Because it takes 20 minutes for a signal from Earth to reach Mars, the separation of the lander from the orbiter and the entire sequence of soft landing the spacecraft were done without ground control.
After release from the Viking orbiter, the lander used a variety of methods to slow down for a soft landing on the surface of Mars. Just before entry into the atmosphere, the spacecraft was oriented so that the heat shield would absorb heat as it decelerated due to atmospheric drag from entry. At 6 kilometers (4 miles) above the surface, a parachute was deployed, and the aeroshell was then jettisoned. Finally, at 1.5 kilometers (5,000 feet), three radar-controlled retro-engines were fired to keep the lander upright and further slow its descent. The landings occurred at a speed of 2 meters per second (4.5 miles per hour). Scientific information on the Martian atmosphere was gathered even before the landers touched the surface. Starting at an altitude of 200 kilometers (124 miles), the instruments measured the composition and layering of the atmosphere. These experiments confirmed that carbon dioxide is presently the major component of the Martian atmosphere and that nitrogen may have been more abundant in the past.
Source: The Viking Mission - NASM
Wednesday, 31-Dec-1969 18:00:00 CST
CSR/TSGC Team Web