Exploratour - The Atmosphere of Mars
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The Viking Mission on Mars
The Viking mission discovered just how large the global dust storms could become. The Viking I and Viking 2 missions were designed to both orbit Mars and land on the planet's surface.
Viking was launched in 1975. Immediately prior to entering orbit around Mars in 1976, Mars became completely covered by a Global Dust Storm, and the surface became invisible for several months. It was the first time that scientists realized the power of Martian dust storms. Eventually, pictures were returned by the Viking orbiter, but scientists sweated about whether they were ever going to get pictures from Mars.