The human race is slowing down.

When the U.S. space shuttle completes its final flight, planned for June, mankind will take another step back from its top speed. Space shuttles are the fastest reusable manned vehicles ever built. Their maximum was only exceeded by single-shot moon rockets.

The shuttles' retirement follows the grounding over recent years of other ultrafast people carriers, including the supersonic Concorde and the speedier SR-71 Blackbird spy plane. With nothing ready to replace them, our species is decelerating—perhaps for the first time in history.

It has been a good two-century sprint, says Neil Armstrong, who in 1969 covered almost 240,000 miles in less than four days to plant the first human footprint on the Moon. Through the 18th century, he noted in an email exchange, humans could travel by foot or horse at approximately six miles per hour. "In the 19th, with trains, they reached 60 mph. In the 20th, with jet aircraft, we could travel at 600 mph. Can we expect 6,000 mph in the 21st?" he wondered.

"It does not seem likely," Mr. Armstrong continued, although he holds out some hope.