In the forties and early fifties scientists varied widely in their guesses as to the probability of meteoroid impact. Fletcher G. Watson, a Harvard University astronomer, predicted in 1946 that at least one of every 25 space ships going to the Moon would be destroyed by collision with a meteoroid. Two years later George Grimminger, a mathematician with the Rand Corporation, estimated that a spacecraft with an exposed area of 1,000 square feet would be hit by a particle with a diameter of ½ millimeter only about once every 15 years. As late as 1951, however, Fred L. Whipple of Harvard, one of the principal American authorities on meteoroids, was rather pessimistic about the chances of avoiding meteoroid penetration and suggested thick shielding on the spacecraft to guard against structural damage.51
The early instrumented satellites sent up by the Soviet Union and the United States did much to dispel the fears of the space flight enthusiasts about meteoroids.  The American satellite Explorer I, launched in January 1958, recorded only seven hits by micrometeoroids - particles considerably less than a millimeter in diameter - during the first month of its orbital life. Apparently none of these pieces of matter penetrated the satellite's outer skin. Data from the much larger Russian Sputnik III, sent into orbit in May 1958, indicated that an orbiting spacecraft with a surface of 1,000 square meters (10,760 square feet) would be hit by a meteoroid weighing at least one gram only once every 14,000 hours. And Explorer VI, orbited by the United States in the late summer of 1959, encountered meteoroid dust particles only 28 times during the first two days it was in orbit.52 These data prompted a human-factors specialist for one of the major aerospace firms to conclude that for low orbital missions in a manned spacecraft "the danger from meteorite [sic] penetration is minor to negligible in comparison to the other hazards of such flights."53 Nevertheless, Project Mercury astronauts would wear a full-pressure suit, a closed ecological system in itself, so that if cabin decompression occurred each astronaut could live until his space capsule could be brought back to Earth.
50 Among astronomers, astrophysicists, and space flight experts there is considerable variation in the uses and definitions of the terms "meteoroid," "meteorite," and "meteor." Throughout this study the authors have employed what is apparently standard NASA terminology: a meteoroid is a solid object larger that a molecule and smaller than an asteroid, moving in interplanetary space; a meteorite is such an object that reaches Earth's surface without completely vaporizing in the atmosphere; and a meteor is the light phenomenon resulting from a meteoroid's entrance into the atmosphere.
51 Hanrahan and Bushnell, Space Biology, 31; George Grimminger, "Probability That a Meteorite Will Hit or Penetrate a Body Situated in the Vicinity of the Earth," Journal of Applied Physics, XIX (Oct. 1948), 947-956; Fred L. Whipple, "Meteoric Phenomena and Meteorites: The Conquest of Interplanetary Space," in White and Benson, eds., Physics and Medicine of the Upper Atmosphere, 137-170. On the meteoroid phenomenon see also Whipple, "Meteorite Material in Space," in Benson and Strughold, eds., Physics and Medicine of the Atmosphere and Space.
52 Hanrahan and Bushnell, Space Biology, 31.
53 A. B. Thompson, "Physiological and Psychological Considerations for Manned Space Flight," Report E9R-12349, Rev., Chance Vought Aircraft, Inc., Dallas, July 7, 1959, 115.