Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Dozens of crew-less spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and India to study the planet's surface, climate, and geology.

Mars Day! is an annual National Air and Space Museum event that celebrates the Red Planet with a variety of educational and fun family activities. Visitors can also talk to scientists active in Mars research and learn about current and future missions.