is a spacecraft
that orbits a planet
without landing on it in order to study the object's surface from a safe distance.
Orbiter is a freeware space flight simulator, being developed since the year 2000 by Martin Schweiger, a PhD and professor of the University College London.
Orbiter allows any user to explore the solar system on a number of spacecraft, such as the Space Shuttle Atlantis. Many other spacecraft are available for download, from the Soviet Vostok spacecraft to Star Trek's Enterprise. Orbiter aims to be a realistic simulator rather than an arcade game. Space physics and orbital mechanics can be complicated, and the learning curve is steep.
The program allows the user to take a spaceship from Earth orbit to Mars, perform a docking to the Mir space station or ISS, rendezvous with a satellite, or just try a suborbital flight.