NASA's legendary Kepler space telescope, which is responsible for the discovery of thousands of freakish and intriguing exoplanets, has officially run out of fuel.
Engineers noticed that the spacecraft was running low on fuel in June of this year (2018) and worked to ensure that the data on board was safely sent back to Earth.
Working in deep space for nine years, Kepler discovered planets from outside the solar system, many of which could be promising places for life.
Launched in March 2009, the $600 million Kepler mission searched the night sky for Earth-like planets using what's called the "transit method".
"As NASA's first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond", said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate in Washington.
Paul Hertz, director of astrophysics at NASA Headquarters, said the Kepler mission "revolutionized our understanding of our place in the cosmos".
NASA's James Webb Space Telescope will also keep a keen eye on exoplanets, even possibly going so far as to peer into the atmospheres of these worlds to understand if they might be habitable. It found inferno-like gas giants, rocky planets, planets orbiting binary stars, Earth-size planets, planets in the habitable zone capable of supporting liquid water on the surface, planets twice the size of Earth, the strangely flickering Tabby's Star, new details about the TRAPPIST-1 planets and, in December, an eight-planet system.
He said Kepler showed mankind how many planets might be out there.
NASA's Kepler planet-hunting telescope now belongs to the ages, with its fuel completely spent and its instruments shut down - but the planet quest continues, thanks to a treasure trove of downloaded data as well as a new generation of robotic planet-hunters.
A new, state-of-the-art planet hunter - the Transiting Exoplanet Survey Satellite, or TESS - was launched last April.
The driving force behind Kepler was Bill Borucki, the now-retired principal investigator for the mission at NASA Ames. However, although these planets tend to be the most enticing, according to Kepler, they are not the most common type of planet out there. We've also found planets that were formed at the beginning of the formation of our galaxy six-and-a-half billion years before the formation of our own star and before the formation of the Earth.
In 2012, Kepler completed its primary mission and was awarded an extension. The first data from TESS is already being sent to Earth and analyzed.
This artist's concept illustrates the two Saturn-sized planets discovered by NASA's Kepler mission. That is, a small dip in the light from a star as the planet passed in front of it.
"We know the spacecraft's retirement isn't the end of Kepler's discoveries", said Jessie Dotson, Kepler's project scientist at NASA's Ames Research Center in California's Silicon Valley.
"While this may be a sad event, we are by no means unhappy with the performance of this marvellous machine", NASA project system engineer Charlie Sobeck said.
Kepler's originally planned $640 million mission went far longer than anticipated, thanks in part to a spacecraft-saving fix that was made in 2013 when a crucial part of the probe's fine-pointing system went out of commission.
"I think we were all extremely impressed with what it was doing for us", Borucki said of Kepler.