The maximum speed at which a human has traveled in space is approximately 24,600 miles per hour or 39,600 kilometers per hour. This velocity was achieved while travelling back from the Moon when NASA's Apollo 10 command module attained escape velocity to re-enter earth's atmosphere. This extreme speed was due to the gravitational acceleration the capsule underwent as it accelerated down out of Moon's gravitational influence before falling within Earth's gravity well in free-fall for re-entry to earth's surface.