In 2009, scientists at the University of Calgary used measurements of atmospheric wind and the flow of charged particles to determine that space begins at 73 miles above the Earth's surface. However, there are competing, and sometimes conflicting, assessments of Earth's boundaries.
According to Space.com, astronauts who have passed the 50-mile mark can claim to have been into space. NASA used the 76-mile mark as the re-entry altitude for the shuttle program, because this was the point at which space craft began interacting with air surfaces. The boundary recognized by the Federation Astronautique Internationale (FAI), an agency that regulates aeronautical standards, is 62 miles up, the height at which atmospheric conditions make the use of conventional aircraft prohibitive.