The Washington Post
What happens in a missile launch:
A plan relies on a promising but unproven system to destroy it with another missile.
See how North Korean nuclear weapons — and U.S. defenses — operate.
North Korea can make a nuclear warhead and has an intercontinental ballistic missile capable of reaching the U.S. mainland. If it launches such a missile, the United States has a $40 billion system designed to destroy the missile in space. What’s unknown is whether it will succeed.
The system, called Ground-based Midcourse Defense (GMD), is a work in progress. It has failed to destroy dummy warheads in six of 10 tests since becoming operational in 2004, but the two most recent tests succeeded. Here’s how it works.
Only an intercontinental ballistic missile could deliver a warhead to the U.S. mainland, which is more than 5,000 miles from North Korea. Rockets would power it in a high, arcing trajectory until gravity pulled its warhead down toward the target.
An ICBM is easy to spot on the way up but hard to intercept because this “boost phase” lasts just a few minutes, and no existing defense system works that quickly. But several ambitious ideas are being developed, such as drones that could zap missiles with lasers shortly after launch.
Infrared sensors on satellites would detect a launch and track the missile’s path by its heat signature until its rockets burn out. Powerful X-band radar on U.S. ships and additional ground radar would track the missile from below. If it appeared to be headed toward the United States, the GMD would be activated.
The ICBM’s trip to the U.S. mainland would take about half an hour, mostly spent outside the atmosphere in the “midcourse” phase. This is where the GMD would try to intercept the warhead.
Burned-out boosters fall away, releasing the warhead, possibly a cloud of debris and any decoys meant to confuse interceptors. Simple aluminized Mylar balloons could be decoys, for instance. Decoys are an example of countermeasures.
After a successful test in May 2017, the Pentagon upgraded its assessment of the GMD, saying it “has demonstrated capability to defend the U.S. homeland from a small number of intermediate-range or intercontinental missile threats with simple countermeasures.”
The military would almost certainly launch two to four interceptors at an incoming ICBM to have a better chance at hitting the warhead. (The GMD has 44 interceptors at installations in Alaska and California.)
Laura Grego, who has extensively researched the GMD for the Union of Concerned Scientists, is among the experts who are skeptical of the system, in part because successful tests have been conducted under controlled circumstances with relatively simple decoys.
“We’re not asking it to defend us in realistic, real-world scenarios,” Grego said, “so we don’t even have the information to judge how well it would do.”
Booster rockets separate as they burn out, leaving the “kill vehicle” to hunt down the warhead. It uses its own sensors and guidance from satellites and ground radar to distinguish the warhead from the decoys and debris.
The latest version of the kill vehicle, which debuted in last May’s test, is one reason some experts are more optimistic.
Tom Karako, director of the Missile Defense Project for the Center for Strategic and International Studies, said the checkered test record shows the progression of an evolving technology. “The systems deployed for GMD today are different than what failed in 2010,” he said.
On-board thrusters steer the kill vehicle into the warhead’s path, shattering the nuclear bomb before it has a chance to detonate. In the May 2017 test, everything went perfectly. But the consequences of less-than-perfect could be catastrophic.
“If North Korea sent six ICBM warheads at the United States and we got five of them, you’d say, ‘Hey, five out of six, not bad,’ ” said Bruce W. MacDonald, former assistant director for national security at the White House Office of Science and Technology Policy. “But if you ended up losing Seattle . . . you’d still feel pretty bad even though it was over 80 percent effective. That’s what you’re dealing with.”