Radio echoes from a manned satellite during re-entry

Abstract

The ionized trails produced during re-entry by the MA-6 Mercury capsule carrying Lt. Col. John Glenn in his first orbital flight on February 20, 1962, were observed from San Salvador Island (Bahama Island Group), approximately 370 nautical miles uprange from the landing point. The apparatus employed was an omnidirectional pulsed radar (of 30.25 Mc/s carrier frequency) similar to those employed by McKinley and Millman in the study of meteor echoes. When the Mercury capsule was passing over the vicinity of the observation station at about 14h 33m EST, five clearly separated ionized trails were observed. The most prominent trail, which may be identified with the wake of the main capsule, was visible to the radar for a total duration of about 20 sec and displayed an equivalent isotropic scattering cross section of about 106 square meters at its peak. The other four trails, which were probably due to fragments of the disintegrating retro-rocket package reported by Glenn, showed only short duration glints (of the order of one second) characteristic of the smooth, rapidly decaying trails of small meteors. Though some of the fine structure of the long duration, scintillating echo from the main capsule trail is not yet understood, the over-all characteristics of the trail can be accounted for by a combination of high light from a strongly reflecting front segment of the ionized wake, which appeared electromagnetically smooth to the 10-meter wavelength employed, and diffused scattering from a turbulent rear segment of the wake, which appeared rough to the wavelength. The fact that the main capsule trail appeared turbulent indicates a Reynolds number of less than 105 (based on the velocity and diameter of the MA-6 capsule, and on the ambient air density) for transition from laminar to turbulent flow in the wake. The appearance of a leading edge echo about two miles ahead of the strongly scattering turbulent wake also suggested the existence of a ‘photoionization halo’ around the capsule due to ultraviolet radiation from the bow shock wave. From these results, some conclusions concerning the origin of instantaneous meteoric head echo may be inferred.