Viking was NASA's first attempt to soft-land a space probe on another planet. This required a great technological advance on other probes. The orbiters had not only to carry the landers to Mars, but also check that the preselected landing sites were safe; position and release the landers to the surface, and then relay their signals back to Earth. The highly successful Mariner design - octagonal, with four protruding solar panels - was adapted to meet these new demands.
