We can, in limited circumstances. Mostly though it’s very difficult. Ballistic missiles travel extremely fast and they’re very small. In order to destroy one, we have to intercept it with something equally fast and equally small and we have to track its speed and position with perfection. If we’re off by even a tiny amount, the intercepting missile will miss. It’s like trying to shoot a bullet out of the air with another bullet.
With ICBMs the issue is that they tend to be launched either from well within enemy territory or from submarines somewhere in the ocean you don’t know at the time. Both of these makes catching the missiles on the way up very difficult or impractical.
Once they are “up” they are in space and outside of the reach of most kinds of attacks. Most missiles can’t even reach space at all or maneuver without air, much less intercept something in a suborbital trajectory.
On the way down a single ICBM can likely split into 10 separately targeted warheads along with another 90 decoys which try to look as much as possible like warheads. This greatly increases the resources needed to devote to intercepting the strike, as if you confuse a decoy with a warhead you are wasting your defensive missiles. On the other hand not shooting down what you think is a decoy and being wrong means a city being flattened.
That is even if you can shoot them down at all. An Iron Dome missile goes about 500 mph, which is roughly the speed of a cruise missile. A typical fighter jet can go around 1500 mph or mach 2. A Terminal High Altitude Area Defense (THAAD) system designed to shoot down ICBMs on reentry has missiles that go up to 6,300 mph or mach 8.2!
But the ICBM warheads are going to be entering the atmosphere at around mach 21 or in excess of 16,000 mph. That is absolutely *screaming* fast and it doesn’t leave a lot of time to react. From launch to hitting the target it only takes about 30 minutes.
Quoting myself some months back in response a similar question:
“Practically, shooting down an ICBM is a bit like shooting a bullet with a bullet. There are systems that can do it on a limited scale, but it’s still not hard to deploy countermeasures like decoy missiles/warheads, or simply overwhelm missile defenses with a huge number of missiles. To your question, one nuke might be stoppable depending on where it goes, but are you willing to accept a 25% chance of your capital city being vaporized? Even 10%?
Also important, missile defense is controversial. The whole theme of the Cold War was mutually assured destruction–if you nuke me, I nuke you, we both die, so why bother. If any nation can nuke another without fear of attack because of missile defense, it now has an overwhelming power advantage. Rolling out a serious missile defense system would be considered an aggressive act that attempts to establish nuclear dominance.
ELI5: You and your brother each have a baseball bat, and have agreed not to hit each other because you know everyone will get hurt. If your brother puts on a helmet and says “Don’t worry, this is just for defense,” would you be concerned?”
We can. It’s just very difficult, and unreliable. And even though missile defense sounds like an innocent, altruistic military policy, (“we’re just trying to defend ourselves!”) it’s frowned upon by other nuclear world powers because it makes them think we’ll be tempted to launch a pre-emptive strike and attempt to defend against a retaliatory strike.
And it’s really expensive. Installing enough missile defense systems to secure the continental U.S. would cost a ridiculously impossible large sum of money, and would not even guarantee protection from a nuclear strike.
Nuclear missiles are hard to intercept, because ICBM missiles, which are the kind that has been stockpiled by world powers since the 1950s actually fly up out of the atmosphere then come down VERY fast onto the target. In order to intercept the missile, you have to detect its launch, determine where it is going to land, and send an interceptor to collide with the missile before it hits the ground.
The USA currently has systems that can intercept ICBM missiles. But these systems aren’t 100% reliable. In the event of a nuclear war, an enemy might launch 5,000 nuclear missiles at us. Even assuming that our systems were 99% effective, this would still mean that 50 missiles would get through and blow up 50 cities.
More recently, some countries have been developing alternatives to ICBMs, such as hyper-sonic glide vehicles, or nuclear tipped cruise missiles. These new technologies can’t be intercepted by existing systems meant to guard against ICBMs.
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