Date:31 July 2009
Where does the Earth’s atmosphere end and space begin?
That depends whom you ask. Various definitions have fought it out. In 1961, Alan Shepard reached an altitude of 186 km to become the first American in space; SpaceShipOne nabbed the Ansari X Prize in 2004 after hitting 100 km.
But scientists at Canada’s University of Calgary say new technology has allowed them to pinpoint the border: it lies 117 km above the Earth’s surface. That’s where relatively gentle atmospheric winds begin to give way to space’s maelstrom- like particle flows, which can exceed 960 km/h. The data comes from the Supra-Thermal Ion Imager, which was sent into space in 2007.