NASA is developing a science-based synthetic world with the Unreal 3 Engine from Epic.
The general idea is that NASA's Learning Technologies Office got a small amount of funding to start a prototype NASA-based game. They are now working with the America's Army development team and the Federation of American Scientists to make a short(10 to 20 minutes) prototype. They are using the Unreal 3 engine from Epic, so they're hoping for commercial quality results.
The prototype is expected to be an introduction to NASA's Vision for Space Exploration - apparently, NASA is planning to head back to the moon by 2020, and by 2035, a human on Mars is planned. The prototype is expected to introduce the vision and give people a chance to appreciate the experience of being on the International Space Station and on the moon - separately, of course.