'Man-made' satellites orbiting Mars include the Mars Global Surveyor and Mars Express. These satellites act as a communications relay for robotic missions on the Mars surface. The satellites pass over the Rover's landing site every two hours in a low-altitude polar orbit. Mission Control must remain in constant radio contact by switching between orbiting satellites.
View a short video showing how radio signals are sent between the Mars Polar Lander, Phoenix, and Mars Odyssey, orbiting Mars. THIS YOUTUBE LINK IS BROKEN
Mars Express in orbit around Mars.
These satellites also carry instruments for studying Mars, including telescopic cameras, and spectrometers and radar that can probe beneath the surface for signs of water, and monitor the temperature and movement of water vapour through the atmosphere.