Edge of Space Found

Hold on to your hats, or in this case, your helmets: Scientists have finally pinpointed the so-called edge of space ? the boundary between Earth's atmosphere and outer space.

With data from a new instrument developed by scientists at the University of Calgary, scientists confirmed that space begins 73 miles (118 kilometers) above Earth's surface.

A lot remains very fuzzy, however, as the boundary is surrounded by a host of misconceptions and confusing, conflicting definitions.

For starters, astronauts can say they've been to space after only passing the 50-mile (80-kilometer) mark.

Meanwhile the boundary recognized by many in the space industry is also a somewhat arbitrary 62 miles (100 kilometers). Scientist Theodore von K?rm?n long ago calculated that at this altitude the atmosphere is so thin that it's negligible, and conventional aircraft can no longer function because they can't go fast enough to get any kind of aerodynamic lift. This 62-mile boundary is accepted by the F?d?ration A?ronautique Internationale (FAI), which sets aeronautical standards.

The United States, however, has never officially adopted a set boundary standard because it would complicate the issue of overflight rights of satellites and other orbiting bodies, according to NASA.

NASA's mission control uses 76 miles (122 kilometers) as their re-entry altitude because that's where the shuttle switches from steering with thrusters to maneuvering with air surfaces, NASA states. Others point out that the "Now Entering Space" sign should be posted way out at 13 million miles (21 million kilometers) because that's the boundary where Earth's gravity is no longer dominant.

(While astronauts experience weightlessness in space, this isn't because there's no gravity there, it's due to the balance of forces acting on them as they orbit.)

In the new study, an instrument called the Supra-Thermal Ion Imager detected the boundary by tracking the relatively gentle winds of Earth's atmosphere and the more violent flows of charged particles in space, which can reach speeds well over 600 mph (1,000 kph).

The ability to gather data in that area is significant because it's very difficult to make measurements in this region, which is too high for balloons and too low for satellites.

"It's only the second time that direct measurements of charged particle flows have been made in this region, and the first time all the ingredients ? such as the upper atmospheric winds ? have been included," says project scientist David Knudsen of the University of Calgary.

The instrument was carried by the JOULE-II rocket on Jan. 19, 2007. It traveled to an altitude of about 124 miles (200 kilometers) above sea level and collected data for the five minutes it was moving through the "edge of space."

The finding, detailed in the Journal of Geophysical Research on April 7, could aid the study of space weather and its impacts on Earth.

The data "allows us to calculate energy flows into the Earth's atmosphere that ultimately may be able to help us understand the interaction between space and our environment," Knudsen said. "That could mean a greater understanding of the link between sunspots and the warming and cooling of the Earth's climate as well as how space weather impacts satellites, communications, navigation, and power systems."

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This article was update at 4:14 p.m. ET.