This week saw a dramatic escalation of tensions on the Korean Peninsula. As North Korea promised to engulf the U.S. territory of Guam in "enveloping fire", President Trump tweeted that the U.S. military is "locked and loaded" should North Korea "act unwisely".
The North's missile and nuclear programs have been shrouded in secrecy for years, but recent tests have shed more light on their capabilities. Here is what's currently known.
North Korean missiles can reach the continental United States.
Any ambiguity about the range of the North's Intercontinental Ballistic Missiles was cleared up in July, with two tests of the nation's new Hwasong-14 ICBM. The missiles flew on a trajectory high into space before returning to Earth. Based on the flight time and altitude, it's clear that this missile could reach points along the U.S. West Coast and possibly even farther into the country.
It has a lot of other missiles too.
Over the past two years, the North has conducted dozens of missile tests of various designs. Many are shorter range, such as the Hwasong-12 missiles which the North has threatened to fire toward Guam. The North has also been testing advanced solid-fuel missiles and submarine-launched missiles, with a mix of successes and failures.
Most experts agree that North Korea could have a nuclear warhead small enough to fit on a missile.
A report that the North had "miniaturized" its warheads caused a public furor, but many experts already believed the North has a compact weapons design.
In 2016, the North Koreans released pictures of leader Kim Jong Un standing before a miniaturized nuclear device. Dubbed the "disco-ball of death" by some analysts, the device was probably just a mock-up. But it nonetheless showed the North had general knowledge of how to build a small nuclear weapon.
Also in 2016, North Korea conducted its fifth nuclear test, which it claimed was of a "standardized" warhead that could fit on a rocket. Most experts agree that the North is using the tests to winnow down the size and weight of its nuclear weapons so that they can fit better onto missiles.
"What we have seen for over a decade, is the North Koreans making a dedicated effort to build smaller warheads that can fit on longer-range missiles, and testing the systems to make that possible," says Jon Wolfsthal a former advisor to President Barack Obama who is now works with Global Zero, a group that seeks the elimination of nuclear weapons worldwide. "If I had ten dollars and I had to bet, I would bet they have it."
Other nuclear-armed states have been able to build compact weapons using far fewer tests, notes Jeffrey Lewis, a nuclear policy expert at the Middlebury Institute of International Studies at Monterey, Calif. In the 1960s, "the Chinese did it with two tests," he says.
But its nukes are nowhere near as powerful, or as numerous, as those possessed by other nations.
According to the recent press report, which remains unverified by NPR, North Korea could have up to 60 nuclear weapons. Other estimates put that number anywhere from 10 to 30. Regardless of the precise number (which depends on the speed at which the North can build these weapons) the arsenal is far smaller than those of the U.S. and Russia, which each hold around 4,500 weapons.
The weapon designs that North Korea has tested so far are in the tens of kilotons of explosive yield. That a tenth the power of many warheads in the U.S. arsenal, which depend on nuclear fusion to create a far larger thermonuclear explosion. In fact, North Korea's weapons are in many ways more closely related to the nuclear weapons dropped by America at the end of World War II than they are to modern nuclear warheads.
Of course, those weapons were still vastly more powerful than any conventional bomb.
And how reliable all these weapons are remains unclear.
North Korea's rockets have a spotty track record. The Hwasong-12 it has threatened to lob toward Guam has failed three out of four tests. And it's unclear whether its compact nuclear warheads are able to withstand the forces and vibrations associated with traveling atop a missile.
It's also unclear whether the North has successfully tested a so-called "re-entry vehicle" that could carry a warhead down from the upper atmosphere.
"I do not believe that North Korea has a nuclear warhead small, light and robust enough to fit on an ICBM to reach the U.S. mainland," says Siegfried Hecker, a former nuclear weapons scientist now at Stanford University. "I think it will take more missile tests and more nuclear tests to have any confidence in such a design."
But others disagree. Jeffrey Lewis says that despite some uncertainties, he believes the North's missiles are reliable enough to be taken seriously as a threat. "I think they probably have 60 missiles with nuclear weapons on them," he says. "Some number of which can reach the United States."
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