The crew of Apollo X spacecraft achieved the maximum speed ever attained by humans during their 1969 journey. On May 26, astronauts Thomas Stafford, John Young, and Eugene Cernan reached a top speed of 24,791 mph (39,897 km/h) or 6.88 miles per second.

According to Einstein’s theory of relativity, time slows down for someone traveling at a high rate of speed relative to an observer. If we take the speed of the Apollo X spacecraft and the length of the journey, and plug that data into Einstein’s formula, we can calculate that the three astronauts actually did age more slowly than people on earth. Upon their return, they were about one-third of a second younger than they would have been if they had not gone on the journey. That author of this publication did meet Gene Cernan in 1995, and he appeared very young for his age.