On Nov. 13, 1971, Mariner 9 became the first spacecraft to orbit Mars. At the time, scientists didn't know much about the Red Planet.

NASA had several Mars flybys before Mariner 9 . Those flybys showed that Mars had a cratered surface , so some scientists thought that Mars looked just like the moon. Then Mariner 9 blew blew their minds.

When Mariner 9 got to Mars , it arrived in the middle of a dust storm. It saw some tall structures sticking through the clouds. When the dust settled, images from Mariner 9 revealed that these structures were the tops of huge volcanoes.

One volcano, Olympus Mons , is the tallest in the solar system. Mariner 9 also spotted an enormous canyon called Valles Marineris . This canyon is five times longer than the Grand Canyon in Arizona.

Mariner 9 worked for one year in orbit. When it ran out of gas, NASA shut it off.…