This article originally appeared in the Jan. 15, 2018 issue of SpaceNews magazine.

Sometime later this year NASA’s Kepler spacecraft, orbiting the sun more than 150 million kilometers from the Earth, will fire its thrusters for the final time. The spacecraft is running out of the hydrazine fuel used by those thrusters to maintain the spacecraft’s orientation. Once the thrusters sputter and shut down, their fuel exhausted, Kepler will no longer be able to control its pointing, and the mission will end.

The project isn’t quite sure exactly when that will happen, since the calculation depends on rates of fuel usage and the challenges of measuring just how much hydrazine is left in the spacecraft’s tanks. “The fuel is expected to last somewhere between the spring and summer of 2018,” said Gary Blackwood, manager of NASA’s Exoplanet Exploration Program, at a Jan. 7 meeting of a NASA exoplanet advisory group. He added that the spacecraft’s manufacturer, Ball Aerospace, “has found very creative ways” to stretch out that remaining fuel.

Kepler is otherwise working well, performing since 2014 an extended mission called K2 that is looking at different parts of the sky for a few months at a time. “The spacecraft is behaving completely nominally,” said Jessie Dotson, K2 project scientist at NASA’s Ames Research Center, at a town hall meeting about the mission Jan. 9 during the 231st Meeting of the American Astronomical Society (AAS) in suburban Washington.

The K2 mission is currently performing a set of observations called Campaign 16, scheduled to run through late February. Mission scientists have plans for Campaigns 17, 18 and 19 that would run through the end of the year in a best-case scenario.

“I’m cautiously optimistic we’ll make it through Campaign 16,” she said. “Anything past that is gravy.”

Enter TESS

But as Kepler approaches the end of its life, NASA’s next mission to search for exoplanets is gearing up for launch. At an Orbital ATK facility near Washington Dulles International Airport, technicians are completing final tests on the Transiting Exoplanet Survey Satellite (TESS), a NASA mission scheduled for launch this spring.

TESS, like Kepler, will look for exoplanets by detecting very small changes in brightness of stars as orbiting plans cross, or transit, their disks. But while Kepler initially examined a single, small area of the sky in an effort to determine the fraction of stars with planets, TESS will instead perform an all-sky survey, focused on the brightest stars nearest to Earth.

That search is intended to find exoplanets well-suited to follow up observations by other telescopes, including the upcoming James Webb Space Telescope, that can help determine their mass and composition, and even study their atmospheres.

“TESS is tiny, but it punches above its weight,” said George Ricker, principal investigator for TESS at the Massachusetts Institute of Technology, during the Kepler town hall. “It’s a finder scope for JWST.”

The spacecraft, 1.5 meters tall and weighing a few hundred kilograms, will ship in early February to Florida’s Kennedy Space Center for launch processing. TESS will launch no earlier than March 20 on a SpaceX Falcon 9 rocket into an elliptical orbit that is in a 2:1 resonance with the moon.

That orbit, Ricker said, is very stable and also allows for high data rates from the spacecraft. However, it limits the days on which TESS can launch in order to phase into the proper trajectory. Ricker said there were about 40 days through June on which TESS could launch.

The four cameras on TESS will map nearly the entire sky over its two-year primary mission. Astronomers expect that TESS will detect thousands of exoplanets, many of which will be ideal for follow-up observations by other telescopes, including the James Webb Space Telescope, to characterize them. Any extended mission, Ricker said, would allow TESS to fill in gaps in observations from its primary mission or do follow-up studies in other parts of the sky.

Shifting focus

TESS has not been without its problems, though. NASA confirmed last July that engineers discovered that the focus in the four cameras on TESS would drift once the cameras cool to operating temperatures after launch. At the time, the agency said that it believed the issue would not be a major problem for the mission, although other astronomers expressed concern it could affect the spacecraft’s ability to detect exoplanets.

Additional testing and analysis since then has given those involved with the mission greater confidence that they understand the focus issue and that it won’t adversely affect the mission’s science.

“Subsequent testing that we did starting this summer and then into the fall indicated that there is a model” for explaining the focus change, Ricker said at a Jan. 9 briefing about the mission during the AAS conference. “This is a very reproducible crystallization effect for one of the materials used to manufacture the lenses.”

Ricker said the mission did four months of testing on a flight spare camera to understand long-term focus effects. Those tests show that the focus of the camera drifts for about one week, then stops. “There’s essentially no measurable change after that,” he said, calling the issue a one-time “focus shift” rather than a more continuous “focus drift.”

That focus shift, he said, won’t affect the ability of TESS to meet its primary, or “Level One,” science requirements, which call for eventually measuring the mass of at least 50 planets similar in size to the Earth. The mission’s primary focus on photometry — measuring very small changes in brightness of stars — also minimizes the importance of a sharp focus.

“This is a photometry mission, not an imaging mission,” he said. “What this means is that it’s not important to have a sharp focus across the entire field of view. This was never part of the design. But it is important that the focus be stable, and that’s what we’ve been able to establish.”

A wide range of science

TESS has also attracted interest from other astronomers wanting to use spacecraft data for other research. Padi Boyd, director of the guest investigator program for TESS at NASA’s Goddard Space Flight Center, said at the briefing that there was a very strong response to a first call for proposals to participate on the mission, with scientists proposing to use TESS data for topics ranging from other exoplanet studies to stellar astrophysics and extragalactic astronomy.

“We were very excited to see how the broader scientific community really responded to this opportunity,” she said, adding that the initial set of guest investigations will be announced in about a month.

While TESS has a two-year primary mission, Ricker said he believed that the spacecraft could operate for much longer. The stability of its orbit, he said, requires no station-keeping, and hence limits the use of thrusters. “The operational life of the mission could very well extend for more than two decades,” he said.

For Kepler, the science will continue long after the spacecraft exhausts its fuel later this year. As with TESS, Kepler attracted astronomers interested in using the spacecraft for more than just exoplanet science during the K2 mission, particular as the spacecraft looked at different parts of the sky.

Dotson said she expects astronomers to tap into the archive of Kepler data for various research for years to come. “While we’re running low on fuel,” she said, “the science is just getting going.”