North Korea's rapid march to develop a nuclear-armed ballistic missile capable of striking the United States has spurred the U.S. military and Congress to ramp up efforts to counter the threat.
The U.S. technological race is happening on the ground, at sea, in the air and in space. But military planners say the greatest benefit of the massive missile defense effort is to deter North Korea from contemplating a strike.
"Missile defense buys you time and opens windows," said Todd Harrison, director of the Aerospace Security Project at the Center for Security and International Studies. "The way you protect yourself from a missile attack is through deterrence. You show your adversary that you can hold them off and strike back at them."
© Korean Central News Agency/Korea News Service via APThis photo distributed by the North Korean government shows what was is said to be the launch of a Hwasong-14 intercontinental ballistic missile, ICBM, on July 4, 2017. Independent journalists were not given access to cover the event depicted in this photo.
North Korea's latest missile launch on July 4 was its first intercontinental ballistic missile. The Hwasong-14 had a maximum range of about 4,163 miles, meaning it could hit targets in Alaska but not the contiguous U.S. mainland or the larger islands of Hawaii in the Pacific Ocean.
Surveillance of the missile left unclear whether it successfully re-entered the earth's atmosphere, South Korea's Yonhap news agency reported.
The North Korean government said its missiles can hit anywhere in the world with a nuclear warhead, but the U.S. government doubts the regime of Kim Jong Un has developed a miniaturized warhead or delivery vehicle needed to accomplish that.
North Korea may be only a year or so away from that feat, according to U.S. estimates, which is why the Pentagon is stepping up its anti-missile program.
Last Tuesday, the U.S. military successfully intercepted a simulated intermediate-range ballistic missile using the Terminal High Altitude Area Defense (THAAD) system similar to one being deployed in South Korea.
The test was the first by THAAD against a missile that is faster and more difficult to target than shorter-range missiles.
In another first, the U.S. Missile Defense Agency used a ground-based interceptor launched from a silo in Vanderberg Air Force Base in California to successfully shoot down a U.S.-launched mock intercontinental ballistic missile fired from Kwajalein Atoll in the Pacific in May.
The U.S. currently has 36 such interceptors deployed and plans to have 44 in place by the end of 2017, based at Vandenburg and in Fort Greely, Alaska.
Congress in 2013 required the Defense Department to research a third site for ground-based interceptors to defend the U.S. East Coast, in addition to the silos in Alaska and California.
This year, House Republicans proposed that the Pentagon conduct research and development on space-based missile defense interceptors - a version of the "Star Wars" system that President Ronald Reagan championed in the 1980s as a deterrent against Soviet nuclear missiles.
Also this year, Defense Secretary James Mattis ordered the Pentagon's Missile Defense Agency to review the nation's overall missile defense strategy.
North Korea's nuclear-capable missile arsenal includes an estimated 1,000 missiles, plus hundreds of thousands of conventional rockets aimed at U.S. and its allies' military and civilian targets in South Korea, Japan, Guam and at sea in the region.
Arrayed against that force is a layered defense of short- medium- and long-range interceptors. These systems are being upgraded to make them faster, with more range and greater accuracy.
Here is what else is in the works:
Next generation satellites
Current satellite technology recognizes a missile launch and a general "fan-shaped" area that it is likely to target, said retired lieutenant general Henry "Trey" Obering III, a former head of the Missile Defense Agency who is now executive vice president at Booz Allen Hamilton.
Satellites "do not provide precision tracking and targeting today," Obering said.
The Missile Defense Agency plans to launch a "constellation" consisting of multiple small satellites. These would augment a series of ground-based monitors and provide enough tracking information to target threatening missiles while they are outside the atmosphere with one of the military's interceptors.
The new satellites would enable multiple attacks on the same threatening missile if necessary, Obering said.
Multiple-warhead kill vehicles
The Missile Defense Agency also is developing multiple kill vehicles that would allow each ground-based interceptor to attack multiple missile threats, said Kingston Reif, director for disarmament and threat reduction policy at the Arms Control Association.
The drawback of such efforts, Reif said, is that they could influence the types of weapons that far more powerful adversaries Russia and China develop. That would lead to an "increased risk of arms racing," he said.
The military is researching the use of chemical rockets or lasers that would fire at missiles from orbiting satellites.
According to a study by the Center for International and Security Studies, such a scheme would require at least 30 satellites for an area the size of North Korea because the satellites would only be in range for a short while on each low-altitude orbit.
Each satellite could be configured to carry multiple rockets and to defend itself from anything North Korea would use to try counter it, Obering said. "Eventually these space-based interceptors can be replaced with a laser," he said.
Obering, who heads the directed energy team at Booz Allen Hamilton, led the Missile Defense Agency in 2010, when it used a chemical laser carried on a Boeing 747 to shoot down a missile in a test.
That program ended because the Defense Department judged it to be impractical: The laser's effective range was too short, the aircraft flew too low, and the program was expensive.
New solid-state electric and hybrid electric-chemical lasers are now smaller, more powerful and lighter, and can be carried on high-altitude drones that can patrol at 60,000 feet above North Korea for days during a crisis, Obering said.
The U.S. is about five years from developing such a weapon, which could attack North Korean missiles in the most vulnerable boost phase, when they're moving relatively slowly and have yet to separate into multiple parts, he said.
"It's based on how much money we're putting into that program," he said.