biological specimens. A Thiokol retrorocket was mounted at the end of the afterbody to decelerate the capsule out of orbit. An 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. Discoverer 18 was launched during a massive solar flare which lasted for the first 13 hours of the 48-orbit, 3-day flight. After the 48th orbit, the recovery capsule was ejected, reentered the atmosphere, and was retrieved in mid-air near Hawaii by a C-119 aircraft at altitude. The Agena B stage remained in orbit until 2 April 1961, when it reentered the atmosphere and burned up. Scientific results In addition to its reconnaissance payload, Discoverer 18 carried a biological research payload, intended to investigate human tissues in space. Since at the time the United States did not publicly acknowledge its reconnaissance satellite programs, this was officially the satellite's primary mission. 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. The onboard film pack "Nuclear Emulsion" experiment comprised two groups of films coated with nuclear emulsions, arranged horizontally and vertically, were used to measure the intensity and direction of cosmic radiation and to distinguish among electrons, protons, and heavier charged particles. These emulsions were sensitive to neutrons, x-rays, and gamma rays. Nuclear track plates were also included in the experiment package. Neutron density was measured by changes induced in a metallic bismuth detector. The experiment was contained in the recovery capsule, which was retrieved in mid-air after 48 orbits. Discoverer 18 also carried a biological package including Neurospora conidia, nerve tissue, algae, human bone marrow, eyelid tissue, gamma globulin, and cancer cells. Results from the flight suggested that biological specimens might withstand radiation from solar flares with a minimum of shielding and that aluminum shielding seemed to offer more protection than lead. References Spacecraft launched in 1960 Spacecraft which reentered in 1961