NASA expert reveals why no astronaut has ever left Earth's atmosphere before

A NASA expert revealed that no astronaut has ever truly left Earth's atmosphere.

When you think of big-time astronauts like Yuri Gagarin, Neil Armstrong and Valentina Tereshkova - or the most recent space tourists like Katy Perry - you might have assumed their space endeavours took them beyond Earth's boundaries. It turns out that might not technically be true.

NASA heliophysics expert Doug Rowland explained that our planet's atmosphere doesn’t have a hard edge, so the boundaries are less obvious.

"As you think of the atmosphere where we live and breathe here on Earth, it doesn’t just stop right above our heads. It doesn’t stop at Mount Everest.

It doesn’t stop where the planes are flying. It goes on and on all the way up and just gets less and less dense the higher you go. And it’s still there at a very high altitude," Rowland explained in a video.

According to the scientist, beyond the known layers of the atmosphere lies an outermost layer called the geocorona. It's a super thin cloud of hydrogen atoms that extends way out into space. And by 'way out,' it's about 391,000 miles from Earth.

The Moon was the farthest that any astronaut ever travelled in space, but sticking to the boundaries of the geocorona, the Moon is actually still within the Earth's atmosphere.

And even though the International Space Station (ISS) is in what we casually call 'space,' it’s technically still flying through the Earth’s atmosphere.

"When you go to where the Space Station is – only a couple hundred miles above the Earth – there’s still enough air there to slow the Space Station down," Rowland added. "And if you didn’t re-boost it with rockets, it would come back to Earth based on the air drag, just like when you’re driving your car."

Furthermore, the problem is with how we define our atmosphere.

There's 'no clear boundary between where Earth’s atmosphere ends and outer space begins,' Rowland described. But most scientists use the Kármán line for reference, which is located 100 kilometres (about 62 miles) above Earth's surface.

There's no clear boundary between where Earth’s atmosphere ends and outer space begins. (DrPixel/Getty)

That said, this gets even more complicated given that Earth and the Moon are both within the Sun’s atmosphere, too.

"So there’s this kind of dichotomy there where you go from the Earth, the Earth’s atmosphere, then you’re in the Sun’s atmosphere," Rowland continued. "And then, at some point, you’re outside that when you reach the heliopause and the heliosphere boundary."

All in all, Rowland said to answer the question: 'Where does space begin?' it depends on your 'point of view.'

He concluded: "If you want to know, 'Where does the atmosphere end?' It’s about 400 miles over your head. But just remember that that space above that is not empty. It’s full of all kinds of interesting stuff."