Block That Star!
How can we find other Earths if their suns keep blinding us?
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Webster Cash remembers the exact moment—it was “about 10 minutes after 5 on a Tuesday evening”—when he found the shape of his starshade.
Along with colleagues at the University of Colorado and three other research centers, Cash had spent months searching for a mathematical function to determine the optimal design for what he calls the New Worlds occulter: a giant screen in space, resembling a flower with 16 petals, placed in orbit 125,000 miles from a space telescope. There, in line of sight with any distant star, the occulter would block enough of the star’s light to let the telescope detect faint Earth-like planets against the glare.
“I must have tried 100 different functions of various kinds,” says Cash, but none would suppress enough light. Then he tried an “offset Gaussian,” and the numbers in his computer model improved dramatically. After another half-hour of tweaking, he achieved (theoretically, at least) the magical 10-billion-to-one contrast ratio that would make an Earth-size planet visible against the blazing light of its host star.
Solution in hand, Cash recalls, “I went home and opened up a bottle of cheap champagne.”
That was in 2004. Astronomers had long been looking for a way to beat the fundamental contrast problem in imaging small planets around other stars. Big, shiny Jupiters are comparatively easy. But what they really want is other Earths, with their potential for life.
It took about a year, says Cash, to convince colleagues that New Worlds was practical in the near term. Sara Seager, a Massachusetts Institute of Technology scientist who studies extrasolar planets, was part of a planning group for NASA’s (now stalled) Terrestrial Planet Finder project; she says she’ll never forget one meeting in which a member of the New Worlds team briefed the group on the starshade concept. “You could hear a pin drop. Every single person was paying 100 percent attention.”
The starshade remains a viable concept, says Seager, although since that initial excitement, “reality has set in.” Deploying a 160-foot-diameter parasol in space, holding it to an exact shape, moving it from one stellar line-of-sight to another, then precisely aligning it with an orbiting telescope 125,000 miles away—after each move—is, technologically speaking, uncharted territory.
Nor is New Worlds the only idea for imaging distant Earths. Other techniques, including those that place the occulter within the telescope itself, are now achieving similar contrast levels, at least in the lab. But internal occulters pose their own challenges, and require mirrors of unprecedented smoothness. Seager thinks it may be 25 years before all the technical tradeoffs are resolved and the money is available to launch a dedicated Earth-hunting telescope; “I’m not happy about it,” she says of the delay.
That kind of schedule makes Cash fidget. He’s angling for a scaled-down, near-term demonstration project. An even fonder hope is that his starshade could be used with NASA’s next big orbiting observatory, the James Webb Space Telescope, due to launch in 2014; that plan would save the billions of dollars required for a dedicated telescope and could advance the timetable for imaging Earth-like planets by a decade or more.
Tony Reichhardt is an Air & Space senior editor.