Jettison cargo
It is possible to jettison cargo and/or fuel from your ships, as long as they are in deep space. Nothing can be jettisoned when a ship is in orbit of a planet. Ships need to have fuel onboard in order to jettison. Cargo is jettisoned during the clientside part of the game: anything you jettison is gone immediately.
What exactly can and can't be jettisoned depends on the client-program that is used: