The landing had been planned for July 4, the American bicentennial, but orbital images showed the chosen site was too rough, and controllers searched for weeks for a safer spot. Viking 1 sent back its first picture minutes after landing, showing its own footpad on a rocky surface. Its experiments tested soil for signs of life, with ambiguous results that most scientists concluded were chemical rather than biological. The lander operated for more than six years.
Viking 1 had entered orbit around Mars in June 1976, and its cameras soon showed the planned landing site was dangerous. The team, including Carl Sagan, spent weeks studying new images before choosing a spot in Chryse Planitia. The landing was set for July 20, the seventh anniversary of Apollo 11's Moon landing.
The lander descended by heat shield, parachute and rockets. Its signals took about 19 minutes to reach Earth, so the landing had to be completely automatic. Soon after touchdown it sent the first image from the surface of Mars, a view of its footpad on sand and pebbles. A panorama followed, showing a rock-strewn landscape. Colour images revealed a pinkish sky, caused by fine dust.
The lander's robotic arm scooped soil for three biology experiments. One, the labelled-release experiment, gave a positive result, but another instrument found no organic molecules, and most scientists attributed the results to reactive chemicals in the soil. Viking 1 worked until November 1982, when a faulty command cut its communications. Viking 2 landed in September 1976.
Key people: Gerald Soffen, Carl Sagan