The deal-breaker for him was the problem of diffraction — light’s tendency to bend around the edge of an optical barrier, thus ruining any hopes of getting unobstructed high-resolution views of a planet. “You can see waves going around a breaker in the ocean,” Cash explains, just as sound waves bend around corners in a building. “Light waves do that, too.”
But Northrop Grumman engineer Jon Arenberg pressed Cash on this point, asking whether there might be a way around the problem. Although he had initially discounted this possibility, Cash soon came around to thinking that Arenberg might be on to something. So far as Cash knew, no one had seriously examined this issue for many years. The more he thought about it, the more he became convinced that it might be worth “trying to crack the problem.” He decided to give it a shot.
How the Shade Was Made
Starting in November 2004, Cash spent six months exploring theoretical possibilities, testing out various shapes in the hopes of keeping diffraction in check — an effort supported by a modest award from NASA’s Institute for Advanced Concepts (NIAC). Finding the optimal shape, Cash explains, boils down to a mathematics problem.
He experimented with 100 different equations, each associated with a different flower-like shape, to see how they did in combating diffraction. He wrote elaborate computer codes and ran lengthy calculations, but none of his results bore fruit. “It’s hard to spend all that time when you don’t know if there’s a better solution to be had,” Cash says.
Inspiration finally struck in April 2005. In all of his prior designs, the flower-like petals fanned out directly from the center of a circle, like a lily. This time he tried something different, placing a dark, round disk in the middle of his shade and having 12 to 16 petals stick out from there — a pattern more like a daisy. Cash showed that this arrangement could, in principle, cut down a star’s glare by a factor of 10 billion, precisely the reduction needed to observe sister Earths near their host star. And a shade made of thin, black plastic, roughly 50 meters across, could do the job. A structure that size could be light enough to be launched and reliably deployed in space, maintaining the “well-defined edge” needed to block a star while laying bare its neighboring planet. “Web is the first, so far as I know, to turn the starshade into a practical idea by showing that it could be smaller, lighter and therefore much more feasible,” says NIAC head Jay Falker.
To Observe New Worlds
Later in 2005, NIAC funded a two-year architecture study of the starshade concept by the University of Colorado, Northrop Grumman Aerospace Systems and Princeton University. The resulting plan called for two spacecraft — the starshade and an orbital telescope — to launch together and detach in space, or to go up separately. Either way, the starshade would use thrusters to maneuver to a spot, about 50,000 kilometers from the telescope, from which it could screen out the star.
After examining a planetary system, the telescope would turn to the next target, with the starshade flying to a new, strategically chosen locale — a feat that would require exquisite, though technologically plausible, coordination between the two spacecraft.