Searching free exploring space game .


I am looking for a free space game, focussing on exploring and expanding. I do not look for a shooter or extensive space combats, but for
- exploring new star system and planets
- trading stuff
- expanding planets or something ...

The game I am looking for must be
- playable under Linux
- playable on a slower system (the game does not need to be cutting edge)
- easy to install, without requiring strange libraries (apt-get install preferred for Ubuntu 13.10)
- easy to play with, without having to read a 1000 page manual
- free

The game should be
- multiplayer

I recall for example a game I played ages ago where you have to send probes across the solar system. The first probe always got stuck in the asteroid belt. This kind of game.

I would appreciate suggestions.