The Best Way to View Terrestrial Worlds

by Paul Gilster on October 24, 2005

Centauri Dreams has been a champion of Webster Cash’s New Worlds Imager for several years now. The proposal, whose initial study was funded by NASA’s Institute for Advanced Concepts, offered a way to find terrestrial planets around other stars and, in its most fully developed configuration, to create startlingly sharp images of such worlds down to the level of continents and weather patterns moving across their surfaces. Now two new developments — related in a phone call from Cash last week — bring New Worlds Imager to the fore as NASA weighs strategies for its Terrestrial Planet Finder mission.

First, Cash has changed the basic design of New Worlds Imager to move away from the enormous ‘pinhole camera’ concept discussed earlier in these pages to an occulter — a design that blocks the starlight from the central star to allow its planetary companions to be visible. The problem with occulters has always been that no matter how scientists worked with their design, they could not get rid of diffracted light. “If you’re trying to make the smallest possible shadow thrower, in fact, you end up doing exactly the opposite,” Cash told me. “You concentrate the light. This has been the problem with occulters all along.”

But Cash, an astronomer at the University of Colorado at Boulder, was in the process of finishing up his Phase I study for NIAC when he solved the diffraction problem. Centauri Dreams will have more to say about this in coming weeks as we take a more detailed look at New Worlds Imager, but the gist of the matter is this: using a star-shade that is much smaller than originally projected, perhaps on the order of 30 to 50 meters tip to tip, Cash’s design can suppress central starlight to an efficiency that not only reveals planets down to terrestrial size, but makes possible spectroscopy on their atmospheres.

Secondly, the new design drops the cost in startling fashion. Working with engineers from Northrop Grumman, Cash now has a New Worlds Imager that can fit inside a single launch vehicle and cost below $1 billion. This at a time when NASA has just announced a sharp cutback in Terrestrial Planet Finder funding for this year as it searches for a cost-effective technology to make it happen. New Worlds Imager offers a clean, practicable solution at lower cost than competing designs.

“Everything is going the right direction,” Cash added. “There are no showstoppers here. I’ve had some really good people at aerospace companies looking at this and we’d be surprised if there are any problems at this point.”

The Jet Propulsion Laboratory’s competing Terrestrial Planet Finder concept, shown on a JPL Web site, is for a two-part mission, both to launch within ten to fifteen years. The first of these, TPF-C, would work in visible light, while the second, TPF-I, would use interferometry with multiple spacecraft to make infrared studies for more precise detections. Cash is proposing a system that seems superior: it does the work of TPF-C and is also capable of doing follow-up spectroscopy to study distant planetary atmospheres. His Phase II work for NIAC will examine a basic ‘observer’ mission and a much more complicated imaging mission, which would use interferometry to share data between five spacecraft examining solar systems for planetary close-ups.

New Worlds Imager offers a low-cost way to achieve NASA’s stated goal of imaging terrestrial worlds and examining them for the presence of life. It does so using breakthroughs in the study of diffracted light that may one day give us new geographies to examine on blue and green worlds that resemble Earth and are probably found in great profusion throughout the galaxy. Centauri Dreams intends to focus on New Worlds Imager closely as this work continues. It is a design that cannot be allowed to vanish beneath bureaucratic red tape as we make crucial decisions for the next phase of exoplanetary exploration.