Very good Software.
STA is an astrodynamics modules and tools collection developed by ESA and European universities. The modules are gathered in a cute (and cross-platform) graphical interface with Celestia as 3D visualizer. This amazing feat brings astrodynamics to the free software world, with the ultimate objective to challenge the famous (and closed) STK, for use in Mission Analysis tasks at ESA and among European companies, universities and students. The program is just at the beginning of his challenging aim but it has already remarkable features! For me this is really an awesome project! At the moment I'm following and studying it and I will try to contribute ASAP. Godspeed STA! ;-)