The hardest part about searching for extraterrestrial planets has nothing to do with the limited power of telescopes; it's the blinding starlight, which overwhelms the reflected light of nearby bodies. So far, the standard solution has been to insert a light shield, called a coronagraph, along a telescope's light-gathering path. But this approach has had only limited success, in part because light waves tend to diffract around the shield.
Astrophysicist Webster Cash, of the University of Colorado in Boulder, thinks he has a better approach: Take the shield out of the telescope and move it out into space. Other researchers have studied similar concepts, but reporting in today's Nature, Cash proposes a flower-petal-shaped "starshade" based on optical principles developed by Augustin Fresnel in the 19th century. The device would be about 35 meters wide and positioned some 20,000 kilometers from the telescope, far enough away to negate diffraction and block offending starlight almost entirely. When astronomers want to focus on a particular system, they would align the shade with their telescope to block the light from that system's star.
Cash says he has tested the petal shape using computer simulations and a 1/1000th scale model, and both have shown that the starshade can reduce starlight interference by a factor of 10 billion, allowing a sufficiently powerful instrument--such as the James Webb Space Telescope, due to be launched by NASA in 2013--to image exoplanets directly. The telescope-starshade tandem could image planets located up to 100 light-years away, Cash estimates, thereby placing thousands of candidate solar systems within its field of view. "There is no undeveloped technology involved," he says. "We could start building this thing today." Cash has teamed with Redondo Beach, California-based Northrop Grumman Space Technology on a $400 million proposal to NASA, which is under evaluation.
"Imaging terrestrial planets suddenly seems difficult, but possible, instead of futuristic," says Marc Kuchner, an exoplanet specialist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who has contributed comments for the NASA review. On the other hand, he says, the starshade concept is relatively unexamined, so unforeseen technical problems could turn up. For example, says space telescope specialist Charles Beichman of the California Institute of Technology in Pasadena, it may be difficult for the starshade to hold its precise position long enough for the Webb telescope to capture a sharp image of a planet. Still, Cash has come up with "a design that has a much better optical performance than previous concepts," Beichman says, "so it's worth looking into."
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