Learn about the pressure of space

Cool marshmallow experiments

In space, outside of the Earth’s atmosphere, there are almost no molecules in the gas that surrounds the International Space Station. The absence of molecules, means that the air pressure is extremely low – in fact it is almost zero, so it is nearly a perfect vacuum.

Humans under pressure

Humans have evolved to live under the pressure of the Earth’s atmosphere, which is pushing down on us all the time. To cope with atmospheric pressure, humans have an internal pressure to equal it. The pressure inside our skin is equal to the pressure outside our skin.

For several different reasons, the vacuum of space is incredibly dangerous for humans unless they wear specially designed Sokol spacesuits that maintain air pressure by being connected to life-support systems within the Soyuz craft. 

Using some marshmallows and a syringe, you can explore one of the amazing effects that low air pressure can have on the human body.

Magic marshmallow experiment

When you visit one of our science centres, you may get to see a demonstration of what happens to marshmallows in a laboratory-size vacuum chamber.

Watch what happens to the marshmallows as the pressure is decreased in the chamber.  

Why do we use marshmallows? Because of their squidgy, spongy texture. The sponginess of a marshmallow is due to it having a lot of air trapped inside it. If you press it or stretch it just a little, it will bounce back to its original shape.

That’s a bit like the flesh of human bodies. But it feels quite unpleasant when our bodies are pressed, pulled, or put in a vacuum so we use marshmallows instead of bodies to look at the incredible impact a vacuum can have.

Here’s how you can create a similar experience at home, on a smaller scale.

What you need

  • Clean plastic syringe - about 10ml or 20ml are often used to give medicine to infants and pets, so you may have one in the kitchen. If not, your local chemist will have one.
  • Small marshmallows - these will act as mini astronauts.

What you do

Close the plunger of the syringe so that is on the 3mm mark. Now press the open end firmly against the skin of the thumb on your other hand. (If you struggle to use a syringe with one hand, press it against the skin on your leg). Gently pull the plunger up to about 8-10mm. 

Do you feel the sucking sensation on your skin? That’s because of the drop in air pressure inside the syringe.

There was a tiny amount of air inside the syringe. When you seal the end of the syringe with your thumb, the air molecules can’t escape and no more air molecules can get in. When you pull up the plunger, you give the trapped air molecules more room to move around and you create a drop in air pressure: a vacuum.

Take your marshmallows and draw a simple smiley face on the side of a couple of your marshmallow astronauts. Use a felt-tip pen to do this.

Now take the plunger out of the syringe and put a couple of mini marshmallows into the syringe. Move the plunger down so that it just touches the top of the ‘smiley’ marshmallow. Don’t squash it, as this will ruin its elasticity. This leaves very little air in the syringe. Place your thumb over the narrow end of the syringe again and pull the plunger out as far as it will go.

Your marshmallow astronauts expand!

If you break the vacuum by removing your thumb, normal atmospheric pressure is resumed and the marshmallows return to their original size.

Now keep the plunger out as far as it will go. Put your thumb over the open end again and push the plunger back in. Look what happens to the marshmallows now: they shrink. 

When the marshmallows expand, it is because their internal pressure is trying to match the low air pressure around it. This is an exaggeration of what would happen to a real astronaut exposed to the vacuum of space because our skin acts as quite a good pressure suit, but the pressure would certainly force all the air out of our lungs.

When the marshmallows shrink, it is because the air pressure they are under increases.

Have fun expanding and shrinking. It will soon become a mushy mess. 

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