The SIM spacecraft will carry out super-precise interferometry to combine the images from pairs of small telescopes mounted along a 10-meter boom into a single image of far higher resolution.
In interferometry, the longer the baseline separating two telescopes, the sharper the combined image that they generate is -- and the Deep Space-3 engineering test mission in 2005 will try to combine the images from two small telescopes on separate spacecraft flying in precise formation at distances of up to half a kilometer.
Optical interferometry requires incredibly precise adjustment of the optical paths connecting two telescopes -- down to a few billionths of a meter -- but recent techniques give us confidence that we can make it practical, even between distant free-flying satellites.
And a version of interferometry called "interferometric nulling" -- first tested on an Earth telescope in 1998 -- may enable us to completely cancel out all but a very tiny fraction of the glare from a central star, and thus directly see even its small planets.
SIM will use this technique to try to cancel out all but 1/10,000 of the light from a star.
Once the telescopes of the next decade have given us a much better idea of what kinds of alien solar systems exist, the next step will be to use the nulling technique not only to locate large numbers of Earth-sized planets in the Habitable Zones of other stars, but to obtain spectral analyses of their atmospheres -- and serious planning has already begun for a very complex American mission called the "Terrestrial Planet Finder" to do this early in the next decade, if the money can be found for it.
(The European Space Agency is considering a similar mission called IRSI -- and, sensibly, consideration is being given to pooling the two efforts.)
TPF would interferometrically combine the images from several 3 or 4-meter telescopes (either mounted on a boom or flying in formation) in solar orbit, to cancel out all but a few millionths of the light from a star.
It could measure the temperature, water and carbon dioxide on all Earth-sized planets within 50 light-years of Earth, thus determining their climate.
And it would also try to detect traces of ozone in their air -- for ozone is made from free oxygen, which can only exist in the air of an Earth-temperature planet if photosynthetic plants are churning it out in large amounts.
(It would also look for methane, which could probably exist in large traces in the air of an Earth-sized planet only if bacteria were producing it -- thus allowing us to look for life on those planets where photosynthesis hsn't yet evolved.) It is thus quite possible that we'll be able to solidly prove the existence of life on a planet of another star before we can prove or disprove it on Mars or Europa!