Viking I, a landing craft attached to a separate vehicle to orbit Mars, was launched August 20, 1975, and entered the Mars orbit June 19, 1976. The lander touched down the following day on the slope of a dry basin. Viking landers essentially were three-legged platforms with numerous instruments atop and within the platform structure. They were powered by nuclear sources, radioisotope thermoelectric generators. The first measurements made by landers came from a set of instructions designed to make profile measurements of the atmosphere during the descent to the surface. Among the Viking surface instruments, the landers carried the first experiments ever sent from Earth with the specific intent of seeking life on another planet.