NASA unexpectedly announced this week that Kepler, the planet-hunting spacecraft, has put itself in a safe mode and gone offline.
One of the orbiting telescope's star-tracker instruments, called a reaction wheel, has failed, rendering the telescope unable to point at target stars or search for planets. Unless engineers can fix the problem, this could spell the premature end of one of the most successful space missions ever.
Kepler was launched in 2009 and has spotted 2,740 potential planets and confirmed 132. Its primary mission ended in 2012, but NASA extended its life to 2016. (Related: "Most Earthlike Planets Found Yet: A 'Breakthrough'")
National Geographic News caught up with William Borucki, principal investigator for NASA's Kepler mission, to get his thoughts on the current status of the spacecraft, its science, and what the future may hold for planet hunting.
What are the chances that Kepler will get back to hunting planets?
We think there is hope, but we have only just begun to characterize the problem. At this point our best guess is that the safe mode was tripped by one of the four onboard reaction wheels.
We are just finishing uploading software that will help the spacecraft move into a position we want and hold it there using just the reaction jets. We expect to have several months to figure out how to fix the wheel. But there is a real chance that using a combination of thrusters and wheels might not be accurate enough to allow photometry measurements of stars.
Could Kepler still reveal major discoveries?
We have two years of data that we have not yet searched. What this means is that we have looked at only half the data, yet just look at the discoveries we've already made. Right now we can't find an Earth-like planet in a habitable zone around a sun-like star. We just have not looked long enough, but we know that it might very well be in the data that is sitting there.
What is the holy grail of the mission?
What we need to know is if planets like Earth are common or rare in our galaxy. If they are common, then our galaxy may be full of life just waiting for us to call out that we are here. If we find that the opposite is true, then we are all alone, and there never will be a "Star Trek" because there's no place to go.
How big does the planet have to be to be Earth-like? How close to its parent star? We are not trying to find an exact match, but Kepler has determined that there are lots of Earth-sized planets out there. We think we've found hundreds of them in just the one small section of the sky [within the constellation Cygnus] we are staring at, which means that there are billions of Earth-sized planets.
If Kepler can be resurrected, what would be its next tasks?
Up to now we have been talking about single planets. We want to look at groups of planets around a single star like our solar system and determine which are water planets or rocky planets.
Now our goal is to get extra data—getting those extra orbits of planets transiting their stars—where we can start characterizing the planets. The longer we look, the more information we can glean about what these newfound planets are like. This is a long process because one orbit may take a few hundred days to complete. Habitable zones are farther out for hotter stars, which means the planets in their orbit will transit in front of their star only once every couple of years or so.
Also the smaller the planet like Earth, the smaller the signal they produce, which means we need to get more signals to verify that they are indeed planets.
What will future planet-hunting missions look like?
The next logical step is to look at stars with planets near our sun. NASA has selected TESS (Transiting Exoplanet Survey Satellite) for a scheduled launch in 2017. It will have an array of telescopes searching nearly the entire sky, examining at least a million nearby stars for Earth-sized planets.
Then the following mission will look to see if those planets have atmospheres.
The next natural follow-up mission will then try to see if these planets are covered with ice, deserts, or oceans. It may be at least a decade off, but people are thinking of how to build such a mission too.
What will Kepler's legacy be?
We have found thousands of planetary candidates telling us that Earth-sized planets are ubiquitous in our galaxy. Kepler has been a tremendous success. But it is time to move on. We've accomplished what we set out to do, which is to determine if Earths are rare or common.
What effect do you think Kepler's demise might have on future missions?
I think Kepler's achievements will encourage a quicker development of these successor missions. People can see what a success it has been. It's only natural to want to go on to the next step. We want to know about their atmospheres. It's in our genetic structure to need to know more.