On July 20, 1969, astronauts Neil Armstrong and Buzz Aldrin walked on the Moon. Four days later, the astronauts were quarantined aboard the USS Hornet for a 21-day isolation period. This was to ensure that no potentially hazardous lunar microbes had hitchhiked back to Earth with them. The NASA scientists found no microbial aliens on the astronauts themselves or in the 50 pounds of lunar rocks they brought back.