New Horizons, the fastest launched probe, is shooting towards a close encounter with Pluto and its three moons on July 14, 2015. As NH will get ~50 metre resolution we can work out the baseline for an interferometer to achieve the same. In visible light, say 0.5 micrometers, the limit of distinguishable detail 50 metres apart needs an aperture of 61,000 metres for Pluto’s distance of ~5 trillion metres. So a near-term interferometer won’t see Pluto better than the probe, not unless the scopes are really, really far apart and their optical signal can be combined as a virtual interference pattern to analyze. A challenge, and though there’s nothing unphysical in the idea it’s not happening soon enough to beat New Horizons.
The planned European Extremely Large Telescope, a massive telescope with a 42 metre wide mirror, will show an image of Pluto about 32 pixels wide, if the pixels are packed to the optical limit. That’s pretty good for a planet so far away. Two E-ELTs 200 metres apart would increase that to ~160 pixels or so.
What’s the extreme of performance possible?
Since 1995 there’s been some NASA discussion of a Exosolar Planet Mapper able to produce ~100 pixel images of Earth-like planets up to ~10 pc away. At that extreme, some 300 quadrillion metres away, an Earth-like planet with a ~130 km resolution needs a telescope some 1,400 km wide. While it’s tempting to say an interferometer that big is doable since radiotelescopes have been combined in bigger interferometers there’s a major problem. The photons reflected off the planet have spread themselves far and wide across an immense spherical wavefront. At 10 pc 1 square metre of reflecting surface of the planet has had its rays spread over ~2.1E+21 square metres. Each photon now has ~2 square metres to itself. To get a decent signal we’d have to catch a lot of photons with a lot of telescopes and keep extraneous noise out. Tricky.