LightningInAJar said:
What limitations does Webb have that Hubble doesn't besides higher resolution? I assume Hubble doesn't need to hide in Earth's shadow? Can Webb photograph earth?
First question:
Neither telescope permanently hides in Earth's shadow (neither Hubble Space Telescope [HST] nor James Web Space Telescope [JWST]).
HST:
HST orbits Earth in a rather conventional low-Earth-orbit. Its orbit isn't as low as the International Space Station (ISS), but it's still pretty low: only about 540 km (330 miles) above Earth's surface. That means it's in direct sunlight a little more than half the time. It has a comparatively small sunshield sort of thing that keeps direct sunlight from entering the optical tube, but that's about it. Maybe some insulation.
By "conventional" orbit I mean that HST orbits around Earth in a elliptical orbit (along with precession and such), as you might expect from most any other orbiting satellite.
JWST:
The sun-shield side of JWST is in direct sunlight all of the time. So from that point of view, JWST is in direct sunlight way more often than HST.
JWST completely relies on its own sunshield to keep it in the shade (from the Sun). There is no hiding in Earth's shadow.
If JWST was directly at the Lagrange 2 (L2) point (which it isn't), the Earth would annular eclipse the Sun (i.e., not a total eclipse).
JWST instead orbits around the L2 point in such a way that it's always in direct sunlight; it never passes through the Earth's shadow (and again, even if it did, there is no umbra of Earth's shadow: penumbra only at that distance).
Second question:
No, JWST cannot image Earth. JWST
must keep its sunshield side on the side where the Sun is. Because JWST always roughly in the vicinity of the L2 point, it means that both Earth and the Sun are roughly in the same direction, more-or-less.
If JWST were to rock itself over to point its optics toward Earth, some of the delicate equipment would end up in the sunlight and bad things would happen.