Where is the boundary between the universe and the sky?

When mountaineers climb Everest, they often have to carry oxygen tanks, devices that allow them to breathe freely at high altitudes. This is necessary because the closer you get to the edge of Earth's atmosphere, the less oxygen is available, especially when compared to the abundant oxygen found near sea level.

This is just one example of how variable Earth's atmosphere is. And it also shows the elemental composition of its layers, from the troposphere, which is near sea level, to the exosphere, in its outermost regions.

According to the National Weather Service, where each layer ends and begins is determined by four key characteristics. It is the change in temperature, chemical composition, density and movement of the gases within it.

So, where does Earth's atmosphere actually end? And where will space begin?

Earth's atmosphere has layers with distinct characteristics.

Each layer of the atmosphere plays a role in ensuring our planet can host all manner of life. It does everything from blocking cancer-causing cosmic radiation to creating the pressure needed to produce water, according to NASA.

'As you move further away from Earth, the atmosphere becomes less dense,' said Katrina Bossert, a space physicist at Arizona State University. Lighter molecules begin to dominate, while heavier molecules stay closer to the Earth's surface."

As it moves up through the atmosphere, the pressure or weight of the upper atmosphere weakens rapidly. Although commercial planes have pressurized cabins, rapid changes in altitude can affect the thin eustachian tubes that connect our ears, nose, and throat. "Here's why your ears can tingle during take-off," says Matthew Igel, an adjunct professor of atmospheric science at the University of California, Davis. .

And eventually, the air becomes too thin for regular planes to fly, because they can't generate enough lift. This is the region that scientists have decided to mark the end of the atmosphere and the beginning of space.

It is called the Kármán line, named after Theodore von Kármán, a physicist with triple American-Hungarian-German citizenship. In 1957, he became the first person to attempt to define the boundary between Earth and outer space.

The Kármán line, because it marks the boundary between Earth and space, not only indicates where the plane's limits lie, but is also important for scientists and engineers to figure out how to keep spacecraft spaceship and satellite orbiting the Earth successfully. 'The Kármán line is an approximate region representing the altitude at which satellites can orbit the Earth without burning up or falling out of orbit before orbiting the Earth at least once,' says physicist Bossert. "

'It is usually defined as 100km from Earth,' Igel added. "It's possible for something to orbit the Earth at altitudes below the Kármán line, but it would require extremely high orbital velocities, which are hard to maintain due to friction. But there's no stopping that."

"It's an imaginary threshold but it's the reality that separates air travel from space travel," he added.

According to Bossert, various factors, such as the size and shape of the satellite, play a role in determining how much air resistance is present and, therefore, in its ability to successfully orbit the Earth. . Typically, satellites in low Earth orbit - the classification usually granted to satellites below 1,000km but sometimes as low as 160km above Earth - will fall out of orbit after a few days. years, because "the drag from the upper layer of the Earth gradually slows down the orbital speed".

Illustration of the layers of the Earth's atmosphere.

That doesn't mean, however, that Earth's atmosphere is undetectable beyond 1,000km away.

'The atmosphere doesn't disappear once you get into the region where the satellites orbit,' says Bossert. there's more. The atoms outside of Earth's atmosphere, the hydrogen atoms that make up its geosphere [the outermost region of the atmosphere], may even extend beyond the moon."

So, if someone made it to Kármán Street, would they notice anything? Do they know that they are basically at the dividing line between Earth and space?

"Nothing has really changed," Bossert said.

'This line is not a physical line, and so one will not notice crossing it, nor will there be any thickness,' added Igel.

What about being able to survive, even for a short time, on the inner line of Kármán? What if you crashed there without a custom spacesuit or oxygen tank attached? If you could get to it, would you be able to breathe at such a high altitude? And can birds reach such heights?

'In principle, it is still possible to fly to the Kármán road,' said Professor Igel. "In practice, however, animals cannot survive at altitudes above the 'Armstrong limit', about 20km above the Earth's surface, where the pressure is so low that the fluid in the lungs boils."