To our ancestors, Mars was a strange, red dot in the night sky. But unlike the stars, it didn't twinkle. What could it be? Through telescopes and spacecraft, we've come to know it's a whole new world waiting for explorers... a rocky planet, in some ways quite like Earth.

Mars is the fourth planet out from the Sun, between Earth and the giant gas planet, Jupiter. It's smaller than Earth, but if you take away Earth's oceans, both planets would have about the same surface area. Some of its features, however, are much larger than anything on Earth.