Astronauts reveal why the 'most terrifying thing an astronaut can ever do' is coming back to Earth

Astronauts have opened up about what's most 'interesting' but also 'scary' about re-entering Earth's atmosphere after visiting space.

You'd think getting up into space would be the scary part but it's actually coming back down to Earth which tests astronauts' nerves the most and for multiple reasons.

A clip from National Geographic documentary One Strange Rock was shared to Instagram earlier this year.

It reveals clips of astronauts re-entering Earth's atmosphere along with interviews with astronauts about why it's 'the most dangerous thing an astronaut will ever do'.

One astronaut explained: "During re-entry, the window seat is, it's interesting."

Another added: "Flying the Russian Soyuz is a little bit like, maybe, flying a meteorite home."

A third revealed that when you look out of the window during re-entry to Earth you can see 'orange and purple flames of 3,000 degrees' which are alarmingly close by.

"That's pretty freaking scary," they added.

Oh, and if the visuals weren't enough already, you're also going '17,000 miles an hour, eight kilometers a second'.

"And somehow you have to get rid of all of that speed. You have to apply the brakes," one astronaut noted.

And it's not taken long for social media users to weigh in.

Astronauts have described what it's like re-entering Earth (National Geographic TV)

One Instagram user said: "How are they no panicking or afraid - my brain would paint all the disaster images and I’d be hyperventilating and crying and my heart would explode from worry. Incredible."

Another added: "WHAT HIT THE WINDOW Y’ALL?!"

A third commented: "Astronauts are brave!"

And a fourth simply resolved: "I would never re entry."

And NASA's website breaks down re-entry from space to Earth in even greater - and slightly terrifying - detail. Prepare to dive into a physics lesson.

It's certainly not for the faint-hearted (National Geographic TV)

It explains: "As a spacecraft re-enters the earth's atmosphere, it is traveling very much faster than the speed of sound. The aircraft is said to be hypersonic.

"[...] The chief characteristic of re-entry aerodynamics is that the temperature of the flow is so great that the chemical bonds of the diatomic molecules of the air are broken. The molecules break apart producing an electrically charged plasma around the aircraft.

"The air density is very low because re-entry occurs many miles above the earth's surface. Strong shock waves are generated on the lower surface of the spacecraft."

The American Space Shuttle, the Russian Soyuz spacecraft, and the Chinese Shenzhou spacecraft are the only aircraft to have flown in this way.

NASA explains the Shuttle uses 'a rocket propulsion system to get into orbit, but during re-entry the aircraft is actually an un-powered glider'.

Re-entering space is risky business (National Geographic TV)

NASA's website continues: "Small steering rockets are used for maneuvering early in the re-entry because the low density of the air at altitudes above 50 miles makes aerodynamic surfaces ineffective. The heat is so great during re-entry that a special thermal protection system is used to keep the spacecraft intact.

"On the Shuttle, special silicon tiles are placed on the aluminum skin to insulate the skin. On the leading edge of the wings, carbon-carbon composite material is used to withstand the heat. The high forces and high heat dictate that the Shuttle has short, blunt wings.

"The Shuttle flies at a high angle of attack during re-entry to generate drag to dissipate speed. It executes hypersonic "S-turn" maneuvers to kill off speed during re-entry. The lift of the wings is only important in the final flare maneuver at touchdown."

The Soyuz Shenzhou, and all of the early Apollo, Gemini, and Mercury spacecraft instead used 'a thermal protection system that is different than the Space Shuttle'.

Each of them use an 'ablative' or 'burning' heat shield which is made of special ceramic materials and designed to 'slowly burn away as it encounters the high temperature plasma flow aft of the bow shock wave'.

"The change of phase from solid to liquid to gas and the convection of the flow away from the spacecraft help to protect the astronauts from the heat of re-entry," NASA says.

Yep. Pretty freaking scary for sure.