Kepler K2
NASA's Kepler spacecraft, launched in 2009 on a mission to search for exoplanets, is in a safe mode amid concerns the spacecraft may be finally running out of fuel. Credit: NASA

WASHINGTON — NASA’s Kepler spacecraft is back in operation despite a problem with one of its thrusters and low fuel levels that may soon bring the mission to an end.

In a brief statement Sept. 5, NASA said Kepler resumed observations Aug. 29. The spacecraft was set to begin what the project calls Campaign 19, the latest in a series of observations spanning nearly three months at a time, in early August, but the spacecraft went into a “sleep mode” after transmitting data collected during the previous campaign.

Alison Hawkes, a spokesperson at NASA’s Ames Research Center, told SpaceNews Sept. 5 that engineers found no evidence of “systemic problems” on the spacecraft other than an issue with one of the spacecraft’s eight thrusters.

“One of the eight thrusters had shown unreliable performance, but the team estimated that simply removing the thruster from use during precision pointing firings could result in acceptable system performance,” Hawkes said. “As a result, the changes were made and Campaign 19 was, as it were, joined in progress.”

Kepler uses the thrusters to maintain its orientation. The spacecraft originally relied on reaction control wheels, but two of the four wheels on the spacecraft malfunctioned by mid-2013, more than four years after launch, forcing an end to the spacecraft’s primary mission.

Engineers developed an alternative pointing approach using the two remaining wheels as well as the thrusters and solar pressure on the spacecraft. This allowed the spacecraft to resume operations under an extended mission called K2, but rather than looking at the same part of the sky, as Kepler did during its prime mission, it instead looks at different regions of the sky in a series of observing campaigns.

The K2 extended mission is expected to end in the coming months when the spacecraft exhausts its remaining hydrazine fuel for its thrusters. Controllers put the spacecraft into a “no-fuel-use safe mode” in early July, prematurely ending Campaign 18 observations, after detecting what NASA called an “anomalous” drop in fuel pressure. Engineers revived Kepler in early August to transmit the Campaign 18 data before putting it back into safe mode.

Hawkes said it’s not clear if the problem with the thruster is linked to dropping fuel levels. “By eliminating the use of the thruster for precision pointing, we are protecting against it being an issue unrelated to the fuel,” she said. “If it turns out to indeed be fuel-related, it is likely that other thrusters will begin to show symptoms.”

Removing the thruster from the precision pointing operations will make the spacecraft less balanced against solar pressure, she added, but the effect that will have on the quality of the observations won’t be known until after the Campaign 19 data is returned to Earth.

The exact amount of fuel remaining on Kepler can’t be easily measured, complicating plans for future observations. “It remains unclear how much fuel remains; NASA continues to monitor the health and performance of the spacecraft,” the agency said in its statement.

Kepler, during its prime mission, searched for exoplanets by looking for periodic, minute drops in brightness caused when those plants pass in front of, or transit, the stars they orbit. Those observations led to the discovery more than 2,300 confirmed exoplanets, along with a nearly equal number of exoplanet candidates awaiting confirmation. The K2 extended mission has discovered several hundred exoplanets while also supporting other astrophysics research.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...