How Do Astronauts Come Back to Earth?
The short answer
They let the air slow them down. A capsule starts home at about 28,000 km/h (17,500 mph). A short engine burn against its direction of travel drops it into the atmosphere, and the air does the braking. By the time Crew Dragon's first parachutes open, it's down to about 560 km/h (350 mph).
The hard part is the heat. Most of it comes from air squashed in front of the capsule, not from friction. A blunt shape and a heat shield that burns away on purpose keep the crew cool.
From orbit to splashdown
In orbit, a capsule is falling sideways so fast that it keeps missing Earth. To come home, it fires its engines against its direction of travel. This is the deorbit burn. It only needs a small cut in speed. That lowers the low point of the orbit into the atmosphere, so the capsule stops missing.
On a Crew Dragon return, the small drogue parachutes open at about 5,500 m (18,000 ft), when the capsule is moving at about 560 km/h (350 mph). The main parachutes open at about 1,800 m (6,000 ft), at about 190 km/h (119 mph). Then it splashes down. 350 mph is 2% of 17,500 mph. So the air has already taken away about 98% of the speed before any parachute opens.
Where the heat comes from
At that speed, the air in front of the capsule can't get out of the way. It piles up, gets squeezed into a shock wave, and squeezed air gets very hot. Friction adds a little heat, but it isn't the main cause. On the Space Shuttle, over 80% of the heating during reentry came from compression.
The heated air turns into plasma. It glows, and it blocks radio signals. That's the reentry blackout: for part of the way down, radio can't get through to or from the capsule.
Why capsules come in blunt
A pointy nose keeps the shock wave right on its tip, so the tip takes the heat. A wide, blunt face pushes the shock wave out ahead and leaves a gap. Most of the heat stays in that air and flows around the capsule. H. Julian Allen worked this out at NACA's Ames Aeronautical Laboratory in the 1950s.
The heat that gets through reaches the heat shield, and the shield is built to burn. Orion's shield is made of blocks of a material called Avcoat. During reentry it burns away, and the gases it gives off carry heat away from the capsule.
Key numbers
| What | Number | Source |
|---|---|---|
| Crew Dragon's speed as it enters the atmosphere from low Earth orbit | about 28,000 km/h (17,500 mph) | Wccftech, SpaceX Crew Dragon reentry footage |
| Share of the Space Shuttle's reentry heating caused by compression | over 80% | Wikipedia, Space Shuttle thermal protection system |
| Height where Crew Dragon's drogue parachutes open | about 5,500 m (18,000 ft) | NASA Commercial Crew blog, 2024 |
| Crew Dragon's speed when the drogues open | about 560 km/h (350 mph) | NASA Commercial Crew blog, 2024 |
| Height where Crew Dragon's main parachutes open | about 1,800 m (6,000 ft) | NASA Commercial Crew blog, 2024 |
| Crew Dragon's speed when the main parachutes open | about 190 km/h (119 mph) | NASA Commercial Crew blog, 2024 |
| Speed left when the drogues open, compared with the starting speed | about 2% (350 of 17,500 mph) | Our arithmetic from NASA Commercial Crew blog, 2024 and Wccftech, SpaceX Crew Dragon reentry footage |
What the film simplifies
- The film says the heat shield "chars and flakes away". Real ablative shields char and wear away on purpose. Pieces flaking off is how they fail, not the plan.
- The film says the heat "isn't mainly from friction" on purpose. Friction does add some heat, just not most of it.
- The voice says "a few hundred miles an hour" for the speed before the parachutes. The 350 mph on screen is Crew Dragon's published speed when its drogue parachutes open.
- "Butt first" is the film's way of saying heat shield first: the wide, blunt base of the capsule faces forward.
- Air, Gravity, the capsule and the little satellite are cartoon characters. Air doesn't talk, and gravity doesn't help or gloat.
For teachers
Runtime 1 minute 22 seconds · Topics: Reentry, Shock waves, Heat shields, Spaceflight · Full script below
Ask your class
Most of the heat during reentry doesn't come from friction. Where does it come from?
Answer
From air squeezed in front of the capsule. It can't get out of the way, so it piles up into a shock wave and gets hot. On the Space Shuttle, over 80% of the heating came from this compression.
Why does a blunt capsule stay cooler than a pointy one?
Answer
The blunt face pushes the shock wave out ahead and leaves a gap, so most of the heat stays in the air and flows around. A pointy nose keeps the shock wave on its tip, and the tip takes the heat.
A capsule starts home at 17,500 mph, and its drogue parachutes open at about 350 mph. What share of its speed is left when the drogues open?
Answer
350 divided by 17,500 is 0.02, so about 2%. The air has taken away about 98% of the speed before any parachute opens.
The script
Host: Getting to space? That was the easy part.
Capsule: Easy?! I'm a capsule now. Where did my fins go?!
Gravity: Coming down? Allow me to help.
Host: Problem. You're still doing seventeen thousand five hundred miles an hour. Sideways.
Host: So you fire your engines backwards. Just a little. Now you stop missing Earth.
Air: Whoa, whoa. Slow down, pal. Nobody gets in that fast.
Host: Here's the twist. The heat isn't mainly from friction.
Host: You're so fast, the air can't get out of the way. It piles up and gets squashed. That's a shock wave.
Host: And squashed air gets hot. Thousands of degrees hot.
Capsule: Is it hot in here, or is it just me?!
Host: That's why capsules come in butt first. Wide and blunt.
Host: The blunt face shoves the shock wave out ahead of you. So most of the heat stays in the air.
Host: What's left hits the heat shield. And it burns on purpose.
Host: It chars and flakes away, and carries the heat off with it.
Host: The air around you glows into plasma. Radio can't get through.
Lil Sat: Capsule? Hello? Are you still there?!
Host: Minutes later, the air has done the braking. You're down to a few hundred miles an hour.
Capsule: Chutes out! Splashdown, baby!
Gravity: You're welcome.
Host: Next time: Earth's core. As hot as the Sun's surface. Follow!
Sources
- A capsule starts its return at about 17,500 mph. Crew Dragon's drogue chutes open at about 350 mph.NASA Commercial Crew blog, 2024
- A short engine burn against the direction of travel starts reentry.FAA, Returning from Space
- Reentry heat comes mainly from compressed air, not friction.Wikipedia, Atmospheric entry
- Air squeezed ahead of the capsule heats to several thousand degrees. Hot plasma around a returning capsule blocks radio signals.ESA
- A blunt shape pushes the shock wave out ahead of the capsule.NASA History, SP 4209
- Heat shields burn away on purpose, carrying heat off with them.NASA, Orion heat shield
- Crew Dragon enters the atmosphere at about 17,500 mph.Wccftech, SpaceX Crew Dragon reentry footage
- Over 80% of the Space Shuttle's heating during reentry was caused by compression, not friction.Wikipedia, Space Shuttle thermal protection system
- A fast vehicle heats the air in front of it and turns it into plasma, which blocks radio.Scientific American, Piercing the Plasma
- H. Julian Allen developed the blunt body concept at the NACA Ames Aeronautical Laboratory.Vincenti, Boyd and Bugos, H. Julian Allen: An Appreciation, 2007