Cassini: The Ultimate Carpet Burn



In news this week, Cassini, the probe that was sent up by NASA 20 years ago sent its final data before breaking up.  Its final moments, minus the data, is a beautiful use of a very basic science principle we see everyday: friction.  You could say that Cassini, in its final moments, had the ultimate carpet burn.

Friction is when two things rub together. We see this on a daily basis when we rub against surfaces fast or hard enough, we will feel warmth on our skin.  In fact if the surface of the other material is rough enough, it will tears bits of our skin away, as anybody who had experienced carpet burn or bitumen burn will tell you.  This is the principle we use when we use sandpaper to smooth a surface.

You say, hang on but there was no sandpaper or carpet when Cassini went through Saturns outer atmosphere.  Its only air!

That is partly true.  Air, while we can’t see it or feel it, it is filled up of particles, like invisible sand in a giant sandpit that is our atmosphere.  When something travels through air fast enough, the particles in air will actually take a layer off the outside of the object if the object is not sturdy enough.  This is partly why aeroplanes are made of sturdy metal, and not made of sponge cakes even though sponge is lighter (hence saving fuel).

Similarly when you lovingly dig a hole in the sandpit, ease the cake in and gently put the sand around and cover it, you will likely end up with minimum damage to the cake.  However if you push the cake in fast like a shovel, bits of the cake is going to start coming off.  Speed determines how much friction is the cake will experience in a given time.

You can test this...

If you stand outside on windy day somewhere exposed, e.g. top of mountain or tall building, cover yourself in clothes.  Initially cover your face with a scarf.  You will fill nice and toasty for a bit.  Take off the scarf now, and stand facing the wind.  Very soon you will feel cold, then very quickly you will start feeling rawness and possible pain.  This is because the wind that is blowing across your face is actually air rubbing your face very hard.  On a windy day, air becomes so fast that your face experience a lot of friction.

This is the very reason why you will see climbers to extreme places, like Mt. Everest, rugs up like a masked bandit.  Not only are they battling frost bites but also friction burn from the strong winds that are likely in those places.

PS Our lawyers told me to tell you to stop as soon as you feel any discomfort, pain or numbness. And don’t do this at all unless you are in the best of health. 🙂  We certainly do not want anybody to lose their face.


Engineers at NASA used this principle to protect Saturn.  Rather than letting Cassini crash against somewhere hard like Titan’s (one of Saturn’s moons) surface when it ran out of fuel, engineers increased its speed.  By increasing its speed, the friction between the air and Cassini increases.  Until they are rubbing so hard against each other that Cassini starts breaking apart like a sponge cake.

Cassini now exists in tiny bits in Saturn’s atmosphere.  One day if we get to Saturn, maybe some space archaeologist will wonder at how on earth the tiny bits got there.

Final drum roll...

Cakes are going to spend a lot of time sleeping with the animals…

Check back in for the final reveal next post!

Edit Note: It has been pointed out that since Saturn is a ball of gas, its not possible to crash against its surface.  It is Saturn’s moons’ surfaces the scientist wish to preserve.  Article has been updated. Thanks for all the feedback!