North Korea’s threat to strike Guam with a salvo of ballistic missiles has raised the stakes for a U.S. missile shield some see as compromised by potentially exploitable seams in its all-important space layer.

Years of program changes, delays and cancellations have created gaps in parts of the space-based layer of the missile defense shield meant to protect the United States and some allies from ballistic missile attacks, say military space analysts, although U.S. missile defense officials dispute such claims.

“The biggest deficiency in current U.S. missile defense plans is the absence of a satellite constellation for reliably tracking ballistic threats during the midcourse of their trajectory,” said Loren Thompson, chief operating officer and space analyst for the Lexington Institute think tank based in Washington.

“Once boosters burn out and warheads are coasting through space, their signatures become difficult to detect — especially if an attacker is using penetration aids like decoys to confuse defenders,” he said.

Of mounting concern, said John Pike, military analyst for GlobalSecurity.com, is the possibility that North Korea or Iran will “get more serious” about their submarine-launched ballistic missiles.

If that happens, he said, “the whole thing will require a complete rethink.”

While intercontinental ballistic missiles lofted high over Alaska might give the U.S. time to fire its ground- and sea-based interceptors, Pike said, “All of this is out the window for [sub-launched missiles] coming in from the Caribbean or from tramp container ships,” he said.

The first set of additional assets the U.S. would need to counter such attacks, he said, are more spaced-based infrared sensors.

Those responsible for defending the U.S. from ballistic missile attack disagree there is any gap against existing threats.

“There is no current vulnerability,” U.S. Air Force Lt. Gen. Sam Greaves, the Missile Defense Agency’s director, told SpaceNews. The U.S. he said, “operates a variety of satellite and ground sensors that actively provide early-warning support and that meet the current threat.”

While other U.S. officials said they remain confident in the existing system to protect against current threats, they bemoan the shortage of space-based sensors and systems.

Speaking to reporters in late July, Air Force Gen. John Hyten, chief of U.S. Strategic Command, said space-based sensors are the most urgent need for U.S. missile defense and he called for increased sensor capabilities.

Discussing the issue with reporters Aug. 8 during a break in the annual Space and Missile Defense Symposium in Huntsville, Alabama, Hyten further underscored the need for such sensors and satellites

“That’s what we really need,” he said. “That’s what it’s all about for that piece — midcourse discrimination. You have that sensor there, and you can do a lot with it, but the driving course is midcourse discrimination.”

Sparking all the resurgent interest and debate is the recent ramp-up in North Korean missile-testing and new assessments about the country’s growing nuclear weaponry capability.

On May 14, North Korea boasted of a successful test for its new Hwasong-12 missile, which some analysts warned could place Guam within range of a missile strike. Then, on July 4, the country claimed the first successful test launch of an intercontinental ballistic missile, the steep flight trajectory of which suggested to analysts that Alaska falls within the weapon’s range. Another missile launched July 28 into the Sea of Japan is believed to have sufficient range to reach the continental United States.

In the days since, U.S. intelligence agencies said they believe North Korea finally has a nuclear warhead small enough to fit atop its missiles. That revelation prompted President Trump to warn North Korea would face “fire and fury” if it continued to threaten the U.S.

The regime of Kim Jong Un met Trump’s verbal volley by threatening Guam with an “enveloping fire.” Trump took to Twitter Aug. 11 to warn that “military solutions are now fully in place, locked and loaded,” if North Korea should “act unwisely.”

Trump also said last week he plans to unveil plans soon for adding “many billions of dollars” to the Pentagon’s missile defense budget.

Whether space-based sensors would

receive any of that promised windfall remains to be seen. However, there is no doubt space-based sensors play a major role in missile defense. The geostationary Space-Based Infrared System (SBIRS) satellites train their unblinking eyes on the Earth’s surface, searching for the telltale signature of a missile launch.

“SBIRS wakes up the entire ballistic missile defense system to say, ‘something’s happening,’” Greaves says.

The satellites alert other sensors and platforms to track missiles through boost phase into space. The need for such a space-based pulse will always be needed, officials say.

“Future conflict guarantees increased necessity for space, missile defense, and high-altitude mission,” Lt. Gen. James Dickinson, commander of the U.S. Army Space and Missile Defense Command, said during the Huntsville symposium.

Industry officials also have touted the importance of space-based sensors. “You just get some advantages from being in space,” said Brad Hicks, Lockheed Martin vice president for mission systems and sensors.

The Space Based-Infrared System program hit a road bump recently when the SBIRS GEO Flight 4 launch scheduled for November was delayed until January due, the U.S. Air Force said, to a reallocation of program resources to help the system better perform its mission.

Air Force officials said there would be no impact to the constellation’s launch-detection capabilities because of the delay.

It is the midcourse part of the equation that gives some pause.

When warheads separate from their launch vehicle in space they are hard to track, being relatively small with modest heat signatures, Lexington’s Thompson noted. The defending system needs to be track them precisely to predict their destinations.

It’s not as if the U.S. isn’t working on developing a sufficiently sensored space layer for missile defense.

