The spacecraft was allowed a pointing error of 1 degree in order to conserve gas. The system kept the spacecraft swinging through this 1 degree of arc approximately once each 60 minutes. As it neared the limit on either side, the jets fired for approximately ¹/₅₀ of a second to start the swing slowly in the other direction. Thus, Mariner rocked leisurely back and forth throughout its 4-month trip. Sensitive photomultiplier tubes or electric eyes in the Earth sensor, mounted on the directional antenna, activated the gas jets to roll the spacecraft about the already fixed long axis in order to face the antenna toward the Earth. When the Earth was “acquired,” the antenna would then necessarily be oriented in the proper direction. If telemetry revealed that Mariner had accidentally fixed on the Moon, over-ride radio commands from the Earth could restart the orientation sequence. PROPULSION SYSTEM The Mariner propulsion system for midcourse trajectory correction employed a rocket engine that weighed 37 pounds with fuel and a nitrogen pressure system, and developed 50 pounds of thrust for a maximum of 57 seconds. The system was suspended within the central portion of the basic hexagonal structure of the spacecraft. This retro-rocket engine used a type of liquid propellant known as anhydrous hydrazine and it was so delicately controlled that it could burn for as little as ²/₁₀ of a second and increase the velocity of the spacecraft from as little as ⁷/₁₀ of a foot per second to as much as 200 feet per second. The hydrazine fuel was stored in a rubber bladder inside a doorknob-shaped container. At the ignition command, nitrogen gas under 3,000-pound-per-square-inch pressure was forced into the propellant tank through explosively activated valves. The nitrogen then squeezed the rubber bladder, forcing the hydrazine into the combustion chamber. [Illustration: _The midcourse propulsion system provides trajectory correction for close approach to Venus._] FUEL TANK NITROGEN TANK JET VANE ACTUATOR (ONE OF FOUR) THRUST CHAMBER Hydrazine, a monopropellant, requires a starting ignition for proper combustion. In the Mariner system, nitrogen tetroxide starting or “kindling” fluid was injected into the propellant tank by a pressurized cartridge. Aluminum oxide pellets in the tank acted as catalysts to control the speed of combustion of the hydrazine. The burning of the hydrazine was stopped when the flow of nitrogen gas was halted, also by explosively activated valves. TEMPERATURE CONTROL Mariner’s 129 days in space presented some unique problems in temperature control. Engineers were faced with the necessity of achieving some form of thermal balance so that Mariner would become neither too hot nor too cold in the hostile environment of space.