R.I.P. Kepler, the planet-hunting telescope | Science News for Students

R.I.P. Kepler, the planet-hunting telescope

After 9½ years, the spacecraft just ran out of gas
Nov 1, 2018 — 11:30 am EST
an illustration of the Kepler space telescope in space

Here’s an artist’s depiction of the Kepler space telescope at work scouting planets orbiting distant stars. Over its lifetime, this spacecraft turned up more than 2,700.


The Kepler space telescope can no longer search for planets orbiting other stars. After 9½ years, NASA’s premier exoplanet-hunter is out of gas.

NASA officials announced the mission’s end at a news conference on October 30.

“Because of fuel exhaustion, the Kepler spacecraft has reached the end of its service life,” said Charlie Sobeck. He is a project system engineer. Sobeck works at NASA’s Ames Research Center in Moffett Field, Calif. “While this is a sad event, we are by no means unhappy with this remarkable machine.”

Kepler’s discoveries have forever changed the way astronomers think about planets in other solar systems. Known as exoplanets, only about 350 were known to exist before Kepler launched in 2009. And nearly all of these were the size of Jupiter or larger.

As of this week, there are now more than 3,800 known exoplanets. And Kepler discovered 2,720 of them. The spacecraft found planets in all shapes, sizes and family structures. For instance, it found seven planets orbiting one star. Some planets had two suns. Other planets orbited their star at jaunty angles. And five that Kepler found orbiting one star were more than twice as old as Earth. “These planets formed at the beginning of the formation of our galaxy,” says William Borucki. “Imagine what life might be like on such planets.” Until he retired in 2015, this astronomer was Kepler’s principal investigator.

Astronomers also have used Kepler’s exoplanet tally to predict that every one of the hundreds of billions of stars in the Milky Way should have at least one planet. And scientists suspect that billions of these planets might have the right size and temperatures to support life.

a graph showing the number of exoplanets (in yellow) Kepler discovered
Here’s a chart graphing the number of exoplanets discovered over time. Kepler turned up the vast majority of them (yellow).
Jessie Dotson And Wendy Stenzel/Ames Research Center/NASA

Second death notice for the spacecraft

Kepler was declared dead once before. In 2013, the telescope lost use of a second of its four reaction wheels. These devices helped to keep the telescope pointed steadily at a selected patch of the sky. That consistent pointing was crucial for Kepler’s planet-hunting strategy. It worked by spotting a slight dip in stars’ light as planets cross in front of them. Five years ago, this seemed to mark the end of Kepler.

But engineers soon revived the telescope. They arranged for it to work in a new observing mode. It now used the pressure of sunlight on Kepler’s solar panels to keep it pointing straight.

“I always felt like it was the little spacecraft that could,” said astronomer Jessie Dotson. She’s a Kepler project scientist at NASA Ames. “It always did everything we asked of it,” she said, “and sometimes more. That’s a great thing to have in a spacecraft.”

Kepler’s official demise came two weeks ago. That’s when the telescope’s fuel pressure dropped by three-quarters in a matter of hours, Sobeck said. Before it shut down, NASA had Kepler send all of its remaining data back to Earth. “In the end,” he observed, “we didn’t have a drop of fuel left.”

Kepler’s legacy lives on, however. TESS, or the Transiting Exoplanet Survey Satellite, launched this past spring. And this planet-hunting telescope has already spotted some exoplanets.

For its final acts, the Kepler team will remotely turn off the telescope’s radio transmitters. The team will also turn off protective systems that might allow those transmitters to be turned back on. “The spacecraft will then be left on its own to drift away in a safe and stable orbit around the sun,” Sobeck said.

Power Words

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angle     The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

exoplanet     Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

Jupiter     (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

Kepler Space Telescope     A NASA mission to search for exoplanets — planets beyond the solar system — especially ones that might be Earth-like. The mission’s development began in 2002, by placing the first orders for the needed instruments that would be used. The mission was named for Johannes Kepler (1571 to 1630), the first person to describe the motions of planets about the sun so that their positions could be predicted accurately. The spacecraft carrying the telescope Kepler spacecraft lifted off March 6, 2009, at 10:49 p.m. from the Cape Canaveral Air Force Station in Florida. It was pronounced dead on October 30, 2018 after having run out of fuel. Before it died, the telescope identified more than 2,700 exoplanets.

Milky Way     The galaxy in which Earth’s solar system resides.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

R.I.P.      An abbreviation for "rest in peace," and is commonly said about someone who has just died.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

strategy     A thoughtful and clever plan for achieving some difficult or challenging goal.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.