The first of the Agena-Discoverer series was launched into orbit on February 28, 1959, with the Thor missile as the booster. The first restart in orbit occurred on February 18, 1961, when the new Agena B configuration was used to put Discoverer XXI into orbit. All of the NASA missions using Agena, beginning with Ranger I in August, 1961, have been flown with the B model. Agena holds several orbiting records for U. S. vehicles. The first water recovery followed the 17 orbits of Discoverer XIII on August 11, 1960. The first air recovery of a capsule from orbit occurred with Discoverer XIV on August 18, 1960. In all, a total of 11 capsules were recovered from orbit, 7 in the air, 4 from the sea. CHAPTER 3 THE SPACECRAFT In the 11 brief months which JPL had to produce the Mariner spacecraft system, there was no possibility of designing an entirely new spacecraft. JPL’s solution to the problem was derived largely from the Laboratory’s earlier space exploration vehicles, such as the Vega, the Ranger lunar series, and the cancelled Mariner A. Wherever possible, components and subsystems designed for these projects were either utilized or redesigned. Where equipment was purchased from industrial contractors, existing hardware was adapted, if practicable. Only a minimum of testing could be performed on newly designed equipment and lengthy evaluation of “breadboard” mock-ups was out of the question. Ready for launch, the spacecraft measured 5 feet in diameter and 9 feet 11 inches in height. With the solar panels and the directional antenna unfolded in the cruise position, Mariner was 16 feet 6 inches wide and 11 feet 11 inches high. THE SPACEFRAME The design engineers were forced to work within the framework of the earlier spacecraft technology because of the time restrictions, but Mariner I and II could weigh only about half as much as the Ranger spacecraft and just over one-third as much as the planned Mariner A. [Illustration: _Mariner spacecraft with solar panels, microwave radiometer, and directional antenna extended in flight position. Principal components are shown._] ROLL AXIS OMNIANTENNA MAGNETOMETER SENSOR PARTICLE FLUX DETECTORS (GEIGER TUBES) RADIOMETER REFERENCE HORNS MICROWAVE RADIOMETER INFRARED RADIOMETER ION-CHAMBER COSMIC DUST DETECTOR EARTH SENSOR SOLAR PANEL COMMAND ANTENNA SOLAR SAIL ATTITUDE CONTROL GAS BOTTLES SOLAR PLASMA DETECTOR DIRECTIONAL ANTENNA The basic structural unit of Mariner was a hexagonal frame made of magnesium and aluminum, to which was attached an aluminum superstructure, a liquid-propelled rocket engine for midcourse trajectory correction, six rectangular chassis mounted one on each face of the hexagonal structure, a high-gain directional antenna, the Sun sensors, and gas jets for control of the spacecraft’s attitude.