Must read: Astro-Mike: In space, everyone can hear you tweet

GREENBELT, Md. - On Twitter, messages of 140 characters may seem confining. Mike Massimino is finding a different type of space even more limiting - outer space.

The astronaut is sending messages to the social networking site from 350 miles above Earth on the space shuttle Atlantis - and what's cramping his 'tweets' is time, not the character limit.

On Earth, Massimino tweeted about having dinner, eating lunch, working out in the gym and watching a movie. In space, he just doesn't have the time to tweet the routine, if you consider anything on orbit routine.

He averaged about four tweets a day before launch. Now that he's in orbit, it's down to one a day.

Still, Astro-Mike, which is Massimino's Twitter name, did make history.

He's the first person to tweet from space: 'From orbit: Launch was awesome!! I am feeling great, working hard, & enjoying the magnificent views, the adventure of a lifetime has begun!'

Massimino's messages, promoted by NASA, are part of the space agency's full-throttle embrace of new social media. There are 36 NASA Twitter accounts, including Massimino's. NASA also has 24 Facebook accounts, 13 YouTube channels, eight Flickr photo accounts and five MySpace sites.

Astronauts' most memorable lines have always been pithy and tweetworthy. Neil Armstrong's 'That's one small step for man, one giant leap for mankind' was 57 characters - if you leave out the 'a' that Armstrong later said he included before 'man.' And Apollo 13 astronaut James A. Lovell's 'Houston we've had a problem' is only 27.

The Astro-Mike tweets come to Earth indirectly. He sends them in e-mails to Johnson Space Center in Houston and officials there post them. More than 270,000 people have signed up to follow his tweets.