"From orbit: Launch was awesome!! I am feeling great, working hard, & enjoying the magnificent views, the adventure of a lifetime has begun!" This was the first Tweet tweeted from space, and it came from astro_mike (http://twitter.com/astro_mike), aka astronaut Mike Massimino, at 4:33 pm on May 12th. But lest you think this was just a publicity stunt, it should be noted that astro_mike has been on Twitter since April 3rd, and he's Tweeted semi-regular updates about his training schedule and the progress of the Atlantis mission that have offered a unique and personal glimpse inside the life of an astronaut.
Mike Massimino is fairly unusual in that he's human, but there are plenty of Twitter accounts of interest to space enthusiasts, most of them devoted to a particular mission or robotic vehicle. In fact, there's a Space Shuttle Atlantis account (http://twitter.com/SSAtlantis) which makes Mike Massimino not only the first person to tweet from space, but the first Twitterer to Tweet from inside another Twitterer! (One hopes.) And there's a Hubble account, too (http://twitter.com/IamHubble). Bio? "I love orbiting the Earth and taking images of space!"
For the space enthusiast, here's a list of the Twitter accounts I know about. I'm sure I've missed some. Please add to the list via comment!
When it comes to space exploration, Twitter does what Twitter always does: it gives us more personal, less filtered access to a person (or project, in this case) that we're interested in but wouldn't normally have direct contact with. If you're a space nut, you owe it to yourself to get on Twitter and start following some of these accounts.