There are only two REAL space simulations on Earth: the Vomit Comet, and the NBL. The vomit comet is a NASA jetliner with a large open cargo area, that flies a parabolic flight path so that there are occasional 30-second moments of actual weightlessness. The NBL is Johnson Space Center's Neutral Bouyancy Laboratory, which is basically a GIANT swimming pool that includes a replica of the station exterior. There, the astronauts in spacesuits are specially weighted and guided by divers to simulate the effects and difficulties of performing maintenance outside the ISS.
Astronauts Robert Satcher, Jr. and Rick Sturkow practiced techniques that will be used by the Expedition 24 astronauts Friday. This effort gives NASA a heads-up to potential problems and responses that might be encountered.
Trivia Question for viewers: Which Gemini astronaut championed the development and use of a swimming pool for space training?