A starshade shaped like a daisy? Centauri Dreams remains entranced with the concept, known as New Worlds Imager and now getting renewed attention thanks to the efforts of astronomer Webster Cash (University of Colorado, Boulder). We’ve seen before that Cash is hoping to land a NASA Discovery-class mission for a starshade that would block the light of a nearby star to reveal the planets around it. The starshade would work in tandem with a telescope mounted on a separate spacecraft 15,000 miles away, with the shade being moved as needed to place it into the line of sight of stars of interest. The result: the ability to image planetary systems including terrestrial worlds, and even to analyze exoplanetary atmospheres.
Cash’s latest thoughts on the subject appear in the July 6 issue of Nature, where he describes a starshade some 50 yards in diameter and its associated space telescope. Both could be launched into an orbit roughly a million miles from Earth, where shade and telescope could be moved as needed to image various star systems. An alternative Cash has also considered is to use the James Webb Space Telescope, scheduled for a 2013 launch, as the ‘eyes’ of the starshade concept, doing away with the need for a separate telescope while enhancing the capabilities of the mission. Cash’s $400 million proposal to NASA is a standalone starshade that would do just that.
Image: A starshade would obscure the light of a star to allow its planets to become visible. Credit: Webster Cash; University of Colorado, Boulder.
We saw several days ago that Greg Laughlin (University of California, Santa Cruz) has worked out the (admittedly burdensome) parameters of using the European Southern Observatory’s HARPS (High Accuracy Radial velocity Planet Searcher) instrument to track potential terrestrial worlds around Alpha Centauri. So it may be that finding our first Earth-sized world can be managed from Earth, but the beauty of Cash’s space-based concept is that it is both inexpensive and capable of extending our reach to numerous nearby stars. A Cash-style starshade would cost a fraction of earlier Terrestrial Planet Finder mission concepts, and would do many of the needed tasks better.
Imagine being able to see all major planets from the habitable zone outward, with a good view of debris disks and possibly comets. “Photometric variations might show the presence of surface features like oceans and continents,” Cash writes. “Follow-up spectroscopy of the detected planets would enable classification by type, and the presence of water would be clearly visible in atmospheric absorption lines. Atmospheric markers (like free oxygen absorption lines) could potentially provide the first evidence of life outside our Solar System.”
The beauty of New Worlds Imager is in the way it suppresses light from the central star. That light can be, as Cash notes in the Nature paper, 1010 times brighter than any Earth-like planets around the star. Handling light suppression within the instrument, as was originally envisaged for the Terrestrial Planet Finder Coronagraph (TPF-C), can help, but diffraction and scattering of light are difficult and expensive to suppress in such a system, and coronagraphs are not as sensitive to outer system planets and debris disks, issues that seem best addressed by a external starshade. Cash’s occulter is small and cheap, but as he says in Nature, “When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs.”
The paper is “Detection of Earth-like planets around nearby stars
using a petal-shaped occulter,” Nature 442 (6 July 2006), pp. 51-53, with abstract available here. Be aware, too, of another key paper: Kasdin, Vanderbei, Spergel et al., “Extrasolar planet finding via optimal apodized and shaped pupil coronagraphs,” which ran in the Astrophysical Journal 582, 1147-1161 (2003). This one showed the possibility of suppressing diffraction close to the needed levels.
Centauri Dreams thanks Anthony Kendall, author of the excellent Anthonares weblog, for his help in gathering material for this story.
It sounds interesting but considering how much space science funding NASA has slashed to cover manned space program cost overruns I’m not optimistic. It’s a pity if it doesn’t come to pass, it sounds a lot more useful than paying hundreds of millions of dollars for a conference call to schoolkids from orbit.
And who knows it might be a useful trial run to build expertise for the deployment of a shade for good ol Sol seeing as we might have to do something drastic to cool this planet this century.
I’m actually quite optimistic about New Worlds Imager. The reduction in cost from the earlier TPF-C concepts out there is enormous, and this is a technology that is more or less ready to go. I think this is going to fly.
Fantastic if it happens. Actual direct detection of terrestrial exoplanets will be a shot in the arm for interstellar studies. Warp drive anyone?