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The Layers of our atmosphere From NOAA via Wikipedia
People tend to think of "outer space" as "out there." Way far away. Light-years away maybe. But, it actually starts much closer to us than we think. The classical definition that describes the interface between our atmosphere and space where space starts is that "outer space" begins where our atmosphere is completely thinned out. That's at 100 kilometers (62 miles) overhead. If you happen to fly over that limit, then you're an astronaut. If you fly below it, you're a high-altitude flier (whether passenger or pilot).
So, once we get above that level, we're in outer space. Where does it end? Well, it stretches on throughout the cosmos, but when you land on another planet, you've left outer space and you're back into a planetary atmosphere. On Mars, for example, you'd be inside the atmosphere at 11 kilometers (about 7 miles).
Earth's atmosphere is pretty darned narrow when you look at it against the limb of our planet. All the life we know about, everybody we know or have known, or who has ever lived on this planet, did it inside a thin envelope of air that starts to thin out a few miles over our heads. The gravity of our planet holds the atmosphere pretty firmly in place, and radiation and particles from the Sun interact with the top of the atmosphere. It's a lively place, this interface between Earth and space.
"Top of the Atmosphere" courtesy NASA.
I've been reading with some interest about the companies that want to start up space tourism. There's a company in Florida that flies people through our atmosphere in a commercial "Vomit Comet" so they can experience several minutes of weightlessness. But those still fly inside our atmosphere. SpaceShipOne, designed by Burt Rutan, made the first non-governmental human spaceflight on June 21, 2004. There will be others, and perhaps in my lifetime, regular people will be able to take off and experience "outer space" for themselves. I hope I can be one of them.
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The Layers of
