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!

Rain Rain Go Away…Relieve Yourself Another Day

With the recent wet weather around the world, it reminded me of a time when a preschooler asked me why does the sky rain? (The explanation I used at the time was a bit NSFP, Not Safe for Polite Company. Its included at the bottom if you need to grab the attention of a restless preschooler.)

Rain basically is water going through evaporation (turning from water to steam, aka boiling) and condensation (turning from steam to water, aka fogging up) cycle. Water gets made into steam in the air from any source of water (e.g. lake, ocean or even just moisture on the pavement). If the environment doesn’t change, it will stay in the air for a long time, hanging around in the form of clouds. However it will usually meet something that causes the steam to cool. This cooling then turns the steam back into water, falling to earth. So at a simplistic level, you can say that cloud drinks water when it is warm, and when something cools it down it rains.

In a way this continues non-stop throughout the world. What is evaporated from the pacific ocean today may fall as rain in Asia tomorrow.

NSFP Analogy

If the preschooler is at a stage when any bodily functions are a hilarious topic, the following may work well.

When its warm, the cloud drinks water (from evaporation, you may have to do some ground work here) just like a person does. Until they drink so much water that their body can’t hold anymore. They have to go to toilet to get rid of the excess water. Then the cloud goes to toilet, producing rain.

Its simplistic… but they won’t forget it in a hurry. 🙂


Let us know any strange analogy you ever had to use to explain a concept to a child.

Sneak Preview - As Promised

Not enough room and WAY too much stock.

It’s National Science Week. Let’s Do Something!

Its that time of the year again!  A national science week where we celebrate all cool things science.  Instead of hearing me rant on about science all around us, you (and your kids) can get a few hours of entertainment while staying out of the chilly Melbourne weather.

I am glad to see that there are a handful of events for pre-school aged children.

As part of this (unofficially), I will be doing a science storytime at my local kinder, helping poor Rexy and Tako find their crown. 🙂  I strongly encourage anyone to put their hands up to do something locally, even if its just making a bicarb volcano!  Anything will help our kids discover science all around us, not to mention looking really cool for making “science magic” happen.

For your nearest events, visit their website.

What’s your favourite science activities/moments you had with kids?  We would love to hear about all your experiences below!

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