Finding planets through a pinhole
An ancient imaging technique may allow astronomers to study planets beyond the solar system as small as our Moon.
October 1, 2004
Take two sheets of white paper. In the middle of one sheet, poke a hole with a pin. Hold the sheets about a foot apart, so the punctured sheet faces the Sun, and you'll see a tiny solar image on the second sheet. This principle, called pinhole projection, was known to ancient Greek and Chinese scholars and, in the middle of the 19th century, morphed into one of the earliest photographic techniques.
Fast-forward 150 years.
The NASA Institute for Advanced Concepts (NIAC) currently is studying a proposal for a space mission that could produce images of planets orbiting a distant star. The plan calls for an opaque, lightweight, football-field-size, orbiting "starshade" with a 30-foot (9 meter) hole at its center. A spacecraft carrying imaging equipment would trail the starshade by tens of thousands of miles. In effect, this setup would be a gigantic pinhole camera.
"To me, one of the most interesting challenges in space astronomy today is the detection of exo-solar planets," said team leader Webster Cash, an astronomer at the Center for Astrophysics and Space Astronomy in Boulder, Colorado. "The beauty of the pinhole as an optical device is that it functions as an almost perfect lens," he explained. "This device would remove the limiting problem of light scattered from the parent star due to optical imperfections."
To acquire an image of an exoplanet, the system — dubbed the New Worlds Imager (NWI) — would carefully align the starshade and imaging spacecraft on its target. According to Cash, NWI would have sufficient resolution to detect exoplanets as small as the Moon.
"In its most advanced form, the New Worlds Imager would be able to capture actual pictures of planets as far away as 100 light-years, showing oceans, continents, polar caps, and cloud banks." NWI also could search for oxygen, methane, and water vapor in exoplanet atmospheres.
The New Worlds Imager was one of a dozen proposals for which NIAC will fund a $75,000 6-month-long viability study. According to NASA, those proposals that win approval for more study will receive up to $400,000 for 2 years.