Not immediately applicable, but …soon: in the Publications of the Astronomical Society of the Pacific, November (vol. 136)-
Lucas, M. Norris, B. Guyon, O. et al. Visible-light High-contrast Imaging and Polarimetry
with SCExAO/VAMPIRES art. 114504 ad89af
The tech of adaptive optics (AO)- compensating for the blurring effect of Earth’s turbulent atmosphere- is progressing, with no physical, absolute limit in our immediate future. We are now in extreme adaptive optics (ExAO, XAO, whatever)- higher-degree correctors, running at shorter wavelengths, for greater resolution and contrast. Sure, much of this is driven and funded by the exoplanet people. But the telescope doesn’t know what the target is; a telescope that can split a tight exo-system can also split nearby but tiny asteroids. Just look at what Vernazza and others are doing with the SPHERE instrument on VLT (Very Large Telescope- soon to be a misnomer).
Using the VAMPIRES instrument system (Visible Aperture-Masking Polarimetric Imager/interferometer for Imaging Exoplanetary Signatures), the group claim a spatial resolution of 20 microarcseconds (20 μas). That’s Hubble-beating, and by a lot, though still not the theoretical limit of the Subaru telescope. (At 8.2 meters aperture, a perfect Subaru and no atmosphere would deliver ~15 μas.) There is still a bit of noise in the system, residual losses, and complexities standing in the way, sorry. And that 20 μas still requires a night of good “seeing” (a night of calm air over the Maunakea observatory site). If there’s a limiting factor, it might be the rather bright mR < 10 limit of the AO correction system’s wavefront sensor… but that’s what laser guide stars are for. It’s a bit funny to think next-gen telescopes will not have laser guide stars.
And that’s the take-home message: telescope tech is not standing still. The extremely large telescopes- the coming European ELT, plus the Thirty Meter Telescope, and Giant Magellan Telescope- will get resolutions 3-4.5 times better than this, and many asteroids can be resolved into a shape from the ground. It’s a bit bold for Vernazza et al. to say space missions will not be needed for shape imaging and morphology, but he’s not wrong, either. It’s time we got EXTREME!