Terrestrial Planet Finder - A,B.C... "c" first
High Contrast Imaging Testbed results from JPL (Trauger, 2004).
From Stapelfeldt's presentation at the 2005 Aspen workshop.
The Terrestrial Planet Finder
Two missions were proposed, TPF-c and TPF-i, I'll briefly discuss the former here.
The Terrestrial Planet Finder-coronograph was a visible light mission, intended to do direct imaging of nearby extrasolar terrestrial planets in visible light.
The essential difficulty in doing this is not that the planet is very faint, it is quite faint but not impossibly so, rather the problem is contrast.
An Earth-like planet a few light years away is very close to its parent star (less than 1" away on the sky for plausible orbits within the "habitable zone") and the reflected light of the planet is approximately a billion times fainter than the light of the star.
So, to see the planet, you must block out the light of the star - this is for two reasons, one is that an exposure deep enough to image the planet would lead to detector burn out from the star light, crudely speaking; and, the diffracted light from the star spreads out, swamping the light from the planet. This is unavoidable for any physical optical system.
You would think that just putting a mask over the star would block the light, but the diffraction of light around the mask will, in general, again swamp the light from any planet.
Fortunately, there are clever ways around that - you can't completely get around the problem of diffraction, but you can do some trade-offs: essentially, if you put in a funky shape "mask" across the telescope beam, you can change the shape of the diffracted light - the simplest pattern is the circular Airy disc, in general the diffraction pattern is the 2-D Fourier transform of the mask, for those of you who think in Fourier space...
In particular, you can choose a finite region within the image plane, where very little light is diffracted, almost none at all, at the price of having more diffraction elsewhere, but you just throw away that part of the image and search the dark patch away from the central bright spike. Then you rotate the telescope (or mask) and map out an annulus around the star.
Problem solved. (see for an early implementation of such masks)
And, as proof of concept that this is doable to the sensitivity level required, HCIT at JPL has shown billion-to-one contrast imaging, of monochromatic laser light in vacuum in the lab.
Rest is engineering (well, not quite, the edges of the mask need to be smooth at the atomic level, so nano-fabing is required, rather amusingly the blocked out mask areas need to be quite thick since a one-part-in-billion leak will ruin the image, there are issues of alignment and centering, the optical path has to be completely clean, and there may be some quantum optical issues, eg different polarization models may reach different levels of cancellations because of edge effects, where the electrical field of the photon interacts differently with the mask edge depending on whether it is parallel or orhogonal to the edge).
And then they were shut down and "indefinitely deferred".
It was a very ambitious mission, large mirror (and funky shape because no current launcher has a payload fairing with large enough a diameter to hold a large enough a mirror), and lots of tech to be developed.
But, it was exciting, doable, and likely to produce breakthroughs.
The engineering teams are dispersed, no new funding in the pipeline and the supporting science defunded with no calls for proposals in the pipeline.
So, what now.
Well. We could put it on the Moon. Heh.
Just kidding. Well, no, actually, not really. Moon is good.
There are new technologies on the horizon which may enable a smaller scope mission which will do something, particularly if results from the Kepler planet finder are promising; and of course the Europeans are turtling along in the inimatable way Darwin - Darwin as proposed is an interferometer, analogous to TPF-i.
More on those later.
There are also new high contrast imaging technologies some of which look rather interesting (like PIAA). We'll discuss those next.
Prospects are a bit depressing right now, but this too shall pass. Its funny, but it always seems to work out in the end, mostly.