Twenty-five years after the demise of President Ronald Reagan’s Strategic Defense Initiative, the Pentagon is once again taking a close look at the possibility of basing missile interceptors in space. Such a project would be technologically feasible, but would strain military budgets and potentially ignite an arms race, analysts say.

The Trump administration’s 2019 Missile Defense Review, released in January, said the Pentagon will “undertake a new and near-term examination of the concepts and technology for space-based defenses” to assess their potential in the evolving security environment.

Assistant Secretary of Defense for Strategy, Plans and Capabilities James Anderson said the analysis, which is expected to be completed later this year, will be comprehensive.

“That will certainly take into account a variety of different factors — the type of different architectures, the potential number of space-based interceptors,” he said during remarks at the Brookings Institution. “It will look at cost, feasibility, practicality, timelines, everything that you would expect from a robust study.”

Having a constellation of space-based interceptors, or SBIs, rather than relying on ground- and sea-based systems, could offer several advantages, defense officials have said.

“As rogue state missile arsenals develop, the space-basing of interceptors may provide the opportunity to engage offensive missiles in their most vulnerable initial boost phase of flight, before they can deploy various countermeasures,” the missile defense review said.

“Space-basing may increase the overall likelihood of successfully intercepting offensive missiles … and potentially destroy offensive missiles over the attacker’s territory rather than the targeted state,” it added.

Undersecretary of Defense for Policy John Rood said systems on orbit could provide “persistent, continuous coverage” and engage missiles “launched by any adversary anywhere on Earth.”

Boost phase intercept in particular is “very attractive” because it “begins to thin out the missile threat before your midcourse and terminal defenses have to deal with it,” he added during a roundtable on Capitol Hill in September, hosted by the Missile Defense Advocacy Alliance.

Intercepting enemy missiles from space would require a number of steps, said Todd Harrison, director of the aerospace security project and defense budget analysis at the Center for Strategic and International Studies.

Sensors such as infrared satellites and terrestrial radars would need to detect the launch and provide precise tracking and trajectory information to a command-and-control apparatus. The architecture would have to automatically calculate which interceptor is going to be in the best position to intercept the missile, and then calculate how it needs to fire its thrusters to divert its trajectory for a successful kill, he explained.

“It would fire, it would start to de-orbit and maneuver towards the missile as the missile is still coming up in flight,” he continued. “Then as that interceptor … gets closer, it’s going to turn on its own sensors and acquire the target missile and then make any final adjustments in its trajectory to make sure that it intercepts.”

All of this would have to be done in a very short timeframe — two to three minutes — to shoot down a missile in boost phase, noted Thomas Roberts, a missile defense expert and program manager at CSIS.

Analysts say developing such a system is technologically feasible.

Harrison noted that the Pentagon already has developed exoatmospheric kill vehicles that ground-launched interceptors can deploy in space.

“We’ve already been working on that for a long time, and every now and then there’s a successful intercept” during testing, he said. “I don’t think it’s a matter of being too technically hard.”

Rebeccah Heinrichs, a missile defense analyst and senior fellow at the Hudson Institute, said a space-based capability is not beyond reach if the political will exists to move forward with it.

“I have received briefings [from defense officials saying] that it is technologically possible,” she said. “Most of the criticisms of it come from the fear that it will be destabilizing or that it will be too expensive, but the technology is not something that is the biggest hurdle here.”

The greatest challenge of creating such a system is the scale required to provide robust coverage of threat areas, analysts say.

“If you invest enough money, you can probably get the kinetic interceptor technology to work, but you’re going to need large numbers … and the costs are going to be very, very high,” said Frank Rose, a senior fellow for security and strategy at the Brookings Institution.

The need for large quantities partly stems from the fact that the weapons would need to be stationed in low-Earth orbit to have enough time to reach enemy missiles before they pass by.

To ensure seamless coverage of a threat area, at least one interceptor must be within range at all times. However, unlike satellites in geostationary orbit, systems in LEO pass over different parts of the Earth as they circle the planet.

“They’re going to move in and out of range pretty quickly for boost phase intercept,” Harrison said.
Roberts noted that each SBI would have a high “absentee ratio.”

“For a satellite in LEO … the vast majority of the time it’s not where it should be” to shoot down a missile from an adversary nation, he noted.

And because the Earth rotates, multiple orbits would be needed to ensure a given region is always covered. To defend against a North Korean missile launch, the United States would need 300 to 400 interceptors in space spread among seven or eight orbits, according to a policy paper by the Union of Concerned Scientists. Covering a larger territory would require even more, it said.

Roberts developed a model showing that a “small” constellation of 98 satellites at a 45 degree inclination could not guarantee boost phase intercept for any region on Earth. A “medium” constellation of 496 systems would ensure at least three to four interceptors would always be within range of North Korea, one to three within range of Iran and zero to four within range of Russia and China.

A “large” constellation of 1,012 systems would ensure at least seven to eight SBIs would be within range of North Korea, two to eight for Iran, and zero to eight for China and Russia. A “mega” constellation of 2,013 satellites would ensure at least 14 to 18 would be within range of North Korea, seven to 16 for Iran, and zero to 18 for China and Russia, according to the model.

The challenge is exacerbated by the fact that gaps would open up in the architecture once SBIs are fired, and they would have to be replaced by backup systems already in orbit or new platforms launched into space, Roberts noted.

