Today I want to talk to you about matter.
Specifically, what happens when it collides with other matter.
This is a bit of equipment I built for this year's CHRISTMAS LECTURES, to discuss the
hazards of micrometeoroid impacts in space.
Whether this is with man-made stuff of natural micrometeroids.
Micrometeoroids are tiny specs of space dust which become very dangerous at high speeds.
So we built this bit of kit here to demonstrate that anything can be dangerous it it's going
fast enough.
Even a carrot.
Using this high-speed footage we were able to clock the carrots travelling at around
100 metres per second.
That's well over 300 km per hour...
which is more than enough velocity to obliterate a lightbulb, just in case you were wondering.
But the carrot is actually travelling pretty slowly compared to things orbiting the Earth.
The speed of something in a low earth orbit, like the International Space Station, is around
8km/s.
That's just under 28,000 km per hour, which is almost 100 times the speed of our carrot.
At this speeds, even specks of dust could be dangerous, posing a huge risk to astronauts
and spacecraft.
This crack in the window of the ISS is about 7mm in diameter, but is thought to be cause
by a tiny fleck of paint, no more than a few thousands of a mm across.
So what happens when matter collides at these extreme speeds and why does it become so dangerous?
First we need to consider what matter is.
That's easy, you might think: matter is made of atoms and molecules, right?
Well, yes and no. In a sense, matter as we're familiar with it is really formed of chemical
bonds.
It's the bonds that determine the properties of matter: how hard it is, how strong it is,
what it looks like.
So when objects collide, really it's the bonds that are interacting.
It's the bonds that absorb and transmit energy through the solid to determine the outcome.
Just about the strongest bonds of all are in diamonds.
Even this tiny thing would require about 12,000 Joules of energy to break all its chemical
bonds.
But even at a speed of 300 km/h, the speed of the carrot gun, its kinetic energy by comparison
would be less than 1 Joule,
and that's why we saw the carrot largely surviving earlier, because bonds take so much energy
to break,
and therefore dominate the way matter behaves in the low-speed world around us.
But at the much higher speeds of space, there's just so much more kinetic energy in play,
the relationship between speed and kinetic energy is a square relationship,
which means that if you're going 100 times as fast, you have 10,000 times as much kinetic
energy.
If this diamond were travelling at around 11 km/s, it would have a kinetic energy of
around 12,000 Joules.
So a collision at this energy would be enough in principle to break every single one of
its chemical bonds.
What this means is that the high energy collisions of space make solid matter behave in a way
that's wholly unfamiliar to us.
Chemical bonds suddenly become much less important because they're just not strong enough to
withstand the enormous energy that each atom carries into the collision.
Really we have to abandon to some extent the concept of a solid, because solidity as a
property of matter is completely dependent on bonding forces.
Essentially we have to consider objects as just a collection of unbound atoms, moving
along together.
As they collide, all of those atoms ricochet wildly off each other in all directions.
Also, because this surrounding material doesn't have time to get out of the way,
you end up with lots of atoms much closer together than they would normally be.
This creates huge pressure, which is a force that doesn't just act along the direction
of impact, but in all directions.
What I'm describing here is basically a high-energy explosion.
When we think of high-speed collisions it's easy to imagine something like a bullet punching
its way through a target, leaving a clean hole.
Instead you get this moving explosion, vaporising matter and blasting much bigger holes in things.
This test footage shows the result of a high velocity impact between a tiny stainless steel
ball and two aluminium plates.
The ball in only 6mm in diameter, but is travelling at a whopping 6.7km per second.
This is still slower than the orbital speed of the ISS.
Here you can see the dramatic impact of a 6.8km per second impact between a small aluminium
sphere and a piece of aluminium.
During these sorts of impacts, the pressure and temperature generated can exceed those
found at the centre of the Earth,
and this is why tiny specks of space dust pose such a risk in space.
So how do spacecraft protect themselves from impacts in space?
Interestingly, they still mostly use an idea devised by astronomer Fred Whipple in the
1940s, before the space age had really got started.
The Whipple shield takes advantage of the way impacts spread out in all directions,
like explosions.
Basically you have a wall made up of several layers. The outermost layer isn't intended
to stop the projectile, just vaporise it.
You then have this big gap before the next layer. This allowsd the vaporised cloud of
material to expand,
so that by the time it gets to this layer its energy and momentum are spread out over
a wide area and can be absorbed easily.
As we can see here, in the first instance with these two pieces of card together, the
carrot makes it through and obliterates the egg.
But with the spacing as in a Whipple shield, we do a lot better.