In 1898, H.G. Wells penned the novel The War of the Worlds, which portrayed an invasion of Earth by technologically superior Martians desperate for water. This story was reversed starting in 1965 with the flyby of the Red Planet by Mariner 4. It was just the beginning of a long series of Mars probes that continues to this day, making Mars the most visited planet in our solar system.
One of the more successful of the orbiting robots has been Mars Odyssey, which set off in 2001 to search for water and survey interesting geological formations on the planet. Odyssey can be thought of as a Martian Swiss Army knife—one tool with a host of purposes, from an instrument which has made a multitude of scientific discoveries on its own, to a transmission relay device for multiple landers and rovers (as well as a device to find landing sites for said landers and rovers), and a champion tag-team partner with other spacecraft that have redefined what we know about the planet.