Interesting and good questions!
I guess that it may be a combination of things. Earth is obviously working as a fill light. I don't think that NASA would use flash, it would need a very big flash for that. Could be HDR or just NASA being able to suck a lot of details out of the shadows.
My experience is that there is an incredible lot of shadow detail to extract from DSLR shots. I tried HDR a lot but never got it working for me, I get the colors messed up and to many artifacts. I probably used the wrong tools, the wrong way for the wrong cases ;.) In my experience I always got better results exposing for the highlights and extracting shadow detail using "fill light".
Some of the "fill light" may come from the solar panels of the space telescope. They are not visible on the picture but satellites are normally powered by large solar panels.
Best regards
Erik
The earth itself should reflect a lot of light and serve as a giant fill.
Now in this particular image, could there be a bit of flash fill in or some HDR?
Cheers,
Bernard