Sending robots to other worlds may almost seem routine today with two rovers on Mars and an orbiter-lander pair at Saturn, but the evolution has been a long process. It was three decades ago when NASA sent the first robots to conduct long-term investigations on another planet.

NASA's Viking program launched four giant robots to visit Mars in 1975, an orbiter-lander combo for each two missions: Viking 1 and Viking 2.

Standing seven feet (two meters) tall and 10 feet (three meters) wide, the Viking landers were the size of a car and weighed a whopping 1,270 pounds (576 kilograms) each - the current Mars rovers Spirit and Opportunity weigh about 400 pounds (180 kilograms) a piece. The Viking robots used retrorockets, thrusters and parachutes to land on the Martian surface. Viking 1 landed at Chryse Planitia on July 20, 1976. Viking 2 set down Sept. 3 on the plains of Utopia Planitia.

In this image, taken by the Viking 2 lander, the robot is looking back across itself to the southwest. The rocks in this picture are approximately 1.5 feet (half a meter) wide. The lander touched down about 124 miles (200 kilometers) south of the Mars crater Mie, leading scientists to theorize that the Viking robot landed on one of the crater's ejecta blankets.

The landers of both Viking missions worked for years, far beyond their 90-day minimum mission timelines, each powered by two radioisotope thermoelectric generators. The Viking 2 lander finally fell silent on April 10, 1980, while the Viking 1 robot lived until November 11, 1982.

The Viking missions were only the first robots to spend years at a time collecting and relaying data from another planet, not the first to ever land on one. A Russian capsule from the Mars 5 mission soft landed on the red planet in 1973 but did not return ground data. Russia also successfully landed a series of Venera and Vega robots on Venus throughout the 1970s and 1980s.

-- Tariq Malik

CREDIT: NASA