That should be useful for stellar coronagraphs for finding extrasolar planets. The idea is to have an opaque disc that occludes the star so you can see the planets. This is usually attached to the telescope, but one could be flown out a kilometer or so with this approach. JPL was looking at this but went with a more traditional design instead.

Presumably the limiting factor for this is density variations in the tiny bit of atmosphere present up there...

And the fix is to fly higher...?

If you know how hard it is to align an interferometer on an optical table, where things are BOLTED to it, this looks absolutely insane.

> The ongoing PROBA-3 (Project for On Board Autonomy-3) mission (Llorente et al. 2013; L. F. Peñin et al. 2020) (launched in 2024) aims to form a solar coronagraph using two satellites approximately 150 m apart with a relative displacement accuracy under a sub-millimeter and pointing accuracy at the arc-second level.

I expected this accuracy at km or greater. At least for radio interferometry. Maybe Optical has smaller scale?

The positional accuracy of fixed point RF interferometers in astronomy are amazing, they can probably measure continental drift (only joking) -So this is saving atmospheric attenuation but with very small angular distance between the arms of the interferometer.

Surely this has to go up to km and 100 of km to be useful? Maybe not in optical.

(the Narrabri Radio Telescope is a giant radio interferometer on a linear railway track. The SKA is fixed position, massive deployments "Square Kilometer" hence the name.)

I'm not sure how different this is, but the Gravity Recovery and Climate Experiment [0] is planning is 3rd mission after 2 currently operating pairs of satellites using interferometry to map gravity anomalies, among other things.

[0]https://en.m.wikipedia.org/wiki/GRACE_and_GRACE-FO

Most definitely a dual-use technology, specifically locating elusive targets via their RF emissions a la the US Navy's NOSS, France's CERES, and some of China's Yaogan military satellite clusters.