An astronomer searching for alien worlds and an outfielder tracking a fly ball on a sunny afternoon face a common challenge. They both run into the problem of “contrast.” Simply put, the object they seek is not only small but exceedingly faint compared with the blazing star nearby. Baseball players have learned to get around this issue by blocking the sun’s light with an outstretched hand. Astronomers might want to try a similar trick.
Of course, it is much harder for planet hunters. In our solar system, for instance, Earth is 10 billion times fainter than the sun in terms of visible light. Trying to see an Earth twin around another star “is like trying to see a firefly fluttering less than a foot from a huge searchlight — when the searchlight is 2,600 miles away,” writes MIT astronomer Sara Seager in her book Is There Life Out There? It would take a perfectly placed screen, of just the right proportions, to block out that searchlight, but not the firefly.
More than 10 years ago, University of Colorado astronomer Webster Cash designed a bit of space apparatus that might pull off such a feat. Akin to a dark, flower-shaped kite minus the string, this rocket-powered screen would fly into position in orbit where it would obstruct light from a target star. An accompanying telescope could then sit in the shadow, inspecting the star’s environment without being blinded. Ever since, Cash has been obsessed with getting his so-called “starshade” off the ground, but he has garnered little encouragement from NASA. (Although Cash coined the term starshade and patented the first realistic design, he did not invent the idea. In a 1962 paper, astronomer Lyman Spitzer suggested the use of an “occulting disk” to aid in the detection of planets around other stars.)
Between 2008 and 2014, NASA turned down 12 of Cash’s proposals in a row. But things are finally starting to look up, he says. In recent months, the agency’s attitude has begun to change, and there’s a good chance an upcoming orbital observatory will be equipped with a starshade accessory. Until the space shields are incorporated into an official planet-hunting mission, though, Cash is determined to test them by any means necessary — whether they’re mounted on the crest of a rocky peak, dangling from a high-altitude balloon or even suspended from a floating zeppelin. “Twenty-first century astronomy will be about exoplanets,” he says. “NASA says they don’t have the money to do it. I say they don’t have the money not to do it.”
