* First observation: Humans have been observing Mars with telescopes since the 17th century. Galileo Galilei was the first to do so in 1610.

* First spacecraft flyby: The first spacecraft to fly by Mars was NASA's Mariner 4 in 1964.

* First successful landing: The first spacecraft to successfully land on Mars was the Soviet Union's Mars 3 in 1971. However, contact with Mars 3 was lost shortly after landing.

* First successful rover: The first successful rover to explore Mars was NASA's Sojourner, which landed in 1997 as part of the Mars Pathfinder mission.

So, depending on your definition of "explored," the answer could be anywhere from the 17th century to the late 20th century.