The Kepler spacecraft came roaring back into the news last week, when scientists announced that the plucky little planet hunter had unearthed hundreds of new exoplanets in our cosmic backyard, despite being literally broken. But that's not all Kepler's been up to -- by a long shot.
The Kepler space telescope was humanity's premier planet-seeking instrument from 2009 until 2013, when two broken reaction wheels brought the mission to an abrupt end. But the spacecraft's engineers were persistent, and they soon found a way to repurpose Kepler. Since the K2 reboot mission began in 2014, the trashcan-sized telescope has busied itself with a breadth of studies, on topics ranging from Neptune to extragalactic supernovae -- all the while continuing to discover small, rocky, potentially habitable worlds. Kepler's success is a testament to the resourcefulness of scientists, and a reminder that missions don't have to go exactly as planned in order to be a smashing success.
"Without a doubt, formulating K2 was the most fun I've ever had," Tom Barclay, an astronomer at NASA's Ames Research Center and Director of the Guest Observer Office for the Kepler and K2 mission told Gizmodo.
When NASA's Kepler mission launched in 2009, it had a single goal: discovering planets beyond our solar system. By staring at a fixed point in the sky and detecting faint, periodic dips in the light output of stars (so-called transit events), Kepler detected over a thousand extrasolar worlds and produced the very first planetary census -- a tally of the number and kinds of worlds orbiting stars. Extrapolating from Kepler's data, astronomers reached a remarkable conclusion: there's at least one planet for each star in the sky; a hundred billion worlds in the Milky Way alone. Even if only 0.1% of those worlds are small and rocky, that's still a hundred million opportunities to find a second Earth.
Artist's depiction of planets around stars in the Milky Way. Via ESO/M. Kornmesser
But after four years of discoveries, two of Kepler's four stabilising reaction wheels had broken, and the telescope could no longer maintain precise focus on a fixed region. The Kepler mission -- as originally envisioned -- was over.
Still, some astronomers saw opportunity. Here was a powerful telescope, already sitting in orbit, with all of its optics fully functional. Even if Kepler couldn't keep its gaze trained on a single region of space anymore, surely we could find some use for it?
That's exactly what Barclay, along with a team of Ball Aerospace and NASA engineers and scientists, decided to find out. In the summer of 2013, the group began running tests on the telescope to figure out what it could still do. At the same time, Barclay and NASA astronomer Steve Howell put out a call to the astronomy community, asking scientists to send test targets. "As we got more and more data we realised that the precision of our observations would really be so much better than we ever expected," Barclay said.
After about six months, the team made a case to NASA for a mission that would stare across the ecliptic plane (the plane in which planets orbit the Sun), observing different regions of the sky. NASA approved the idea in near-record time, and K2 officially launched in the spring of 2014.
Unlike the Kepler mission, K2's objectives aren't handed down from NASA. Rather, they're sourced from astronomers who submit requests to look at a specific region of space. If the science case is compelling enough, the request is approved, and it gets an 80 day campaign. K2 recently wrapped its seventh campaign, and -- pending funding -- is expected to run at least 18.
Kepler was already one hell of a planet hunter -- now it's proven itself a jack of all astronomical trades.
In astrophysics, the K2 mission has already made discoveries about star clusters. Stars typically form in densely packed groups before growing up and drifting apart. Because clusters contain many stars of the same age, astronomers can use them to do population studies. For instance, it's long been known that a star's rotational rate varies with age -- older stars spin slower -- but the exact nature of that relationship is poorly understood.
The beautifully crowded star cluster Messier 68, via ESA/Hubble & NASA.
"The calibration between how fast [stars] spin and how old they are is really challenging," Barclay said. "But with K2, we can look at these nearby, bright, and extremely well studied clusters," and start to put some constraints on that relationship.
"Once we've calibrated it, we can look at any star in the sky, see how fast it rotates, and get an estimate of how old that star is," he added. That could allow astronomers to quickly deduce whether a star system might have planets, and whether any of those planets could be habitable.
K2 will also help scientists study the early stages of supernovae -- the violent explosions that rip stars apart, seeding the cosmos with planetary building blocks. "By mapping the very early stages, right when the supernova rises, you can look at how the shape of the star changes," Barclay said. From there, astronomers can begin to work out the underlying forces behind the epic cosmic outburst.
Video showing K2 observations of Neptune, in a dance with the two moons Triton and Nereid.
In keeping with its origins, K2 is still discovering exoplanets -- troves of them. As we reported last week, the spacecraft raked in 238 exoplanet finds in 2014, though only 100 of these have been confirmed so far.
Unlike Kepler, K2 is looking at targets right in our cosmic backyard -- worlds tens of light years away, instead of hundreds to thousands. The planets the K2 missions discovers are among the very first we'll be able to get a deep look at with the James Webb Space Telescope (JWST). When JWST launches in 2018, its unprecedented seeing power will allow astronomers to peer into the atmospheres of nearby worlds -- and search for signs of life.
"This is a really exciting area," Barclay said. "We're starting to find the first super-Earth sized planets that are going to be the first James Webb targets."
"K2 fortuitously fulfils what we didn't even identify as a gap," he added, referring to the years between the original Kepler mission and the big exoplanet surveys to come. "I've heard people say this is the best thing that happened to Kepler."
Top image: Artist's concept of the Kepler Space Telescope, via NASA/Ames/JPL-Caltech