ICBMs or Intercontinental Ballistic Missiles.

On an individual level, like there's not a whole lot that I can do about North Korea

but sometimes it helps to

understand the things that we're like

kinda freaked out by, so we wanted to talk about how these weapons of mass destruction work,

why they're so difficult to engineer, and why they're so dangerous

Missiles are guided, rocket-propelled weapons and ICBMs are the longest-range missiles out there.

They can travel at least 5,500 kilometers (about the distance between New York and London). Some designs can go twice as far or even farther.

ICBMs can also carry warheads, the toxic or explosive stuff that makes them especially dangerous

and a big concern these days is having an ICBM that carries nuclear warheads.

Over the last few years North Korea has been testing missiles that can go farther and farther

and on July 4th they launched their first test ICBM

and they've already followed up with a second on July 28th.

Experts say that, well, it doesn't seem like they've completely figured out how to make functional ICBMs;

they are getting closer.

Now the US and Russia have had ICBMs since the late 1950s,

but building them is really hard.

It's literally rocket science.

At the core of the problem is a trade-off between mass and distance.

If you want a missile to go really far, it's easier if it weighs less,

but you need fuel to propel it those long distances,

and the warhead is pretty big too which means it kind of has to weigh a lot,

so you need to engineer your way out of the problem and find a balance.

That's led weapons designers to build ICBMs

with multiple stages instead of just one fuel tank and set of engines.

They work a lot like the rockets we use to get the space.

Missiles with multiple stages lift off of the blast from the main stage or booster.

But, once that fuel is spent that part of the missile with the heaviest engines that produce the most thrust can be let go.

The lower mass then makes it easier for the rest of the missile to accelerate

and travel farther propelled by engines and fuel in higher stages.

Multiple stages can really extend the missile's range by gradually sloughing off some of the weight,

but the nuclear warheads they're designed to carry are still very heavy.

They're made out of enriched uranium or plutonium; some of the heaviest elements, so weapons engineers try to nuclear warheads as tiny as possible.

We're talking several hundred kilograms instead of a thousand

That's known as miniaturization.

Less material usually means a smaller, lighter weapon,

but there are ways to organize metals and other unstable materials to make the reactions extremely powerful.

I'm not going to get into too much detail about this for obvious reasons,

but one of the most space efficient designs is a thermonuclear

weapon; one that uses a fission reaction and then triggers a secondary fusion reaction.

Fission reactions release energy by splitting atoms,

basically, neutrons it kicked out of the nucleus of an unstable atom hit another nuclei

setting off a chain reaction that releases lots of heat and X-Rays.

When you build your weapon the right way that release of energy can set off a secondary fusion reaction which releases energy

by putting atoms together and producing more neutrons,

and if there's more uranium or plutonium wrapped around the fusion fuel the fusion reaction can trigger more fission,

using up more material and releasing even more energy.

The combination of fusion and fission makes a really powerful bomb

Nuclear weapons are so destructive because messing with subatomic particles can unleash a lot more energy

then just breaking chemical bonds like typical explosives do.

More energy means more immediate destruction

plus there's the ionizing radiation that can damage tissue and cause radiation sickness.

While the basics of designing a compact thermonuclear bomb have actually been well known for more than half a century,

which is why I'm able to talk about it right now,

putting all these engineering strategies into practice is a lot easier said than done.

You have to make the materials and figure out how to trigger

the fusion reaction and keep the chain reaction going through as much material as possible.

And even if you stick with a simpler bomb that only uses fission

you'd need to keep it tiny and keep those reactions going instead of just fizzling out.

To make an ICBM you have to do all that and then send the missile into space and back.

And another huge challenge for missile engineers is re-entry through the Earth's atmosphere.

Even though other ballistic missiles go into outer space,

ICBMs have it the hardest because they're traveling so fast when they're coming back into Earth's atmosphere,

at more than 21,600 kilometers an hour, and when you re-enter the atmosphere that fast

you generate lots of heat like even above the surface of the sun temperatures, it definitely melts metal,

so without some kind of protection these missiles would just disintegrate before reaching the ground.

One way to protect the warhead is to have a big, bulky re-entry vehicle that slows it down before it hits the lower denser parts of the atmosphere,

Where the heating gets to be the most intense, but big and bulky feeds into the weight issue.

We talked about before and it's harder to hit a target with this slower type of design.

Engineers can build heat shields that are relatively lightweight and are designed to break up during reentry

These chunks carry away the superheated gases and heat from the surface of the missile,

keeping things relatively cool, but this can also raise new challenges for getting the missile to where you want to go.

It's hard to use computer models to predict how an object might veer off course when pieces of it are burning off,

especially because they can burn off unevenly, and that's even harder for missiles like

ICBMs that spend a lot of time in the atmosphere because tiny trajectory errors can get compounded over longer distances.

So because ICBMs travel so far and fast, everything is harder.

You need it to have powerful engines and survive the atmosphere,

but you also need it to be as lightweight as possible,

and of course you need to be able to aim the thing and modelling a missiles trajectory is its own huge challenge.

It takes a lot of work and engineering and brainpower all for something incredibly dangerous that pretty much everyone agrees should never be used.

To be clear, we're pretty sure that North Korea doesn't have working thermonuclear ICBMs yet,

but they might someday.

As more countries make destructive weapons,

it's easy to feel powerless or afraid but it can help to at least

understand the technologies that humans are using to threaten each other, and we do have ways to protect ourselves

against the threat of an ICBM which we will get into in a future episode in the meantime

Don't freak out too much,

and thank you for watching this episode of Scishow News.

If you'd like to see our other episodes about the science of nuclear weapons or if you're interested in learning about more things in science,

generally you can go to youtube.com/SciShow and subscribe