spaceflightnow.com
1550 GMT (11:50 a.m. EDT)
Atlantis has was opened at 11:45 a.m., and mission specialists Joe Tanner and Heidemarie Stefanyshyn-Piper have climbed out of the middeck.
1531 GMT (11:31 a.m. EDT)
After a dramatic, down-to-the-wire debate, NASA's Mission Management Team called off the shuttle Atlantis' countdown today and delayed launch at least 24 hours because of concern about an apparently faulty low-level hydrogen fuel sensor in the ship's huge external tank.
Read our full story.
1530 GMT (11:30 a.m. EDT)
The Orbiter Closeout Crew and the AstroVan have arrived back at pad 39B to assist the astronauts out of Atlantis.
Meanwhile, NASA is planning a post-scrub news conference no sooner than 12 p.m.
spaceflightnow.com
1614 GMT (12:14 p.m. EDT)
The AstroVan is driving away from launch pad 39B now -- a little more than four hours after delivering the astronauts to board Atlantis.
1608 GMT (12:08 p.m. EDT)
Commander Brent Jett has exited the shuttle. All six astronauts will be heading back to crew quarters for dinner and await tomorrow's launch attempt.
1606 GMT (12:06 p.m. EDT)
Rookie shuttle pilot Chris Ferguson has emerged.
1602 GMT (12:02 p.m. EDT)
Steve MacLean from the Canadian Space Agency has egressed.
1600 GMT (12:00 p.m. EDT)
Flight engineer Dan Burbank has exited Atlantis.