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Earths Atmosphere Stretches to the Moon!

I'm often asked by students about how high the earth's atmosphere stretches. It can be hard to answer: they want a clear-cut answer, suggesting a sharp boundary between 'air' and 'no-air', whereas it just gets progressively thinner and thinner without a clearly defined edge. I usually refer to a few easy-to-grasp concepts. At 10km, there's too little oxygen for people to breathe (only super athletes can climb Everest without oxygen at just over 9km). Over 40km, the air is too thin to support even specialist airplanes. At 100 km the air has effectively disappeared - and that's often taken as the 'edge'.

I found this useful list in Wikipedia:

  • 50%of the atmosphere is below 5.6 km.

  • 90% is below 16 km

  • 99.99997% is below 100 km

But now we have a new outer limit to consider. Data from SOHO, an ESA craft that has been hovering at a distance of 1.5 million km from earth (towards the sun) for over 20 years was used to find the extent of the earth's geo-corona: a layer of very low pressure hydrogen atoms that surround the earth, trapped by our gravitational field. And apparently it extends as much as 600,000km from the earth - meaning it includes even the moon within its limits!

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