Discoverer 18 was a satellite placed in a near-polar orbit to test spacecraft engineering techniques, to continue evaluation of the Agena B vehicle, and to attempt separation, deceleration, reentry through the atmosphere, and recovery from the air of an instrument package. The satellite's scientific experiment package of radiation dosimeters, infrared radiometers, and microwave band detectors was identical to that of Discoverer 17 (launched November 12, 1960), with the addition of photographic film packs sensitive to neutrons, x-rays and gamma rays, and nuclear track plates. The spacecraft carried external lights (approximately a sixth to seventh magnitude star) for optical tracking from ground camera stations. The cylindrical Agena B stage carried a telemetry system, tape recorder, receivers for command signals from the ground, a horizon scanner, and a 300-lb recovery capsule. The capsule was a bowl-shaped configuration 33 in. in diameter and 27 in. deep. A conical afterbody increased the total length to about 40 in. The recovery capsule payload included the photographic film packs, nuclear track plates, and biological specimens. A Thiokol retrorocket was mounted at the end of the afterbody to decelerate the capsule out of orbit. A 40-lb monitoring system was included in the capsule to report on selected events, such as firing of the retrorocket, jettisoning of the heat shield, and others. The recovery capsule was ejected, reentered the atmosphere, and was retrieved in mid-air near Hawaii by a C-119 aircraft at 14,000 ft altitude after 48 orbits. The Agena B stage remained in orbit until April 2, 1961, when it reentered the atmosphere and burned up. Useful scientific data were obtained from the satellite.