Editor's Note: This story was updated on Nov. 28 at 7:00 p.m. E.T.
With tensions brewing between the United States and North Korea — highlighted by a flurry of nuclear missile tests and fighting words by both countries — the possibility of nuclear war seems closer than it has been in years, according to experts.
Pentagon officials announced today (Nov. 28) that North Korea conducted a nuclear test launch of an intercontinental ballistic missile with the range to reach Washington D.C. South Korean officials believe its hostile neighbor to the north could have the ability to pair such a missile with a nuclear warhead sometime in 2018, CNN reported.
Though North Korea doesn't currently have the ability to nuke the United States capital, the mere possibility of a nuclear attack put people around the world on edge. In the event that North Korea did decide to attack the states, is there any way to stop nuclear missiles once they've been fired? [7 Strange Cultural Facts About North Korea]
One option that has been floated — and refloated — over the years, is to somehow create a shield or defense system to protect people from nuclear attacks. From the earliest uses in 1959 of the intercontinental ballistic missile (ICBM), which is designed to deliver nuclear weapons, the U.S. has been working on methods that would protect people from such an attack. Yet decades later, the country still has only a flawed system that most experts believe would not reliably protect Americans against a nuclear attack, said Philip E. Coyle III, a senior science advisor with the Center for Arms Control and Non-Proliferation and the former director of operational tests and evaluation with the Pentagon, who has extensively evaluated missile defense systems.
But why has it taken so long to get a nuclear missile shield up and running? And is there any possibility that this technology might work in the future?
"This is the hardest thing the Pentagon has ever tried to do, as our nearly 70 years of trying shows," Coyle told Live Science.
The first attempts at building a nuclear missile defense program started up almost as soon as intercontinental missiles were invented in the 1950s, though most of those projects were put on hold in 1972, after the U.S. and the Soviet Union signed the Anti-Ballistic Missile Treaty, which limited the number of missiles each side could retain. A number of wacky ideas have been proposed over the years, including Operation Argus, which aimed to create a protective radiation belt above Earth by detonating a nuclear weapon in the atmosphere, and Project Seesaw, which explored using particle beams to zap nukes, according to "The Imagineers of War: The Untold Story of DARPA, the Agency That Changed the World," (Knopf, 2017)
In the 1980s, President Ronald Reagan said he was uncomfortable with "mutually assured destruction" (that is, the idea that both the United States and Russia had enough nuclear weapons to destroy each other in the event of a nuclear war) as the only protection against the U.S.S.R. He pushed for the development of the Strategic Defense Initiative, or the Star Wars program, in which nuclear-powered lasers placed in space would zap nuclear weapons. The program was an expensive flop, in part because the whole concept was too fantastical, said Laura Grego, an astrophysicist and expert on missile defense and space security at the Union of Concerned Scientists. [Top 10 Ways to Destroy Earth]
Challenges to nuclear missile defense
In some ways, the failure of these projects isn't surprising: Intercepting an intercontinental ballistic missile is really hard, Grego said. An ICBM launches, spends 15 minutes traveling through the vacuum of space and then reenters the atmosphere before hitting its target. So an ICBM could be intercepted at just a few points on its journey: when it first launches, once it's out in space, and as it reenters the atmosphere and is zooming toward its target. Each of these approaches has its limitations.
For instance, "the launch phase is a minute to a few minutes long," Grego told Live Science.
That doesn't leave much time for a rocket to intercept and "kill" a nuclear missile, she added. What's more, historical United States rivals, such as Russia and China, have large land masses. They would likely keep their missiles far inland, meaning sea-based interceptors couldn't get to a missile during its launch phase.
So killing a missile early in the course of its flight would require hovering over likely launch sites, Grego said. Early on, the military proposed placing giant Boeing 747s with bomb-killing lasers in the skies above Russia and China.
"Pretty quickly, you can see the operational difficulty with that," Grego told Live Science. "Are you going to have several large 747s just hovering indefinitely for decades, just waiting for something to happen?"
Beyond that, there are other problems with the "launch-phase" approach. If the interceptor doesn't hit exactly the right spot on the missile, the missile "may not quite make the target it was intended. It will fall somewhere else, like Canada, which Canada will not like," Grego said. "You really have to be explicit and target the payload at the tip of the missile."
Using unmanned aerial vehicles has also been as an option, but they lack the firepower to destroy a missile, she added.
The second option, and the most viable one, is to intercept the missile during its longest flight course — in space. An advantage of that approach is that, because most U.S. enemies are west of the Pacific, they would all likely program their missiles to take a path above the poles, meaning that just one ground-based interceptor could be placed in Alaska and likely protect the whole country.
But intercepting a missile in space also has its problems.
"The incoming missile is going 15,000, 17,000 miles an hour [24,000 to 27,000 km/h]," Coyle said. "And going that fast, if you miss by an inch, you can miss by a mile."
There's another problem, too: There's no air resistance (or drag) in space. That means a decoy like a balloon that's shaped like a nuclear warhead could travel in the same way as the true warhead, making it difficult for a missile to distinguish the real missile from the decoy. And because balloons are so light, a sophisticated warhead could easily launch 20 or 30 decoy balloons to obscure the path of the warhead, Grego said.
Finally, the last-ditch effort would be to intercept as the missile reenters the atmosphere, before it hits the target. An advantage of this approach would be that air resistance would prevent decoys from distracting a system. On the other hand, "you don't have very much time to defend, because it's rapidly coming toward you, so it's not a workable strategy," Grego said. And jamming the electronics in nuclear warheads with something like an electromagnetic pulse (EMP) would likely not work; the weapons are designed to be robust enough to survive the effects of EMPs from other nuclear weapons nearby, Grego said. [Flying Saucers to Mind Control: 22 Declassified Military & CIA Secrets]
As a result, the military has in recent decades focused on attacking an ICBM during its midcourse, known as ground-based midcourse missile defense. The military developed a prototype under the Clinton administration that saw early success. But under Bush, the military pushed the weapon from an early prototype and rushed it to operational status. Since then, it has missed the target in 9 out of 17 tests, according to the military.
Between 2010 and 2017, it has missed the target in 3 out of 4 tests. (However, in late May the U.S. military announced it had achieved a successful test of the mid-course missile defense system.)
"The failure in flight-intercept tests is all the more surprising, because these tests are highly scripted to achieve success. If these tests were planned to fool U.S. defenses, as a real enemy would do, the failure rate would be even worse," Coyle said.
What's more, "it counts one of those failures a success if the interceptor hit the target with a glancing [blow] but did not destroy it," Coyle said. "Close only counts in horseshoes and not in nuclear war."
Part of the problem is that the systems were rushed through the engineering process and suffer from design flaws, both Coyle and Grego said. In addition, the military needs to develop additional technology infrastructure, such as radar in different wavelengths, or better satellites to detect missiles, that could do a better job of locating and visualizing the target.
But even if the projects were redesigned from the ground up, with careful thought and the best use of existing and new technologies, some challenges with nuclear defense may be insurmountable, Grego said. For instance, so far, no one has come up with a way to solve the problem of nuclear warhead decoys in space, she said.
And focusing on "strategic defense" that can protect American cities half the time may be much more expensive and ultimately more dangerous for the world, compared to using those resources for more effective war-deterrence strategies such as diplomacy, Grego said.
Editor's Note: This story was originally published on May 2, 2017. It was updated to add new information on North Korea's successful test launch of an intercontinental ballistic missile capable of reaching the U.S., along with additional information on the U.S. mid-course missile defense tests conducted in May.
Originally published on Live Science.