Why can't we just using existing space telescopes for planetary science?

[Camacha]

Yep, resolution is a function of aperture (1.22Lambda/D), or the distance between 2 apertures (interferometry, which is mostly, but not only done with radio at the moment).

While true, it makes more sense to launch two separate telescopes into space, rather than a binocular one. That way you can either use them in conjunction, or separately for double capacity.

Edited July 26, 2015 by Camacha

[YNM]

Something else you should take into account - each telescope specification has it's own excellence.

A very important part of a telescope specification, other than "bigger, stronger, faster, brighter" is their f/D :

* f/D < 6 are called fast telescope, intended for survey of large parts of the sky at once. Examples are Schmidt telescope, which AFAIK HST are one of them, which means it's better for them to capture a huge area of dim objects than a small bright object HST are Ritchey-Cheretien telescope with f/D of 24 - guess the problem is in the really sensitive CCDs then. But yeah, HST do captures Jupiter, Moon, Saturn and fair amount of other bodies, not to mention nowadays deep-space observation of HST takes a long time, for example this, which is really cool.

* 6 < f/D < 10 are general purpose telescope. You can pretty much do anything with them, but they're quite "jack of all trades, master of none". Research telescope are mostly not in this range, amateur as well (though it'd be perfect).

* f/D > 10 are slow telescope, they excels in resolving features of bright object before the detector goes saturated. Useful for planetary objects as well as individual astrometry, photometry and spectroscopy of each stars (say, binaries, or even exoplanet-harboring objects). Though sometimes with the advances of blocking plate they can be replaced by fast telescopes (think of SDSS).

So, next time you're looking for things, even when buying a telescope on your own, consider these values, and compare that with what you want to observe.

EDIT : JWST is a three-mirror anastigmat (often said as Cassegrain w/ corrector mirror, not corrector lens) with f/D of ~20. There're free telescopes from NRO (one use here, mind the emphasize on name, which is the same for HST I suppose), but exact specifications aren't clear.

Edited July 26, 2015 by YNM

[DMagic]

I just hope after Hubble falls back to Earth and after the JWST we get another visible spectrum telescope like HST.

Apparently we might get a bigger, UV/Vis/IR version of Webb. You know, as long as someone wants to pay for it...

http://arstechnica.com/science/2015/07/whats-after-webb-maybe-a-12-meter-space-telescope/

http://www.hdstvision.org/

[LordFerret]

Apparently we might get a bigger, UV/Vis/IR version of Webb. You know, as long as someone wants to pay for it...

http://arstechnica.com/science/2015/07/whats-after-webb-maybe-a-12-meter-space-telescope/

http://www.hdstvision.org/

http://i.imgur.com/tYN4Iav.png

Imagine two of those out there in orbit out past (what) Jupiter? Pluto? ... 180 degrees in opposition ... what that would do for the Hipparcos catalog!