Outer space is one of those things you know when you see it. The sky is dark and quiet, with not enough air to sustain life. But just where space begins is hard to say – there’s no single definition that’s accepted by one and all.

In the last century, the National Advisory Committee for Aeronautics – the forerunner of NASA – said that space begins 50 miles up. At that altitude, the air pressure is only about a hundred-thousandth of the pressure at sea level. NASA and the Air Force have used that definition to award astronaut wings, beginning with the pilots of the X-15 rocketplane in the 1960s.

Later, engineer Theodore von K‡rm‡n devised a slightly higher boundary: 100 kilometers – about 62 miles. At that altitude, the air pressure is just one-millionth of the pressure at sea level. In such thin air, a craft must move at orbital velocity to stay aloft.

That altitude is known as the K‡rm‡n Line. Today, it’s the most commonly used definition for the edge of space. Anything that stays below that line is an aircraft, while anything that goes above it is a spacecraft. Hundreds of people have flown above that line – qualifying them as space travelers.

A smidgen of atmosphere extends above even the Karman Line – up to several thousand miles. It’s so thin, though, that it’s basically a vacuum – a region that certainly qualifies as “outer space.”

We’ll go deep into outer space tomorrow.

Script by Damond Benningfield