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. But the spacecraft remains one of the most famous Mars voyagers of all time.
Here's why it mattered
Mariner 9 cemented its place in space history when it entered into orbit around Mars, the first human-built spacecraft ever to do so.
The NASA spacecraft actually beat the Soviet Union's own Mars 2 spacecraft to the Red Planet, even though Mars 2 "had an 11-day head start," NASA said in a mission description. (Mars 2 would go on to be the first human-built craft to impact the surface of Mars when it crashed into the Red Planet.)
But it was the science from Mariner 9 that transformed Mars from a mysterious Red Planet into a full world for humans to explore.
Mariner 9 carried four primary instruments to study Mars, including a camera system for imaging, an ultraviolet spectrometer, an infrared spectrometer and an infrared radiometer. The probe mapped 85% of Mars with those tools during its nearly one-year mission.
The Mariner 9 spacecraft discovered 20 giant volcanoes on Mars (including Olympus Mars) and made key observations to map Valles Marineris, which runs 2,500 miles (4,000 kilometers) long acros the Tharsis region and is 120 miles (200 km) wide.
"The canyon was named in honor of Mariner 9," NASA said in an overview.
In all, Mariner 9 took 7,329 photos of the surface of Mars, mapping the planet down to a resolution of between 1/2-1 mile (1-2 km). It also snapped at least 80 photos of Phobos and Deimos, the two small moons of Mars.
By February 1972, Mariner 9's supply of gaseous nitrogen, which it used to feed its attitide control thrusters, had run dry. NASA finally shut down the mission in October 1972.
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