Adversaries could also overwhelm a system by firing thicker salvos of missiles, analysts say.

“What it would provoke them to do is have a shot doctrine that says, ‘If you’ve got a satellite constellation that can intercept some [X] number of missiles at once, I am going to fire X plus one,’” Harrison said.

Additionally, the platforms would be vulnerable to anti-satellite weapons, according to experts.

Some analysts question the cost effectiveness of acquiring a robust space-based interceptor layer.

“Such a system would easily become one of the most expensive military projects of all time,” the Union of Concerned Scientists’ policy paper said.

Roberts said the scaling for ground- or ship-based interceptors is linear as the number of missiles that must be intercepted increases, but for the space-based model it’s essentially exponential.

Harrison explained: “If they fire two at the same time, now you’re going to need twice as many [interceptors in the constellation] to always have two within range. What if they fire four? What if they fire 10? … The biggest hurdle is that a boost phase intercept system from space does not scale in a favorable way with the threat.”

Independent cost estimates for notional systems vary widely depending on the scope of the architecture envisioned and other assumptions.

The Institute for Defense Analyses in 2011 estimated a 24-satellite constellation would cost $26 billion over 20 years, while a global constellation of 960 satellites would cost $282 billion.

A 2012 report by the National Academies of Sciences, Engineering and Medicine estimated a system designed to counter North Korean missiles would cost at least $300 billion.

A 2017 report by CSIS put the price tag at $67 billion to $109 billion.

Analysts at the International Institute for Strategic Studies recently estimated the cost of protecting against an attack from North Korea would exceed $100 billion. A more ambitious architecture that provides defense against ballistic missiles launched from anywhere on the globe would be prohibitively expensive, they concluded.

Undersecretary of Defense for Research and Engineering Mike Griffin has pushed back against the higher estimates.

“I get tired of hearing how it would cost $100-or-more billion to put up a space-based interceptor layer,” he said during a roundtable on Capitol Hill. The entire cost of a system with 1,000 SBIs could come in at about $20 billion, he said. “We’ve paid a lot more [for other technologies] and gotten a lot less in the Defense Department over the years,” he added.

Harrison said it’s tough to come up with even a rough estimate of how much a new space-based interceptor program would cost because it would depend on a number of variables including operational requirements.

“Are we just trying to cover North Korea? Are we trying to cover North Korea and Iran? Or are we trying to do something that’s more global?” he asked.

The specifics of the interceptor design, including thruster size and the amount of propellant they would need to carry is another factor, he said.

“All of that goes into figuring out how many satellites you need in the constellation and how much mass you’re going to have to launch into orbit,” Harrison said. “All of those are big cost drivers and you can vary this quite a bit.”

Whether the weapons are intended for boost phase or midcourse intercept would also change the calculus, he noted. “You’ve got to get some parameters on exactly what kind of system are we talking about building, otherwise the costs could be anywhere.”

Heinrichs, who favors developing and deploying this type of technology, noted that a system could be rolled out over time to mitigate some of the affordability challenges.

“We do not have to have a space-based interceptive layer that … right out of the gate has to provide this global defense,” she said. “We can have an initial capability, … adapt it and learn from it and provide just a qualitatively different kind of boost phase [defense] capability … in some parts of the globe. That is a completely cost-effective way to do that.”

However, new space-based weapons could compete for funding with other high priority Pentagon programs. The Defense Department is moving to modernize all three legs of its nuclear triad. Meanwhile, the Air Force is pursuing new fighters, bombers, tankers and trainers; the Army is looking for funding for a new generation of long-range fires, combat vehicles, helicopters, networks and soldier systems; and the Navy is trying to grow to a 355-ship fleet.

“Adding one more new major weapons acquisition program to the mix would just not be fiscally sustainable at this point,” Harrison said.

Within the missile defense portfolio, the Pentagon could get more bang for its buck by investing in ground- and sea-based systems and space-based sensors, he said.

Rose said pursuing space-based interceptors is not a good idea, not only because of the investment required but also the potentially destabilizing effects it could have with regard to the United States’ strategic relationships with China and Russia, which would likely view the technology as a threat.

What conclusions will be drawn from the study and what policy impact it will have remains to be seen. Rose said some powerbrokers appear to be leaning toward building new systems.

“If you look at statements from senior officials in this administration … there seems to be a desire to move toward at least research and development of space-based interceptors,” he said.

For fiscal year 2020, the newly established Space Development Agency requested $15 million to develop and demonstrate a proliferated low-Earth orbit communications and data transport layer and its sub-constellations. The effort is focused on developing a “government reference architecture” for a space-based kinetic interceptor layer for boost phase defense, according to budget documents.

After the Pentagon study is completed, Missile Defense Agency Director Lt. Gen. Samuel Greaves said there will be discussion and debate within the Defense Department, the Trump administration and Congress about what happens next.

If the military’s R&D community is given the green light to build a new system, the focus will be on “ensuring that we have the technology we need to pursue whatever the requirement is, that we adequately work with industry to develop it, we test it, demonstrate it in the lab, demonstrate it on the ground and demonstrate it wherever it needs to be” deployed, he said during a Q&A session at CSIS.

Heinrichs said there is institutional resistance in Washington to the idea, but she believes the United States will eventually deploy space-based interceptors because of the operational benefits they would offer.

“I think we’re going to do it at some point,” she said. “The question is when.”