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Most of us are hoping that any nuclear threats are just empty threats, and getting at the facts about ICBMs can be difficult. But what would actually happen if someone launched a nuclear weapon?

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Sources:

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http://www.cnn.com/2017/02/14/politics/us-north-korea-aegis-ballistic-missile-defense/index.html
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https://www.quora.com/How-many-steps-are-required-to-arm-and-trigger-an-aircraft-fired-missile-and-what-safeguards-are-there-to-prevent-accidents
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http://www.businessinsider.com/nuclear-bomb-accident-goldsboro-nc-swamp-2017-5
http://www.nytimes.com/1981/05/26/us/us-discloses-accidents-involving-nuclear-weapons.html
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Images:

https://commons.wikimedia.org/wiki/File:Dauntless_bomb_drop.jpg
https://en.wikipedia.org/wiki/File:Trident_II_missile_image.jpg
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https://en.wikipedia.org/wiki/File:YAL-1A_Airborne_Laser_unstowed_crop.jpg
https://en.wikipedia.org/wiki/File:Military_laser_experiment.jpg
For the last couple decades, nuclear weapons have mostly stayed out of the news, as the Cold War has kind of started to fade to the back of our minds.

But they’re back in the news now, and back on our minds. Most of us are hoping that any threats are just empty threats.

But at some point you might start to wonder what would happen if there were a nuclear missile heading for us. Of course, the latest in military defense is classified. Like any sort of defense, the more people know about it, the easier it is to circumvent.

But we do know a lot of the basics, and the good news is that an incoming nuclear missile doesn’t necessarily mean doom. Because we have some options when it comes to the most important part of stopping an incoming attack: hitting it before it hits us. The only nuclear bombs ever used in a war were dropped from planes.

That could definitely still happen, but most people today aren’t worried about planes. They’re worried about missiles. Missiles are faster and harder to stop than planes, and some types can launch from pretty much anywhere.

The most famous are Intercontinental Ballistic Missiles, or ICBMs, which rocket into space before speeding down at their target. They can hit something halfway around the world within an hour after launch, and if you’re curious about the challenges involved in building one, you can check out our video about that. Other types of missiles are less famous, but each is designed to go a different distance and carry a different amount of weight.

No matter what type it is, the best time to stop a missile is before it even launches, so countries constantly use spy planes and radar and satellites to monitor each other for any signs of missiles being prepared. But sometimes, the first hint of a missile is the launch itself. Missile launches are bright and hot, and the missiles stay hot while they’re screaming through the atmosphere.

So the US has a worldwide network of satellites on the lookout for unusually hot objects, known as the Space Based Infrared System. SBIRS and its predecessors have monitored missiles for decades, although its heat-seeking eyes can track wildfires and watch volcanoes for signs of impending eruptions when there isn’t a missile to watch. But if one sees a missile, you can /bet/ those other jobs go out the airlock.

Other countries have their own missile defense systems, but they pretty much all share the same goal: making sure that if a missile is being launched anywhere, we know about it. Once a missile is in the /air/, there’s really only one way of stopping it: Hit it with something. Usually another missile.

It isn’t ideal to have a nuclear bomb explode up in the atmosphere and rain fallout on the world below, but that’s still way better than letting it reach its target. Plus, the nuke might not even explode when the rest of the rocket does. A lot of modern bombs don’t bring the explosive parts near each other, known as arming, until they’re about to land, to make sure that nothing goes wrong during the launch or the flight.

So the nuke might be destroyed by the explosion, or it might end up like the unarmed hydrogen bomb that accidentally fell into a North Carolina swamp in 1961. Nothing happened when it landed, and its uranium core is probably /still/ fifty or so meters beneath a farm. To track their target, missile interceptor systems use radar and the same heat signature that SBIRS uses, but hitting a missile with another missile definitely isn’t easy.

It’s like shooting a bullet with a bullet, except each bullet is moving six or seven kilometers per second, and missiles are way more complicated to build, aim, and launch than bullets are. To make it even harder, missiles often carry decoy parts that fall off during descent to distract defensive systems from hitting the real thing. With so many factors to take into account, it’s not too surprising that missile defenses need a lot of testing.

But you don’t want to test any defense too many times, even if it’s not quite perfect. To an outside observer, every test reveals a bit more about how it works — /and/ how to get around it. It’s expensive, too: Each ICBM defense test costs tens of millions of dollars and takes months to prepare for.

Altogether, the US is probably equipped to stop a couple dozen ICBMs at once, but it’s hard to know for sure because the military tends to not go around telling people exactly how many missiles it would take to overwhelm our defense systems. We /do/ know that single networks and groups of submarines in places like South Korea and Japan are capable of tracking a hundred smaller missiles at once. But even they aren’t perfect; missiles are so hard to hit that it’s next to impossible for every interceptor system to stop every missile 100% of the time.

And military strategy is always evolving, so some places will end up less defended than others. But the military is always trying to develop new technologies to block those holes, and one technology that was first imagined back in the seventies and eighties might make a comeback: Lasers. Back then, people figured that satellites with ultra-powerful lasers could overheat and blow up missiles mid-flight.

It was incredibly impractical at the time. For one thing, lasers powerful enough to overheat a missile from so far away just didn’t exist. But the American military recently announced it was testing laser-mounted drones for missile defense.

Today’s lasers are a lot stronger and smaller than those from decades ago, and drones could quickly get within ten or a hundred kilometers of a missile without endangering lives. Powerful lasers still aren’t quite compact enough to be carried by a drone, but a laser defense system might not be too far off. Or maybe we already have one and it’s just not public information.

Either way, generations of brilliant scientists and engineers have spent their careers trying to make sure that no nuclear weapon ever hits another city, and we have their defense systems in place. Let’s just hope we never need them. Thanks for watching this episode of SciShow.

If you’d like to learn more about the science of nuclear weapons, or the science of anything else, you can go to youtube.com/scishow and subscribe.