[sbierwagen]

I'd like to see a deep space version of the Gaia astrometry space telescope.

It measures the parallax shift of stars, and is basically the one reliable way of directly measuring how far away a star is from us. Unfortunately, it's at L2, and therefore has a baseline of 1 AU. Another Gaia way out at 20AU would have capacities no Earth-based telescope could ever have.

[lkbm]

I somehow didn't realize we already had pre-Webb stuff at L2. Luckily, Wikipedia has a list: https://en.wikipedia.org/wiki/List_of_objects_at_Lagrange_po...

[mulmen]

This seems like the kind of thing where two would actually be useful. Is there any benefit to making both observations at the same time? Or are the scales so great that it doesn't really matter?

[sbierwagen]

You'd almost certainly want to launch several. You get one data point per half-orbit, when you're at opposite sides of the Sun. This is tolerable for the Earth, where an orbit is one year. But a full orbit out at 20AU takes eighty four years! Collecting a useful number of samples with one spacecraft would take centuries, while two spacecraft in opposition on the same orbit can measure parallax instantly.