”The specification for our pointing accuracy is 9 milliarc seconds of drift over 15 minutes,” Troeltzsch says. That drift is only about 1/56th of the angle resolved by a good ground-based telescope. There aren’t many disturbances in space, except for the solar wind pushing up against Kepler ’s photovoltaic panels. To make sure the telescope doesn’t move, four CCDs in the camera monitor the position of 24 stars continuously. If those stars move in the field of view, a control system repositions the telescope.
Building a 1-meter-diameter telescope with a field of view 33 000 times as great as that of the Hubble Space Telescope was no easy undertaking, but that large field of view was absolutely necessary. Planetary orbits are randomly oriented in the sky. In order to see a transit, your line of sight must precisely match up with the plane of the orbit. The more stars you see, the more likely you are to find one with the right orbital plane. The array of CCDs in Kepler ’s camera form a focal plane of 900 square centimeters.
Behind that focal plane is a 0.5-meter box containing more than 20 000 electronic components. ”The difficulty was designing all those parts to be low noise and maintain performance in a radiation environment,” Troeltzsch says. ”It was eight years of putting designs together and seeing if they’d work.” Part of the solution was to operate the camera at –85 °C, which helped reduce the noise.
Once a month, Kepler will turn toward Earth to align a fixed high-gain antenna and transmit its data to NASA’s Deep Space Network (DSN). The DSN is an international network of antennas that supports the agency’s interplanetary spacecraft missions and radio and radar astronomy observations. Scientists at the Ames Research Center will then analyze the results to verify any possible planets. Kepler will also rotate once every 90 days to keep its solar panels directed at the sun.
Planet hunting has been around for more than a decade, but it’s been done mostly from the ground. Kepler isn’t even the first spacecraft to search for planets. In 2006, the French space agency, CNES, launched the Convection Rotation and Planetary Transits (COROT) mission, but it was never expected to find Earth-like planets. Unlike Kepler , COROT orbits Earth, which partly obscures the view of the sky from the craft. This means that COROT can observe a patch of sky for only weeks at a time, not years. ”In order to have a firm detection of a planet, we need to see at least three orbits so we can verify that we see a body moving in a fixed orbital period,” Fanson says. ”To find planets like Earth, which take a year to go around a star, you need to observe the same stars for three to four years.” Also, compared with Kepler, COROT is much smaller, less sensitive, and has a smaller field of view.
Fanson, Duren, and Troeltzsch say that even if Kepler finds no Earth-like planets, that itself will be a monumental discovery—the fact that planets like ours are rare. But the researchers don’t think that will be the case.
”I’m an optimist, and I hope the galaxy is filled with many Earth-like planets,” Fanson says, adding that the Kepler mission is the highlight of his long career at the agency.
”I’ve been working for NASA for 25 years and have been involved in many exciting missions, including being on the team that fixed the Hubble Space Telescope,” Fanson says. ”But this is the most exciting one, because we have the chance to answer a question that has been in the minds of people for as long as we have records.”
