Mariner 9 was the first spacecraft to orbit another planet. It arrived at Mars in 1971 and changed our perceptions of the Red Planet.
Mariner 9's cameras snapped close-up views of the planet's major features, including Mars' polar caps, the vast Valles Marineris canyon, and the Martian moons (Phobos and Deimos). It also spotted evidence of flowing water sometime in the ancient past.
The spacecraft ran out of attitude control gas in 1972, after nearly a year in orbit, but its pictures provided NASA with enough data to send the Viking 1 and 2 spacecraft out to Mars four years later.
An extended operation
NASA had visited Mars several times before, but those missions were just flybys. In the very early days of planetary exploration, the agency's strategy was to launch pairs of spacecraft to Mars so that if one of the spacecraft failed, the other could pick up the torch.
Mariner 4 was the first spacecraft to zip by the Red Planet in 1965, taking the first pictures of a planet from up close. Its twin, Mariner 3, failed during launch.
Four years later, Mariners 6 and 7 paired up to fly by Mars within a few days of each other. Mariner 7 even nabbed a picture of Phobos, one of the Martian moons.
The missions were successes, but to reach a better understanding of the planet, long-term observations would be necessary. NASA could then watch the changing seasons, and take the time to do detailed measurements of the atmosphere, magnetic fieldand surface features.
NASA added more weight and instruments to Mariners 8 and 9, which were supposed to orbit the planet. They would need more fuel, and also require a more robust propulsion system for extended operations. Each spacecraft weighed more than Mariners 6 and 7 together.
Mariner 8 lifted off on May 9, 1971, from Cape Canaveral, Florida, on a mission that lasted just six minutes. Due to a problem with the upper stage's main engine, the spacecraft arced over the Atlantic Ocean and crashed in the water about 350 miles north of Puerto Rico.
Managers then sent the Mariner 9 spacecraft aloft on an Atlas-Centaur rocket on May 30, 1971. This time, Mariner 9 made it into space and set a course for the Red Planet.
After 167 days flying in space, Mariner 9 reached Mars on Nov. 14, 1971. It fired its rocket for just over 15 minutes to put itself into orbit – a major accomplishment for NASA after the previous flyby missions.
In space, though, things rarely go as planned. Mariner 9 was ready to take pictures, but the planet was apparently not willing to show its true face yet. A dust storm had kicked up in September, and by the time Mariner 9 arrived, the planet was engulfed.
This in itself was a discovery – astronomers had suspected these dust storms existed, but it was the first time we saw them up close.
The dust began to clear in the coming weeks, settling down by January 1972. When the curtain lifted, it revealed a planet full of change and perhaps, also with a colorful past.
"Some of the observed features included ancient river beds, craters, massive extinct volcanoes, canyons, layered polar deposits, evidence of wind-driven deposition and erosion of sediments, weather fronts, ice clouds, localized dust storms, morning fogs and more," NASA wrote in a summary of the mission.
Along with finding evidence of flowing water in the past, "the question of the existence of life on Mars was intensified," NASA wrote of Mariner 9.
"It was clear that Mars had brought about many more questions which a lander would be best suited to answer."
The spacecraft snapped 7,329 pictures and managed to image 80 percent of the Martian surface in less than a year.
A vast canyon and other discoveries
Mariner 9's most prominent discovery was a vast canyon stretching 2,500 miles (4,000 kilometers) across the planet – nearly 10 times as long as the Grand Canyon – and reaching as deep as four miles (seven kilometers). The massive rift stretched a quarter of the way around Mars.
The feature was later named Valles Marineris, after its spacecraft discoverer. In the decades since, scientists have debated the canyon's origins.
At least one group of researchers believe Valles Marineris points to evidence of plate tectonics on Mars. A separate study looking at a nearby canyon suggested salts in the surface layers of Martian regolith collapsed when heated, opening a rift in the surface.
Mariner 9 also beamed back pictures of the Martian south pole, whose layers hinted that the Red Planet could have had different environments in the past.
NASA has kept a close eye on the pole in other missions. Mars Global Surveyor found evidence that carbon dioxide deposits on the pole were shrinking. Mars Odyssey spotted vast tracts of water ice, and Mars Reconnaissance Orbiter saw "dry ice" snowflakes falling from clouds near the pole.
Additionally, Mariner 9 shot pictures of apparent river beds winding their way across the surface, and got relatively close-up views of the Mars satellites Phobos and Deimos.
Mariner 9 showed researchers that Mars is a rapidly changing place, and helped point the way for other missions in terms of picking interesting science targets.
Legacy of Mariner 9
After almost a year in orbit, Mariner 9 simply ran out of attitude control gas. NASA turned the spacecraft off on Oct. 27, 1972. To this day it is presumably drifting, cold and silent, in circles around Mars.
NASA expects Mariner 9 is orbiting high enough to keep flying around the planet until at least 2022, but at some point the spacecraft will enter the atmosphere and crash into the surface.
Donna Shirley – who worked on several NASA Mars efforts and is best known for managing its Mars exploration program during the Pathfinder and Sojourner missions – has said that Mariner 9's orbiting mission was essential before sending a lander.
"If we had launched something to land in 1971, it would really have been silly. I mean, we didn't know much about the atmosphere," she said in a 2001 NASA oral history interview.
"The Martian atmosphere varies a lot between summer and winter, much more than the Earth's atmosphere because it's thin and it gets puffed up very easily when it gets warmer and then collapses down in the winter. So there were just enough uncertainties that doing an orbiter first was definitely the intelligent thing to do."
NASA program managers used pictures from Mariner 9 in planning the Viking 1 and 2 landers, which both touched down successfully on Mars in 1976.
— Elizabeth Howell, SPACE.com Contributor