The NASA\u27s next missions to Mars are to land two unmanned spacecraft on the surface in 1976 to perform scientific experiments. This pair of missions called Viking consists of two identical spacecraft each launched from a Titan/Centaur in the summer of 1975- Each spacecraft consists of a Lander and an Orbiter combination and together carry 13 scientific investigations. The Orbiter carries its Lander to the planet and then acts as a relay to return the telemetered data to Earth. Following the launches in the summer of 1975\u3e there is an 11-month cruise to the planet. The spacecraft are injected into highly elliptical synchronous orbits about the planet. The periapsis of about 1500 km is selected to be over the appropriate preselected landing site. The missions are separated by about 2 months so that the missions can be handled consecutively. For each spacecraft, after the landing site is certified and the orbit is trimmed, the Lander is released. The nominal landing target is an ellipse about 100 by 500 km. The Lander enters the Martian atmosphere at about 15° traveling about 5 km/sec. The initial deceleration is from the aerodynamic drag of its 12-foot aeroshell. At about 5 km above the surface, a parachute is deployed as the second braking system. At 2 km as sensed by an onboard radar, three retrorockets are fired and the Lander is soft landed onto the surface (see Fig. 1).
The scientific experiments aboard the Lander require several months of operation; the nominal length of the mission is 60 days. Power for this period on the Lander is obtained from two 35-watt radioisotope thermoelectric generators. The data from the experiments are telemetered by relay up to the Orbiter and back to Earth or may be telemetered directly from the.Lander to the Earth at a lower rate. Commands to both the Lander and Orbiter will be sent over the deep space net. A tape recorder aboard the Lander will allow storage of data for subsequent transmission, thereby permitting events to be recorded independent of the positions of the Orbiter or Earth
