On November 29, 19.61, the Mercury-Atlas 5 launch at Cape Canaveral carried chimpanzee Enos into orbit for a scheduled three-orbit mission. Because he attitude-control system malfunctioned, retrororockets were fired on the second orbit. The Mercury spacecraft was recovered 1 hour 25 minutes after the water landing, and Enos was recovered in seemingly excellent condition except that the extreme heat had obviously plagued him.³

During the postflight medical examination of Enos, there was to be considerable concern over the variations in cardiac rhythm which had been recorded by the instruments developed for this flight. The critical question posed was whether plans should proceed for the first manned orbital flight. Yet on the date of Enos’ flight. President Kennedy had announced that Lt. Col. John Glenn would be the prime astronaut for the first manned orbital mission to take place shortly, with Lt. M. Scott Carpenter as backup. Capt. Donald Slayton would be the prime astronaut for the second manned orbital mission with Lt. Comdr. Walter Shirra as backup.⁴

During the following days, however, it appeared that this announcement might have been premature in the light of medical findings of the Enos flight. The tension of those days has been described unofficially to the author. It must have been a period of uncertainty as to the proper course of action to take, for the first manned orbital flight was scheduled for December 1961.

Fortunately this potential medical crisis did not become full blown. Eminent cardiologists were asked to review the records and biological data obtained during the orbital flight of Enos, to determine the reason for the arrhythmia, if possible, and to separate it from the influences exerted by weightlessness in space flight. It was found that the difficulty lay with the instrumentation, and that the data were therefore invalid. Accordingly, it was recommended that the manned orbital flight proceed as scheduled.⁵

3.  L. E. Stringly, "Countdown and Procedures for Project Mercury. Atlas-5 Flight (Chimpanzee Subject); Final Technical Documentary Report." Aeromedical Res. Lab. Tech. Documentary Rep. 62-17, Holloman AFB, 1962.

4.  Aeronautical and Astronautical Events of 1961, Report of the National Aeronautics and Space Administration to the Committee on Science and Astronautics, U.S. House of Representatives, 87th Congress, 2d sess., June 7, 1962, p. 68.

5.  Based on classified documents and off-the-record discussions by the author.