Mariners 6 and 7: More Mars secrets revealed
Sen—Following the success of Mariner 4, which returned the first photos showing a cratered Mars in 1965, NASA followed up with another twin mission four years later when the Red Planet and Earth were again due to come close.
The two probes Mariners 6 and 7 were launched a month apart, Mariner 6 lifting off from Cape Canaveral Air Force Base, Florida, on Feb. 24, 1969, and Mariner 7 from an adjacent pad on March 27. Each was lifted aloft by an Atlas-Centaur SLV-3C launch vehicle to begin an approximately five-month journey across space. In fact, although Mariner 7 was launched five weeks after its sister ship, it took a shorter route to catch up with Mars, and arrived only a week or so later.
There was a slight glitch with Mariner 7, a week before closest encounter, when mission control at the Jet Propulsion Laboratory, Pasadena, lost contact with the probe when its link failed with a tracking station in South Africa. There were fears that it might have been struck by a meteoroid, causing it to lose its lock onto the bright star Canopus that kept it oriented. Fortunately the spacecraft's sensors found Canopus again and the link was reestablished. The incident took it slightly off course for a while, resulting in Mariner 7 arriving at Mars ten seconds later than scheduled.
Up until now, space scientists were still working with a basic map of Mars that showed the elusive canals that a number of observers had been tricked into thinking they were seeing through their telescopes. But there was no trace of any of these on the 143 of images that the spacecraft took on their approaches, or on 59 taken when at their closest, at distances of around 3,430 km from the surface.
Diagrams showing the design of the Mariner probes and their instruments. Image credit: NASA
The 26 images taken by Mariner 6 at its closest on July 31, 1969, showed what NASA described as a chaotic and heavily cratered surface plus some of the dark features that had been seen from Earth. Resolution in the images was better than Mariner 4 had achieved, partly due to improved technology but also because the probes got closer to Mars. They showed that Mars' surface was less like the Moon than Mariner 4 might have suggested.
Their missions carried them over the Martian equator and the south polar regions, but both missed some of the most dramatic features that later orbiters would discover—the huge rift that forms a "Grand Canyon" and the giant volcanoes in the northern hemisphere. (A bright ring indicating the position of Olympus Mons, the largest volcano in the Solar System, can be seen in the images taken from afar.) The two probes also used their sensors to study the Martian atmosphere.
Planetary scientists used what they had learned from Mariner 6 to instruct its sister craft to take additional photos of the south pole. Mariner 7 also managed to picture Mars's larger moon Phobos, which may be a captured asteroid, in one of its 33 close-up images as it sped past on Aug. 5, 1969.
Mariners 6 and 7 each had a mass of 413 kg, and their imaging systems comprised wide and narrow-angle cameras with digital tape recorders. Other instruments included an infrared spectrometer and radiometer, and an ultraviolet spectrometer, to study Mars at different wavelengths. Their missions over, the two probes are now dead and orbiting the Sun.