Spaceflight Insider

Kepler Space Telescope concludes mission that discovered thousands of far-flung worlds

Artist's depiction of Kepler Space Telescope and exoplanets circling distant stars. On Tuesday, October 30 it was announced that Kepler had run out of fuel Image Credit NASA

Artist’s depiction of Kepler Space Telescope and exoplanets circling distant stars. On Tuesday, October 30, 2018 it was announced that Kepler had run out of fuel and that its mission had concluded. Image Credit: NASA

The Kepler Space Telescope has been on duty for more than nine years. Its mission(s) came to an end when the spacecraft expended the last of its fuel. It leaves behind a wealth of data about exoplanets that has revolutionized humanity’s understanding of the galaxy.

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. The last of Kepler’s fuel was used up approximately two weeks ago.

Kepler contained 12 kilograms of monopropellant hydrazine which was pressurized by a nitrogen gas air bladder. While it was operating, Kepler’s fuel was used to offset the pressure of sunlight that affects Kepler’s ability to point precisely at designated targets.

Some of the final activities to shut the telescope down included turning off its radio transmitters and the disabling of its fault protection systems. At present, Kepler is trailing the Earth at a distance of about 94 million miles (151,278,336 km).

“This was not unexpected,”said Paul Hertz, Astrophysics division director at NASA Headquarters. “Kepler has had a successful nine-and-a-half year mission. During that mission it has revolutionized our understanding of our place in the cosmos.”

The Kepler Space Telescope has discovered an array of planets that scientists never imagined existed during its missions that last more than nine years/ Image Credit: GFycat

The Kepler Space Telescope has discovered an array of planets that scientists never imagined existed during its missions that last more than nine years/ Image Credit: GFycat

Kepler’s time on orbit has seen the spacecraft weather the hostile environment of space which has taken its toll on the spacecraft. Over the years mission managers have altered its tasks accordingly.

Kepler encountered problems with one of its four reaction wheels (wheel 2) in July of 2012, hampering its operations. On May 11, 2013, a second wheel (wheel 4) also failed. 

In 2013 a modified mission proposal for Kepler was submitted named K2 “Second Light.” It proposed that the telescope be used as best as possible under the circumstances. K2 collected data on the formation of stars, supernovae as well as bodies within our own solar system. Approval for this new phase of Kepler’s life was given by NASA on May 16, 2014.

Kepler has encountered problems during the course of its mission. When the telescope’s handlers contacted the spacecraft in early April of 2016, they found that the telescope was in “emergency mode.” This configuration consumes a larger amount of fuel than other modes and also decreases the spacecraft’s operational capabilities. On the next day, Kepler’s status improved as it entered “safe mode.” Although this shortened the length of the research campaign it was working on, it did not take the spacecraft out of service.

“Data from the Kepler Space Telescope will be studied for years to come,” Hertz said.

One of the interesting facts that Kepler has revealed is that there are a number of terrestrial worlds in the galaxy that are larger than Earth but smaller than Neptune. This type of planet does not exist in our solar system.

In June of 2016 NASA announced that K2 would be extended by three years – which placed it beyond the spacecraft’s fuel reserves. Pushing the vehicle to its limits not only ensured that the public got the most for its money, but that those who worked on the mission could expand our knowledge about the worlds that inhabit our galaxy.

Kepler_Space_Telescope NASA image posted on SpaceFlight Insider

The Kepler Space Telescope. Image Credit: NASA

“I’ve always been interested in astronomy,” William Borucki, retired Kepler principal investigator said. “In 1983, when I started my search for methods to detect planets orbiting other stars, the only planets that were known were those in our solar system.”

Borucki detailed how difficult Kepler’s mission was, not just for the telescope itself, but for those operating it as well.

“It was like trying to find a flea crawling across a headlight when the car was 100 miles away,” Borucki said. “In 2001 we submitted our fifth proposal to NASA. This time they were satisfied that this mission would be successful and gave us the go ahead to develop the Kepler spacecraft.”

NASA has estimated that the mission cost some $700 million (for its planned operational life of 3.5 years). It was announced in 2012 that the mission would be extended through 2016, the agency provided roughly $20 million per year for its operation. 

Representatives with NASA repeatedly mentioned that other space-based observatories such as the Transiting Exoplanet Survey Satellite (TESS) and the upcoming James Webb Space Telescope will pick up where Kepler left off. Kepler far outperformed expectations and was viewed with pride by those who spoke about the close of its mission.

“We are by no means disappointed in the performance of this spacecraft,” Charlie Sobeck, project system engineer at NASA’s Ames Research Center said. “We knew when we launched, that the spacecraft would ultimately be limited by its fuel load.”

NASA has tallied the number of exoplanets that Kepler has discovered at 2,681. This is the single largest amount of these far-flung worlds that has been discovered by a single spacecraft to date. 

The lessons that Kepler has taught those who guided the telescope during her time on orbit will now be used on the next generation of exoplanet hunters.

“There’s several different ways in which it will be employed. In the closer term, many of the planetary systems that we found with Kepler will be observed by the James Webb Space Telescope to try to understand are there atmospheres and to learn more to characterize those planets,” Jessie Dotson, Kepler project scientist at Ames told SpaceFlight Insider. ”

Kepler was launched atop its United Launch Alliance Delta II (7925-10L) rocket from Cape Canaveral Air Force Station’s Space Launch Complex 17B on March 7, 2009. The telescope’s mission was to search for exoplanets that were roughly the size of Earth and is named after famed German astronomer Johannes Kepler

“When we talk about building these new missions they are often very specialized and require that when they look at a planet they spend a lot of hours of operation yet they have a finite lifetime, ” Borucki told SpaceFlight Insider. “We want to know what’s out there. Where do you point that telescope so that you get the maximum science from it, because it won’t be able to look at 100,000 stars. Many of these designs talk about looking at maybe only a 100 or 50 stars – so we need the information as to what kind of stars/ What kind of planets? What are the odds? How do we set these systems up so that we get that extremely valuable data that we get even though we have to look for very long periods of time at each of these objects?”

Kepler_Logo image credit NASA posted on SpaceFlight Insider

Image Credit: NASA





Jason Rhian spent several years honing his skills with internships at NASA, the National Space Society and other organizations. He has provided content for outlets such as: Aviation Week & Space Technology,, The Mars Society and Universe Today.

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