wheels.
This new phase, called K2, is imperfect, and NASA originally believed it would only allow for 10 observation campaigns with the remaining fuel. Yet it works, and allows Kepler to observe patches of space for roughly 83 days at a time.
Per NASA, while the original Kepler mission discovered 2244 candidate exoplanets and 2327 confirmed exoplanets, the extended K2 mission has managed to identify 479 candidates and confirm 323 others. The craft is currently on its 18th K2 observation campaign.
NASA is unable to determine the exact amount of fuel left within Kepler, as there is no onboard gas gauge. However, since it is in deep space trailing the Earth's orbit at roughly 151 million kilometres away and there is no risk of it hitting another potentially life-bearing astronomical body such as an icy moon, the agency is free to keep working the spacecraft until it gives up and dies.
According to NASA, Kepler staff have put the craft into hibernation mode until August, when the plan to turn it back on and use NASA's Deep Space Network to transfer mission data back to Earth. If that is successful, they plan to start a 19th observation campaign with the remaining fuel.
As it turns out, Kepler has discovered key insights about the unique nature of our stellar neighbourhood, which have included blazing-hot gas giants in perilous proximity to their host stars, binary star systems and red dwarfs orbited by numerous rocky worlds. It's also found a planet that uncannily mirrors many of Earth's characteristics.
While it is still in limited operation, NASA has already launched a successor, the Transiting Exoplanet Survey Satellite, which sent back an incredible image of thousands of stars in March 2018.