How do spacecraft return through the atmosphere?

1. A capsule circles the Earth at about 28,000 kilometers an hour. That is fast enough to cross an ocean in ten minutes. To come home it must lose almost all of that speed.
2. First the engine fires against the direction of travel. The capsule slows just a little, and its path bends down into the air. Then it drops the engine section and turns its wide heat shield to face forward.
3. The capsule hits the air so fast that the air cannot get out of the way. It is squashed in front of the shield and glows at about 1,600 degrees Celsius. That squashed air pushes back hard and slows the capsule.
4. The heat shield is made to burn away slowly. Its outside turns to black char and tiny bits fly off, carrying heat with them. Behind it, layers keep the heat out, so the cabin stays comfortable.
5. After a few minutes the air has taken away almost all the speed. A small parachute opens first to steady the capsule. Then three huge main parachutes open and slow it to the speed of a bicycle.
6. Splash! The capsule lands in the sea, close to a waiting ship. The air did nearly all the braking, the shield kept out the heat and the parachutes made the landing gentle. Welcome home!

Tap each part of the capsule to find out how it helps the crew get home.

• Heat shield. The wide, rounded bottom of the capsule faces forward on the way down. It is made of material that chars and burns away slowly, taking the heat with it.
• Side walls. The sloping side walls sit in the shadow of the heat shield, so they get much less heat. A thinner layer of protection is enough here.
• Crew cabin. The crew rides in a small sealed room filled with air. Even while the outside glows, the cabin stays at about room temperature.
• Seats. The astronauts lie on their backs, facing away from the heat shield. When the capsule slows down hard, they are pressed evenly into their padded seats.
• Steering jets. Small gas jets puff to turn the capsule. They keep the heat shield facing forward and can tilt the capsule a little to steer toward the landing spot.
• Parachutes. The parachutes are folded tightly under a cover at the top. Small ones come out first, then the big main parachutes for a gentle landing.
• Hatch and window. The hatch is the door of the capsule. It seals tightly so no air leaks out, and it has a small window of very thick glass.

Drag the arrow to set how steeply the test capsule dives into the air. Then start the return and watch the heat, the slowing and the landing.

Experiment: What happens if the capsule comes in at a very shallow angle, almost skimming the air?

1. Set a very shallow path and start the return. Watch how long the capsule glows.
2. Set a very steep path and try again. Watch the g-force dial.
3. Find a path in between that lands the capsule safely.

What we found: A shallow path is gentle, but the capsule stays in the thin, hot air for a long time, so the heat soaks in and the shield wears out. A steep path is over quickly, but the slowing is far too hard. A safe return lies in between.

Turn on JavaScript to try it yourself.

Aim the test capsule so it comes down next to the recovery ship. Open the parachutes when the speed dial is in the green.

A spacecraft in orbit is not floating: it is moving sideways at about 28,000 kilometers an hour, so fast that it keeps missing the Earth as it falls. To land, it has to get rid of nearly all that speed. Carrying enough rocket fuel to slow down would make the spacecraft enormous, so engineers use something free instead: the air.

When the capsule dives into the air, the air in front of it cannot get out of the way in time. It is squashed, and squashed air gets hot, just as a bicycle pump warms up when you pump hard. Here the air becomes so hot that it glows. Most of that heat stays in the air and streams away behind the capsule.

The heat shield deals with the rest. It is wide and rounded, which keeps the hottest air a little way off its surface. Its material slowly chars and burns away, and each little bit that leaves takes heat with it. Behind the shield, insulation keeps the cabin comfortable.

The angle matters a lot. Too steep, and the capsule slows so hard that the crew would be squashed into their seats. Too shallow, and it stays in the hot, thin air for too long. Our model is simplified, but real missions also aim for a narrow path in between. Near the ground the air has done its job, and parachutes finish the trip.

Key words

Reentry

Coming back from space into the Earth's air.

Drag

The push of the air against something that moves through it.

Heat shield

The layer on the front of a spacecraft that keeps the heat of reentry out.

Ablation

Protecting something by letting its outer layer slowly burn away and carry heat off.

Shock wave

A thin wall of squashed air in front of something that moves faster than sound.

g-force

How hard you are pressed when speeding up or slowing down. 1 g feels like your normal weight; 4 g feels like four times as much.

Check your thinking

1. What does most of the work of slowing a returning capsule?

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   The air pushing against it. The air takes away nearly all the speed. Parachutes only do the last little bit.

2. Why is the heat shield wide and rounded instead of pointed?

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   A blunt shape keeps the hottest air a little way off the surface. A blunt shape pushes a cushion of squashed air ahead of it, so most of the heat stays in the air.

3. Why does the air in front of the capsule get so hot?

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   It is squashed very hard and very fast. Squashing air heats it, like a bicycle pump. The capsule squashes it enormously.

4. An entry is made steeper. What happens to the peak g-force and to the total heat absorbed?

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   The g-force rises and the total heat falls. A steep entry meets dense air while still fast, so braking is violent but short. Less time in the heat means less total heat soaks in.

5. Why can't the parachutes open at the start of reentry?

   Show the answer

   The capsule is far too fast and hot: they would tear and burn. Cloth cannot survive that speed and heat. The air and the shield go first; parachutes finish the job.