The motive power of Mariner itself was limited to a trajectory correction rocket engine and an ability, by means of gas jets, to keep its two critical faces pointing at the Sun and the Earth. Therefore, the spacecraft had to be boosted out of the Earth’s gravitational field and injected into a flight path accurate enough to allow the trajectory correction system to alter the course to deliver the spacecraft close enough to Venus to be within operating range of the scientific instruments. The combined Atlas-Agena B booster system which was selected to do the job had a total thrust of about 376,000 pounds. With this power, Atlas-Agena could put 5,000 pounds of payload into a 345-mile orbit, propel 750 pounds on a lunar trajectory, or launch approximately 400 pounds on a planetary mission. This last capability would be taxed to the limit by the 447 pounds of the Mariner spacecraft. THE ATLAS BOOSTER: POWER OF SIX 707’S The 360,000 pounds of thrust developed by the Atlas D missile is equivalent to the thrust generated by the engines of six Boeing 707 jet airplanes. All of this awesome power requires a gargantuan amount of fuel: in less than 20 seconds, Atlas consumes more than a propeller-driven, four-engine airplane burns in flying coast-to-coast nonstop. [Illustration: _Photo courtesy of General Dynamics/Astronautics_ _This military version of the Atlas missile is modified for NASA space flights._] The Atlas missile, as developed by Convair for the Air Force, has a range of 6,300 miles and reaches a top speed of 16,000 miles per hour. The missile has been somewhat modified for use by NASA as a space booster vehicle. Its mission was to lift the second-stage Agena B and the Mariner spacecraft into the proper position and altitude at the right speed so that the Agena could go into Earth orbit, preliminary to the takeoff for interplanetary space. The Atlas D has two main sections: a body or sustainer section, and a jettisonable aft, or booster engine section. The vehicle measures about 100 feet in length (with military nose cone) and has a diameter of 10 feet at the base. The weight is approximately 275,000 pounds. No aerodynamic control surfaces such as fins or rudders are used. The Atlas is stabilized and controlled by “gimbaling” or swiveling the engine thrust chambers by means of a hydraulic system. The direction of thrust can thus be altered to control the movements of the missile. The aft section mounts two 154,500-pound-thrust booster engines and the entire section is jettisoned or separated from the sustainer section after the booster engines burn out. The 60,000-pound-thrust sustainer engine is attached at the center line of the sustainer section. Two 1,000-pound-thrust vernier (fine steering) engines are installed on opposite sides of the tank section in the yaw or side-turn plane.