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FARGO — What is "outer space," and where does it begin and end? This question has been debated for decades. Space begins where the atmosphere ends, but this is difficult to measure.
Earth's atmosphere is concentrated by air pressure near the ground and decreases with height until there is no more air. This gradient is why your ears pop when you ride in an elevator and why airplane interiors are pressurized. Traditionally, NASA has defined the edge of space as a sphere about 60 miles above the ground at the lowest perigee attained by orbiting space vehicles.
In 2009, NASA's JOULE-II rocket carried the Supra-Thermal Ion Imager, an instrument developed at the University of Calgary in Canada. It was able to detect a thermal boundary at the outer edge the atmosphere located 73.3 miles above the Earth's surface. Beyond this boundary, space is not completely empty, but very nearly so.
