Mariner 9: First detailed survey of Mars
Sen—With appetites truly whetted by the images of Mars sent back by Mariner 4 in 1965, and Mariners 6 and 7 in 1969, NASA scientists were keen to learn more about our neighbouring outer world. They planned to build on what they had learned from the previous brief flybys by putting twin space probes in orbit around the Red Planet.
That aim was lost as soon as the first probe, Mariner 8, was launched by an Atlas-Centaur SLV-3C booster on May 9, 1971 from Cape Canaveral, Florida. The Centaur's upper stage failed, sending it and the probe plunging out of control into the Atlantic Ocean.
All rested on the success of sister ship Mariner 9 which launched three weeks later on May 30, 1971, atop a similar rocket. This time all went well, and the probe was sent on course for a 398 million km direct flight to Mars, deploying its four solar panels and locking onto the star Canopus to keep itself oriented correctly. With a planned mid-course manoeuvre completed on June 5, the probe arrived at Mars on Nov. 14, 1971, where an engine burn lasting 15 minutes and 23 seconds put it into orbit. Mariner 9 thus became the first man-made spacecraft to circle another planet, making one orbit every 12 hours.
A cutaway diagram showing the design of Mariner 9. Image credit: NASA
All technical operations were therefore a huge success, but planetary scientists had to be patient before they could get their first close views of the Martian surface from Mariner 9. As luck would have it, a dust storm had begun to blow up in September 1971 and by the time the probe arrived it had become one of the largest such storms ever observed, engulfing the whole planet. Only the summits of Olympus Mons, which is the largest volcano in the Solar System, plus Mars's three Tharsis volcanoes could be observed above the dust cloud.
Mission controllers ordered their spacecraft to wait patiently while the storm blew itself out. Fortunately it began to subside during November and four weeks after its arrival, Mariner 9 was able to begin its main mission goal of taking high-quality images of the planet's surface.
The probe carried several instruments fitted to a scan platform at the base of its octagonal frame. These were wide- and narrow-angle TV cameras, and infrared radiometer, an ultraviolet spectrometer, and an infrared interferometer spectrometer.
This close-up view of the caldera at the peak of Mars's Olympus Mons volcano was remarkable for its time. Image credit: NASA/JPL
Since, by chance, the previous probes had photographed areas that were not representative of Mars as a whole, the images that Mariner 9 began returning came as a surprise to the scientists. Rather than simply being cratered like the Moon, Mars had not only those volcanoes but canyons and what resembled ancient, dried-up riverbeds cut into its arid landscape. The largest canyon, which today we know as Valles Marineris after the probe that discovered it, stretches an amazing 4,800 km across the planet, and is up to 600 km wide and 8 km deep.
Mariner 9 was a huge success for NASA, managing to map the entire surface of Mars in 7,329 images during nearly a year of operations, plus the first close-up photos of its two potato-shaped moons, Phobos and Deimos. The probe completed its final transmission of data back to Earth on Oct. 27, 1972. Though now dead, it continues to orbit Mars and is not expected to enter its atmosphere and crash until the early 2020s.
A view of an ancient channel called Nirgal Vallis, taken by Mariner 9. Image credit: NASA/JPL-Caltech
The Soviet Union also targeted Mars during the 1971 window when the planet was closest to Earth, but had less success. Kosmos 419 was launched on May 10, two days after Mariner 8, but failed to get beyond Earth orbit and re-entered the atmosphere a couple of days later.
Two other Soviet missions, Mars 2 and Mars 3, reached the Red Planet in November and December 1971. Each combined an orbiter and a lander. The orbiters sent back 60 photos between them as well as data about the atmosphere and temperatures within the Martian soil. But the Mars 2 lander crashed on Nov. 27 after its parachute failed, becoming the first object from Earth to reach the planet's surface.
Mars 3 did make a successful soft-landing, on Dec. 2, and began to transmit an image in the dust storm. However, transmission ceased abruptly after just 20 seconds and the probe was never heard from again. The sudden failure of Mars 3 is still a mystery, but in April 2013, NASA revealed that the HiRISE camera on its own Mars Reconnaissance Orbiter (MRO) may have photographed the lander, its parachute and heat shield.