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This is an artist's conception of the sequence of events that will take place just prior to landing a life-detection laboratory on the surface of Mars on July 4, 1976. Above right, the Viking spacecraft, composed of an orbiter and a lander, has been in orbit around the Red Planet since June 19, 1976, taking pictures of the planned landing site to ascertain its safety before releasing the lander (top, left) for its threeto five-hour descent. Protected by aeroshells, the heat-sterilized lander hurtles into the thin Martian atmosphere at a speed of about 10,000 mph, to be slowed first by aerodynamic drag until the shell is discarded, then by parachute (center) and finally by retrorockets to assure a gentle landing. Instruments will study the structure and composition of the Martian atmosphere as the lander drifts down. Viking 2 is scheduled to arrive at Mars on Aug. 7 and touch down on the surface on Sept. 4.