Photojournalists have strict guidelines as to what is acceptable use of Photoshop. The program is a powerful image manipulation tool, but we only use a fraction of its capabilities. REUTERS has their guidelines spelled out very clearly. I'm sure NASA has similar guidelines in place to assure that no one is mislead by photos they publish but they still get the spectacular images that are expected.
For us at The Catholic Spirit and the other papers we publish, at the very minimum, images need to be toned to printer specifications, light and dark areas of the image need to be at certain levels so they don't get too much ink (in the shadows) or no ink at all (in the highlight areas). And often there are dust spots that need to be removed (I have this problem a lot!).
As far as color goes, there are many reasons why the image you get out of the camera doesn't look exactly like what you saw when you were taking the picture. Maybe it is under exposed, or more commonly the white-balance was set wrong. There is nothing wrong with tweaking the color to make it look more like the scene actually looked.
What we don't want to do is alter the image to be something other than the scene that was photographed. No taking people out, removing unwanted items like poles, cars, etc...
Here's an example of a photo I shot recently that needed a little work:
A usable version: