The Supra-Thermal Ion Imager
By Hamish Johnston
Where does Earth’s atmosphere end and space begin?
The answer is 118 km above sea level, at least according to physicists in Canada and the US.
In 2007 the team sent their Supra-Thermal Ion Imager into space aboard the JOULE-II rocket. The instrument was able to detect the precise altitude at which the rocket left the cocoon of Earth’s atmosphere and was subject to the blast of charged particles found in space.
They have just published a paper describing their findings. You can read more about it here