At the U.S. Government Accountability Office pointed out in a May report, the Defense Department “has spent several billions of dollars” since 1984 “on space-based missile tracking programs — such as Space-Based Infrared System (SBIRS)-Low, Space Tracking and Surveillance System (STSS), and most recently, Precision Tracking Space System (PTSS) — but has yet to operationally field such a capability.”

“There must be some underlying factors that explain the perpetual churn on this front,” Pike said, referring to the many starts and stops that have marked these efforts. Missile defense officials, for their part, said they are still refining the requirements for such missile-tracking satellites.

U.S. missile defense officials initially had big plans for the Space Tracking and Surveillance System, the new name given to SBIRS-Low after the troubled program was transferred in 2001 from the Air Force to the Missile Defense Agency (MDA).

STSS was envisioned as a constellation of 21 to 28 satellites equipped to detect and track missiles, relaying necessary cueing data to other elements in the missile defense system. By orbiting the Earth at low altitudes (about 1,350 kilometers), the satellites would provide better missile-viewing angles and higher resolutions than the SBIRS sensors in geostationary orbit. Each STSS satellite would feature two infrared sensors: an acquisition sensor to watch for bright missile plumes during the boost phase and a tracking sensor to follow the missile through midcourse and reentry.

However, the STSS program would result in the launch of only two demonstration satellites in 2009. The Northrop Grumman and Raytheon-built satellites operate with a 58-degree inclination, with 120-minute orbital period, MDA says.

Greaves said STSS “continue[s] to be a viable MDA asset” even though the two satellites were built to last just four years.

The satellites can detect visible and infrared light and serve as experimental space-trackers for the U.S. missile shield. MDA uses the satellites for missile defense tests

and to reduce the risk in developing a future missile defense space tracker,” it said.

For now, the MDA tasks the satellites to do the following: provide U.S. missile interceptors with accurate tracks of midcourse reentry vehicles; detect boosting targets with an acquisition sensor; autonomously hand off targets to a track sensor; report position and amplitude estimates of 2-D line-of-site targets to the ground; provide accurate 3-D tracking to ground sources and the ballistic-missile shield system.

The satellites, MDA says, have completed on-orbit testing, including demonstrating the fully calibrated performance of both satellites, their crosslink systems, and the acquisition and track sensor payloads. They have also finished the first stereo collection of a birth-to-death missile flight and provided the tracking data to the missile defense system in near real-time. They demonstrated on-orbit receipt of an external cue and the use of the satellite crosslink to transmit that cue to the out-of-view satellite, resulting in stereo midcourse tracking and observation of missile intercept from low Earth orbit.

The STSS satellites also provided tracking data for a ballistic missile defense remote engagement test with a U.S. Navy ship equipped with an Aegis combat system that led to a successful missile intercept.

MDA says it is still looking for the satellites to validate remote sensor and fire control integration to influence the design and operations of the next-generation tracker as well as provide data supporting trade studies and analysis for missile defense systems.

The MDA also expects the constellation to help integrate space capabilities into the ballistic missile defense system as well as to participate in MDA tests.

“The STSS satellites demonstrate MDA space-layer capabilities and reduce risk for future systems,” agency budget documents say. “These two satellites provide valuable risk reduction for threat object acquisition, tracking, and discrimination functionality to include stereo data fusion, cueing radars over the horizon and over-the-horizon fire control.”

Greaves said, “The data we have gathered is being used to support follow-on system development to perform a variety of missions, including space situational awareness.”

The satellites were developed, he said to reduce the risk of developing future MDA tracking and surveillance systems.

“The follow-on system and its capabilities are still being studied,” he said. The MDA did not provide a timeline for such a follow-on system.

“STSS supports other missile defense mission areas when needed,” Greaves said. “The two satellites provide valuable support to MDA programs.”

But despite all the data-collection, testing success and limited mission support, there are only two satellites in low Earth orbit since 2009, out of a constellation that initially was to be about two dozen or more. Thompson, Pike and others contend that simply is not enough coverage, with nothing now to fill in the gaps.

The Precision Tracking Space System was meant to help bolster that space-layered missile defense mission. The proposed space-borne sensor constellation and ground system would close the “fire control loop with the…shooters, specifically the Aegis ballistic missile defense and Ground Based Interceptor weapon systems,” GlobalSecuity.com notes.

The planned constellation also focused on tracking large raids of regional Medium-Range Ballistic Missiles, Intermediate-Range Ballistic Missiles and Intercontinental Ballistic Missiles from today’s regional threats, GlobalSecurity.com says.

The proposed system was canceled in 2013, in part, because of affordability concerns, the Government Accountability Office pointed out in May. “The program was projected to cost as much as $22.5 billion over the life of the program.”

And, as the Union of Concerned Scientists warned in a report last year, that space-tracking layer of U.S. missile defense appears to be quite thin for the foreseeable future.

But the MDA says the current system can shield against any existing threat and the agency is searching for future improvements, including in space-based midcourse tracking. Greaves pointed out the Ballistic Missile Defense Review is underway to further define requirements. The study is due by year’s end.

“As the threat evolves, we have plans in place to develop enhanced space systems,” Greaves said, “We continue to explore, develop, test and refine concepts of operations for the next-generation space tracking system.”

This article originally appeared in the Aug. 14 issue of SpaceNews magazine.