Bringing the Starshade to Reality

The goal of detecting a terrestrial class exoplanet has burned bright in the imagination ever since the discovery of the first planets orbiting main sequence stars. In a recent SEED Magazine story, Lee Billings (one of the most graceful science writers now working) harkens back to then NASA administrator Daniel Goldin’s 1996 speech at the American Astronomical Society meeting in San Antonio, Texas. Goldin talked about seeing Earth-like exoplanets up close, speculating that in 25 years we might be able to obtain images with a resolution to see clouds, continents and oceans.

I’m going to use a different Goldin quote than Lee did, from a later speech, but the idea is clear enough in either iteration. Here Goldin is speaking about the classrooms of the mid-21st Century and what they might look like:

When you look on the walls, you see a dozen maps detailing the features of Earth-like planets orbiting neighboring stars. Schoolchildren can study the geography, oceans, and continents of other planets and imagine their exotic environments, just as we studied the Earth and wondered about exotic sounding places like Bangkok and Istanbul . . . or, in my case growing up in the Bronx, exotic far-away places like Brooklyn.

This is a classroom I’d love my grandchildren to spend some time in, but exactly which generation gets to do that depends on how timely we are in creating the kind of terrestrial planet finder missions that can do the job. With a star outshining a terrestrial exoplanet by a huge factor, the challenge of just finding the planet is, as Billings notes, “…like photographing a lit match on the cusp of a detonating hydrogen bomb.” And in any case, we seem to have set up a built-in generational gap here. We’re making huge progress today at detecting exoplanets, but funding for the follow-up investigative tools is sadly lacking.

This is why I’ve often looked at starshade concepts on Centauri Dreams, thinking the technology of an external occulter could simplify the detection process and, in doing so, lower the costs of the project. Webster Cash (University of Colorado at Boulder) has spent the last five years working on a starshade that could function with the upcoming James Webb Space Telescope. The cost: $700 million to image alien Earths around nearby stars, a fraction of the price of the original TPF-C and TPF-I concepts worked up at the Jet Propulsion Laboratory.

We’re not talking about those Goldin-style images of Earth-like worlds on the schoolhouse wall, at least, not with the earliest generation of starshade. But Cash does believe we can use the early starshades to get a spectrum of an Earth-like exoplanet within the next ten years. That could flag the presence of oceans and even reveal signs of life.

Here’s Billings’ description of the operative technology:

Cash’s starshade would resemble a many-petaled sunflower—if sunflowers were matte-black and about half a football field in diameter. Its special shape is designed so that waves of starlight will diffract around it, lapping against and nullifying each other to cast an ultra-dark shadow, ensuring that only an exoplanet’s light falls on the JWST’s huge mirror. Equipped with small thrusters, the starshade would fly some 70,000 kilometers in front of the JWST, precisely aligning to block light from a target star so that its accompanying planets could be seen.

Read the article for more on this (and, if you’re looking for further background, run a search on this site for stories on Cash’s work). And keep this in mind. Astro2010 is a decadal survey of astrophysicists put together by the National Research Council, one that will soon release a report on research priorities for the coming decade. Cash’s starshade has been submitted to the committee, and so has a competing starshade concept by David Spergel and Jeremy Kasdin (Princeton University), former Cash collaborators. Will the committee support a starshade?

The lack of support from NASA on bringing such projects home is a natural consequence of the funding crisis the agency faces, one that may or may not change no matter what the upcoming Astro2010 report has to say. “The problem here is not the technology, but the lack of money to demonstrate it one way or another — and there’s something wrong with that,” Cash tells Billings. “This wouldn’t be just helping me, it would be NASA helping itself. NASA has a unique opportunity to conduct an experiment whose results, if positive, will never be forgotten.”