It's surprising to see this large of a discount on a great simulation that was just released in February. You can watch a review here: http://www.youtube.com/watch?v=AzFYL7g_K7M
It's also available DRM-free at Desura for the same price: http://www.desura.com/games/lunar-flight
Sale ends May 25th.
A modern take on the classic arcade game Lunar Lander, Lunar Flight extends the experience to a fully fictionalized and accessible lunar module simulator providing a variety of mission types involving transporting cargo, acquiring data at survey locations and locating lost cargo.
Completing missions earns experience points and money to be used to refuel, repair and upgrade the lunar module. In addition to missions there are time trial challenges complete with online leader boards and a globally ranked pilots leaderboard, ranked by experience. To round out the game there are a variety of achievements to provide additional challenges and objectives.