Mars is the fourth planet from the Sun and Earth's outer neighbor in the solar system. Named after the Roman god of war, Mars appears as a bright reddish point of light in the night sky, earning it the nickname “the Red Planet.” That rusty color comes from iron oxide (basically rust) covering much of its surface.

Mars is about half the size of Earth and has dramatic features that would astound any visitor: Olympus Mons, the largest volcano in the solar system, towers three times higher than Mount Everest. Valles Marineris, a canyon system, stretches so long it would reach across the entire United States. The planet has polar ice caps, dust storms that can engulf the entire planet, and seasons like Earth (though they're about twice as long).

Scientists have sent numerous robotic rovers and spacecraft to explore Mars, searching for signs that liquid water once flowed there and investigating whether life ever existed on the planet. Mars fascinates people because it's the most Earth-like planet we know, and many scientists and engineers dream of humans eventually visiting or even living there. When someone talks about colonizing Mars or a mission to Mars, they're discussing one of humanity's most ambitious goals: becoming a species that lives on more than one planet.