The budgetary demise of Terrestrial Planet Finder has cast a pall over some researchers, but it may have energized an entirely different solution. What if I told you that in the 2013-2015 time frame we may get conclusive images that tell us whether or not there are terrestrial worlds around Tau Ceti, Epsilon Eridani, and Centauri A and B? Images that allow us to examine the habitable zones of as many as 100 stars over a three-year period? With TPF gone, the idea sounds like a fantasy, but my recent conversation with astronomer Webster Cash revealed it to be anything but.
Cash (University of Colorado at Boulder) has been involved in the development of a concept most recently called New Worlds Imager, one that began as an enormous ‘pinhole camera’ design, as I discussed in Centauri Dreams (the book) in 2004. Within the last 18 months, the design has morphed into a low-cost mission using an occulter (call it a ‘starshade’) to mask the light of the star being observed so as to reveal planets. Observing the occulter from some distance away would be a second spacecraft equipped with a telescope. The key to Cash’s work is to suppress diffraction, for it is an odd fact of optics that when you put an occulter directly in front of a bright object like a star, you still have to deal with diffraction effects because of the wave nature of light.
In other words, you can’t just block starlight with the right-sized object and expect to see nearby planets, because a good deal of the star’s light still bends around your occulter and disrupts the image. Optics students will recognize the classic case of this phenomenon as Poisson’s Spot, although it grew out of work originally performed by Augustin Fresnel and was confirmed by Francois Arago early in the 19th Century. In any case, diffraction has played havoc with earlier attempts to suppress light sufficiently to discover terrestrial worlds.
And so a great deal of energy has gone into solving the diffraction problem, with what can only be described as middling results. What motivated Cash to attack it again were conversations with engineers from Northrop Grumman about his original design and the possibility of modifying it to use an occulter. Cash continues to pursue this work with funding from a Phase II grant from NASA’s Institute for Advanced Concepts, which had provided a Phase I venue for his original ideas.
The result? Cash now tells me the diffraction problem is well on the way to solution, meaning that his new theoretical and now laboratory work is demonstrating that it will be possible to screen out starlight effectively — we’ll be able to get a look at those terrestrial planets if they’re there. Moreover, a mission to make this possible is definitely feasible even given NASA’s budgetary constraints. Read on…
The mission built around Cash’s concepts would involve two spacecraft, a telescope and a free-flying occulter (powered by ion propulsion) that would block starlight and move from star to star; the telescope would thus acquire data from a multitude of stars as the occulter craft changes position.
And here’s what makes this realistic: Cash has made an arrangement to use the James Webb Space Telescope as the ‘eyes’ of the planet-finding mission. This means the cost of producing what would have been a two-spacecraft mission has been dramatically reduced, for all Cash has to supply is the occulter vehicle, which will involve a starshade and the necessary ion propulsion and electronics to move the shield in relation to the telescope. Both would be at the L2 point, with Cash’s occulter (launched three months after JWST in 2013) approximately 30,000 kilometers away from the space telescope.
The observing method? Move the occulter into position so that it masks the light from the star to be studied. Then turn the JWST instrument toward it to collect data. Imagine observing runs happening about once a week as the relative configuration of occulter and telescope changes. And imagine picking up planetary images from the inner habitable zone all the way out to the edge of the observed solar system. Cash believes the mission can acquire rich data from about 100 candidate stars in a three-year run.
As it turns out, NASA made an announcement of opportunity for Discovery missions in December that could allow the Cash design to happen. The Discovery program has already produced such spacecraft as Deep Impact and Mercury Messenger. Cash believes he can offer a proposal that will come in at or under the $425 million cap NASA places on such missions (bear in mind that $130 million is lost up-front in the cost of an Atlas V). He’ll be competing against other strong proposals, but with Terrestrial Planet Finder now out of the picture, the chance to do the entire TPF-C mission at a fraction of its earlier estimated cost seems too good to ignore.
Terrestrial Planet Finder had been assumed all along to be a two-part mission, with some designs for TPF-C using a visible light coronograph to observe terrestrial worlds, and the subsequent TPF-I (launched some years later) built around an interferometer using multiple spacecraft for infrared observations. Cash’s vehicle, operating in tandem with JWST, would be able to handle the observations originally demanded for TPF-C and at least some of the spectrographic analysis of TPF-I thrown in for good measure. “On some of the better targets, we can use some longer wavelength photometry and look for the water bands,” Cash added. “So we can not only find these things in the habitable zone but see whether they have methane or water atmospheres.”
Success at this mission might lead to renewed interest (and funding) for the interferometry mission that could follow. We’ll know more come April, when proposals for the Discovery program have to be submitted. Cash will enter his proposal with the backing of a consortium including Princeton University, Northrop Grumman, Ball Aerospace and Goddard Space Flight Center. Meanwhile, the more public interest that can be generated re saving the hunt for terrestrial worlds, the more likely this mission is to fly. If it does, then the budgetary disaster that swallowed the original TPF will be seen to be a blessing in disguise, allowing us to move to what appears to be a superior technology using existing equipment with a low-cost occulter. And that just might find us our first terrestrial exoplanet, far sooner than we had expected.