exosphere

The outermost layer of the atmosphere, without well-defined limits. Most authors place it between 500 and 10,000 kilometers of altitude, but there is no unanimous agreement. The recent analysis of data taken in the 1990s by SOHO (the Solar and Heliospheric Observatory) has revealed the presence of neutral hydrogen atoms of atmospheric origin at 630,000 kilometers of distance from the Earth's surface. In the exosphere air density is very low - negligible for practical purposes -, with barely any collisions occurring between the gaseous molecules of that highly rarefied air, some of them - the lightest ones - escaping the Earth's gravitational field. In Meteorology, it is common to consider the top of the atmosphere to be at 300 kilometers of altitude, so that what lies above is considered a vacuum. For its part, the International Aeronautical Federation sets the limit between the atmosphere and space at just 100 km, which is known as the Kármán line.