Charlie Sobeck is project manager for the Kepler Mission at the NASA Ames Research Center in California. Launched in 2009, the Kepler space observatory, now in a heliocentric orbit, was designed to look for Earth-size planets around other stars. The Kepler team received this year’s National Air and Space Museum Trophy for Current Achievement.
Associate Editor Diane Tedeschi talked to Sobeck in January 2015.
Air & Space: What are the advantages of Kepler’s simple design?
Sobeck: What we’re basically looking for is the shadow of planets as they circle their stars. As the planets orbit their host star, if the alignment between the star, planet, and telescope is correct, we will see that planet transit right in front of the star, and we can measure a dimming in the brightness of the star. Conceptually, this is a very simple principle to understand.
Are there any disadvantages to such a streamlined design?
No one telescope is going to do all things. For what Kepler was designed to do—find these planets and characterize them—we were able to do that job really well with what we had. The next thing we would like to do in understanding exoplanets is to understand whether there are atmospheres, and if there are, what might be in those atmospheres. To do that, you would need to be able to take a spectrum: be able to separate the light coming from a star into different wavelengths. Kepler does not have that ability.
Can you describe the moment when it was confirmed that the Kepler hardware was working after launch?
The first couple months of operations after launch is a phase we call “commissioning,” where we turn on the spacecraft slowly. We operate it in its various modes, and we gradually work up to full operation. For a telescope like this, a major event is what we call “first light,” where all the hardware in the telescope—you verify it’s working. You open up the aperture, you open up the cover of the telescope, and you take your first image of starlight.
For this telescope, it wasn’t all that dramatic a moment because we had to take the data, we had to downlink it to the ground, send it off for processing. It took probably a week of processing until we had an image we could look at. But then we have to use the focal plane to do what we call “fine-pointing”: point the telescope where we want to within a millionth of a degree. And that’s something we simply couldn’t test on the ground. We did a lot of analysis, but being able to command [the telescope] into fine-point mode and see it go to fine-point was a very tense moment where people were standing there looking at the displays. And when it did happen, there was applause.
Kepler has exceeded its original lifespan of three and a half years. Do you know how much longer it will return data?
The spacecraft itself is still capable of working quite well, and what’s going to limit its operation right now is fuel. We do burn fuel slowly, and we have a limited amount, but we believe that we have enough fuel to last another maybe three years.
It’s not operating for the Kepler Mission because it can no longer point at the Kepler field of view [an approximate 100-square-degree section of sky between the constellations Cygnus and Lyra]. We’ve lost two of our four reaction wheels [which point the spacecraft and control its attitude]. But we’ve been able to repurpose the spacecraft and the instrument and look at other parts of the sky. And we call that mission “K2.”
How many Earth-size planets has Kepler detected to date?
I’m guessing a few hundred Earth-size planets. Not necessarily in the right orbit to be Earth-like at all. Many of them are going to be very close to their stars, so they’re going to be very hot planets. What we’re really interested in is in the Earth-size planets in the habitable zone around stars like our sun. And we’re only now starting to uncover that within the data. So we don’t have any confirmed planets in that category. We do have now six candidates, and we have to follow up on those to confirm if they are real planets. That would be the ultimate prize: to find that kind of a planet—one that resembles the Earth.
Are the numbers of Earth-size planets that you’re detecting about what you expected to find?
When we wrote our first senior review, we were thinking [we’d find] something between five and 15 Earth-size planets. We have six candidates. And we’re not done yet. We expect that as we continue to process the data, that number will likely grow.
And how much longer will the data processing continue?
It’s about two more years.
What would be a good follow-on mission to Kepler?
Well, there are good follow-on missions, and they are in fact in progress. They’re already approved. People didn’t have to wait for the Kepler results before they knew what the next step was. Kepler demonstrated that planets are common out in the universe. The next mission is going to say: “Well, now that we know there are planets out there, where are our nearest neighbors?” And that’s the TESS mission—it stands for Transiting Exoplanet Survey Satellite. So that is going to look at all the sky—not just one part of it like Kepler did. And it’s going to look at nearby stars. So it’s going to say: “Where are the planets within our neighborhood?”
And then following that, we have the James Webb Space Telescope, which is going to take spectra of stars. And once we find the planets, and inform James Webb what stars to look at, be able to measure that spectra and perhaps get the information on atmospheres. That would be so exciting.