Space exploration has always been one of the most challenging dreams for mankind. On top of the obvious benefits for our evolution, the maths and physics behind space travelling provide the ultimate playground for geeks like us, which seek the truth through the most advanced scientific knowledge. Unfortunately I didn't have the pleasure of witnessing the space race triggered by the cold war which culminated with the Apollo program since I'm not old enough. Nevertheless I've been able to closely follow most of the Space Shuttle program as well as all the probes we've been recently sending to the solar system and beyond.
Having said that, there's a difference between watching something like an STS mission live on Nasa TV following it closely through Nasa's website and actually participate on such a mission albeit virtually. That's exactly what Orbiter allows you to do without any risk other than spending days of your life glued to the computer until you can finally accomplish your mission in space. But geeks are passionate about such things and every single second I spend in Orbiter means extra knowledge, not lost time.
Though this wonderful piece of software isn't opensource, it is freely available as freeware on the Internet and rivals proprietary ones like Microsoft Space Simulator much like FlightGear rivals Microsoft Flight Simulator. Orbiter isn't a game in the sense that you don't have a predetermined goal to achieve or levels to pass, rather you have a complete simulation of our Solar System's physics and a wide range of spacecrafts which you can fly, namely real ones like the Atlantis Space Shuttle, the complete Apollo program or imaginary ones like the great Delta Glider. Thus you can choose your favorite spaceship and plan the trip of your choice as long as you're prepared for a steep learning curve, you must make some calculations as this is a fully fledged simulator featuring Newton's Mechanics.
I've already gone through some of the basic maneuvers with the Delta Glider like taking off and landing on the Kennedy Space Center (this spacecraft behaves much like a normal airplane while under the influence of the atmosphere) or establishing a simple circular orbit around the Earth which implies cutting off your engines when the desired apogee altitude (the highest point or apoapsis of an Earth orbit) is reached followed by a prograde (Earth's rotation direction) burn so that the perigee altitude (the lowest point or periapsis of an Earth orbit) rises matching the apogee.
Further I've also successfully finished more advanced missions like docking with the ISS which requires orbital plane alignment (burning normal or anti-normal at orbital nodes), orbit synchronization and radio assisted docking maneuvers. More recently I've reached the Moon after performing a TLI burn (Trans Lunar Injection burn which puts the spacecraft into an eccentric orbit towards the Moon) while on a low Earth orbit with its orbital plane aligned to the Moon's orbital plane. Next I'll try to land on the Moon, which requires precise management of the hover engines on the Delta Glider as well as the RCS (Reaction Control System) since there's no atmosphere.