[2] Discoverer 18 was the third of the KH-2 Corona spy satellites, which was distinguished from the predecessor KH-1 series in its incorporation of the improved C' camera, which replaced the C model carried on KH-1 missions.

The satellite had a mass of 1,240 kilograms (2,730 lb),[1] and was equipped with a panoramic camera with a focal length of 61 centimetres (24 in), which had a maximum resolution of 7.6 metres (25 ft).

[5] Discoverer 18 was placed in a near-polar orbit to test spacecraft engineering techniques, to continue the evaluation of the Agena B vehicle, and to attempt separation, deceleration, reentry through the atmosphere, and recovery from the air of an instrument package.

[1] The cylindrical Agena B stage carried a telemetry system, tape recorder, receivers for command signals from the ground, a horizon scanner, and a 136 kilograms (300 lb) recovery capsule.

An 18 kilograms (40 lb) monitoring system was included in the capsule to report on selected events, such as the firing of the retrorocket, jettisoning of the heat shield, and others.

[8] The satellite's scientific experiment package of radiation dosimeters, infrared radiometers, and microwave band detectors was identical to that of Discoverer 17, 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.