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.

As Old As Time – or is it?

We are all familiar with the expression, “as old as time”, but did you know that time as we know it today only existed about a couple of centuries?  It was only in the past couple of hundred years when the world gradually agreed to one standard measure of time that time became a universal concept.  Until then people worked and rested by the rise and fall of the sun.

The (clear) glass we are used to are brought about by the government confinement of glass craftsmen to the island of Murano (to stop fire and the skills from escaping…).  Otherwise we will probably still be using vases that looks like it has been chiselled out of quartz.

This is something that we should remind ourselves, as we increasingly look to technology sector as miracle workers that both disrupt and benefit society. No invention occurs in isolation.

Innovation/problem solving/’scientific discoveries’ started when caveman wondered what that burning bush struck by lightening is.  It’s an essential skill in today’s increasingly fast paced world.  A kinder teacher I know, instead of googling answers herself, involve students in working out the answer to their questions.  This means a lot more work for her, but I look forward in 10 years time to the next young batch of inquisitive young scientists/engineers/makers.

For more information about time and glass, Steven Johnson has an excellent series called “How We Got to Now” episodes of which is available now on ABC iView.  Time and glass episode have expired now, but cold and light are still on.  I am sure you can get the DVD/downloads from the usual retailers if you want to catch up.

PS I am coining the term “Science Anthropology”.  Let us know if you think there’s a better (or more official) name out there.


This is not a sponsored post, nor do we have any association with ABC or Steven Johnson.

Why the short post!?

This post may seem out of style with our usual posts, but we are madly working on 2 concurrent exciting projects at the moment (yes yes, I know what people say about multitasking…).  Stay tuned for sneak peeks and news over the next few weeks!

Why are Fridge Doors Magnetic?

I was explaining to a child recently how the fridge door works (two magnets or a magnet and piece of metal stick together to hold the door closed).  The child then asked the inevitable question, “Why a magnet?”

This started me on a journey that reminds me of how much human oriented design every product should be…

Warning! Contains content not safe for children. Click here for full post.

In the good old days, fridges came with a handle and a latch.  It was the best way to make sure that the door was sealed tight against the fridge body, keep the cool air in.

However that had an unintended consequence….it created a airtight seal between the outside and inside.  Good right? Only until children realised that fridges were the best places for playing hide and seek in…

The airtight seal kept fresh air out and screams from inside the fridge in.  After a few accidental deaths, the US government brought in legislation that require fridge doors to be able to opened from the inside.

So what did the designers do?  After all the old design latch was good at holding the door closed.  After due consideration, they went back to experiments on how much force a child can push with from inside a confined space.  With that data, they hit upon the magnet that generated enough force to hold the door close, but not too much…


This is a classic tale of how a product evolved because of human behaviour.  It is not a consequence that any designers could have foreseen.  The latch was, and is still, the best solution to hold doors close. However it is good design principle to design with human behaviour in mind, even if it means compromises. After all there is no point in putting out a product that nobody can/will use.

How do we get there? Now that’s a whole different discussion altogether…

Do you know of  times when design had to unexpectedly bend around human behaviour? Tell us below.

Composite Sushi Roll?

Hokkaido University have recently come up with a new material that is 5x stronger than carbon steel. They did this by combining hydrogel (a wobbly jelly like substance) and glass fibers, creating a composite material.

What is a composite material?

Composite material is a blend of two (or more) materials, where the materials work together to each give the new material their type of characteristics that they are good at.

Think of a sushi roll as a composite material. The outer seaweed layer stops the rice from spilling out everywhere. The rice stops the seaweed getting wet, while tightly binding the middle ingredients together. The filling, especially if you got a vegetarian one, gives the sushi a hard core to stay up straight while we chomp away.

Take away any one of these materials, you will end up with a sushi roll that you can’t eat without a big mess:

  • No seaweed – rice spills everywhere at the pressure from being picked up (Ok this doesn’t apply if you are a sushi master.)
  • No rice – sloppy wrap that will tear from moisture and you end up with a soggy mess of vegetable
  • No filling – a roll that will flop over at first sign of wobble

Next time you look at a sushi roll, think of the ingenious composite that is your lunch.

What about Nigiris?

Ok… so how does Nigiris (a cube of sushi with filling on top) stay together without the seaweed? Nigiris are often made with a higher level of stickiness in the rice. The stickiness means that you do not need to have a seaweed wrapper. The stickiness of the rice makes the seaweed’s characteristic of holding the rice together redundant.

This emulates an evolution of composite in real life. If one material of a composite gets more desirable characteristics as time goes on, the composite can become simpler (usually also cheaper) to manufacture. Eventually, some composites become obsolete when another material is discovered that replaces all the useful characteristics of the original composite. (Newspaper as wall insulation, anyone?)