Experts in interplanetary navigation have hit upon a novel scheme for maintaining communications with Mars, should a need to do so eventuate - as in the case of a manned mission to the red planet, for instance.
The problems in communicating with Mars come when the Sun lies between it and Earth, which happens at intervals of a bit more than two years. Direct radio communications are cut off for weeks.
A natural solution would be to use a communications relay satellite in a suitable orbit, which could carry signals from planet to planet around the sun much as Earth-orbiting satellites carry them around the world.
More: http://www.theregister.co.uk/2009/10/16/mars_ion_commsats/