Mariner 9 Became the First Craft to Orbit Another Planet on Nov. 13, 1971

What happened when it arrived?

Mars greeted the spacecraft with a global dust storm. The entire planet was shrouded, which threatened to blank every camera on board. But engineers had built Mariner 9 for longevity, and that design choice paid off. Instead of rushing its observations, the orbiter simply waited out the storm, then started its science campaign once the air cleared.

The Soviet Union's Mars 2 and Mars 3 spacecraft, which arrived around the same period, did not fare as well against the same conditions. The contrast made Mariner 9's patience look even better in hindsight. Waiting, in this case, won.

What did it actually find?

Over roughly a year of operations, Mariner 9 returned 7,329 photographs — well past its target of about 3,000 — and mapped 85% of the martian surface. The numbers alone were impressive. The pictures rewrote what researchers thought Mars was.

Before Mariner 9, many scientists pictured Mars as a static, moon-like ball of rock. The images said otherwise. The orbiter revealed a dynamic and active world, one that had clearly changed over time. Among its discoveries:

• shield volcanoes, the broad, gently sloping kind built by flowing lava
• dry river valleys, hinting that liquid water once moved across the surface
• ice at the martian poles

The probe also delivered close-up views of Phobos and Deimos, Mars's two small moons. Those images gave researchers their best look yet at the surfaces of both bodies.

Put simply: Mariner 9 turned Mars from a dot in a telescope into a place with a geological history. Dry riverbeds and polar ice raised questions about the planet's past climate that researchers still chase today. The mission's photo haul — 7,329 frames and 85% of the surface — set the baseline every later Mars orbiter built on.

One probe, launched as a backup for a failed twin, ended up defining how humanity first saw another planet. That's the legacy of Nov. 13, 1971.