28 June 2018
The United States has plans to develop two new missile defense programs in
the space domain: a space-based sensor architecture and a space-based
missile intercept layer. Both proposed systems rely on a network of
satellites in low Earth orbit to offer full or partial coverage of the
Earth’s surface, precisely tracking a missile during its flight in one
case, or shooting it down entirely in the other. A space-based sensor
system could expand current capabilities for monitoring missile launches
and warrants further study. The deployment of a space-based missile
intercept layer, however, would require launching hundreds or thousands of
weapons into space – an expensive, inefficient, and provocative idea. The
technical discussion surrounding space-based interceptors should be
decoupled from that of space-based sensors – a much more plausible
proposal. Despite decades of support from influential policymakers, the
resources required to deploy space-based interceptors would be better
spent on other layers of US missile defense.
In one way or another, US missile defense systems are heading toward space. The 2018 National Defense Authorization Act (NDAA) passed late last year outlined plans to develop two new missile defense programs in the space domain: a space-based sensor architecture and a space-based missile intercept layer (Conference Report 2017). Both proposed systems rely on a network of satellites in low Earth orbit to offer full or partial coverage of the Earth’s surface, precisely tracking a missile during its flight in one case, or shooting it down entirely in the other. A space-based sensor system could expand current capabilities for monitoring missile launches and warrants further study. The deployment of a space-based missile intercept layer, however, would require launching hundreds or thousands of weapons into space – an expensive, inefficient, and provocative idea.
While the two plans depend on similar space architectures, they come with vastly different price tags and enjoy different camps of support among missile defense advocates in Washington.
Space-based sensors vs. space-based interceptorsThe space-based sensors proposed in the defense authorization act would be designed to precisely track a missile’s trajectory from space and discriminate between a warhead and other objects an attacker might use to obscure the warhead during flight, including debris, decoys, or other countermeasures. Currently, the United States operates several space-based infrared sensors in geosynchronous orbit – including the Defense Support Program (DSP) and the Space-Based Infrared System (SBIRS) – but those systems are only capable of tracking missiles during boost phase, when a hot plume is visible from space. When a missile enters its midcourse phase, after burnout, infrared sensors lose track of it. Ideally, a new system of space-based sensors could work together with ground- and sea-based radars already in operation to pinpoint a missile’s warhead mid-flight and reduce the number of interceptors needed to defend against the attack (Karako 2017). Whether stationed on Earth or on a satellite in low Earth orbit, radars require an unobstructed line of sight to the target in order to function. Space-based sensors orbiting hundreds of kilometers above the Earth’s surface can see further than those near sea-level, and therefore can offer more coverage to threat regions.
Space-based sensors have been proposed by Republican and Democratic administrations over the past 30 years, but no comprehensive system has yet been deployed. In the past year, both Lt. Gen. Samuel Greaves, the director of the Missile Defense Agency (Dowd 2018), and Gen. John Hyten, Commander of US Strategic Command, have also supported the development of such a system, urging quick action in an increasingly contested missile threat landscape (Freedberg 2018).
Kinetic hit-to-kill space-based interceptors (SBIs), on the other hand, are weapons in space designed to descend from orbit and strike a missile during flight. Like a low-altitude space-based sensor architecture, an SBI layer has also never been deployed. Due to their potential ability to intercept missiles during boost phase – the first phase of flight, while the missile’s engines are still firing and the warhead remains inside the rocket body – SBIs are a particularly attractive option to some missile defense advocates. Because kinetic hit-to-kill SBIs need to physically intercept their targets, as opposed to just observe them remotely, they need to be much closer than space-based sensors to work. Using current technologies, interceptors must be within a few hundred kilometers of a target missile to ensure they have enough time to maneuver towards the target missile before the end of the boost phase.
Since they were first proposed in the 1980s, plans for SBI development have primarily surfaced in Republican administrations, with the current administration being no exception. SBIs also see consistent support from Republicans in Congress. In February of this year, Sen. Ted Cruz of Texas wrote a letter to Defense Secretary James Mattis urging the 2018 Missile Defense Review (MDR) to “recommend the development and eventual deployment of a space-based missile defense layer.” Similarly, 15 House Republicans – including Mike Rogers, Mike Turner, and Elise Stefanik – wrote to Vice President Mike Pence to urge him to make sure the MDR “comes out strongly, and unequivocally, in favor of the boost phase missile defense plan and space-based missile intercept layer plan” (Gould 2017). Some military leaders, however, have shown less support. When asked by Cruz at a Senate Armed Services Committee about the path forward on boost-phase missile defense in space, Hyten replied: “I am convinced that space-based sensors are required. I am not convinced at this time that the space-based interceptor is required”(Hyten 2018).
Due to the limited time frame in which a missile is in boost phase – about three minutes for a solid-fueled intercontinental ballistic missile (ICBM) – SBIs face a simple, but significant technical challenge: being in the right place at the right time (Barton 2004. Due to this constraint, kinetic hit-to-kill SBIs must be placed in low Earth orbit, since higher altitudes would leave the interceptors too far away to reach a missile while it is still in boost phase. A space-based interceptor layer would therefore need to be composed of a dense constellation of satellites in order to offer full- or even partial-Earth coverage.
Satellite constellationsAlthough satellites can be organized into constellations using infinitely many orientations, the most frequently cited models are those by Walker, Ballard, and Rider. Some of the most common types of constellations – used by GPS, Iridium, and Globalstar communications systems – are Walker’s star and delta constellations.
A star constellation features satellites in equally spaced, polar or near-polar orbital planes, which together can offer full-Earth coverage. Satellites in polar orbits pass over the north and south poles. Star constellations offer their densest coverage at the Earth’s poles and their sparsest coverage at the Earth’s equator. The Iridium satellite constellation – a commercial network of 66 communication satellites – utilizes this orientation to achieve full-Earth coverage (Iridium Iridium Global Network 2018).
A delta constellation features satellites in equally spaced, inclined orbits. Inclined orbits do not pass over the Earth’s poles, covering only lower latitudes. Therefore, unlike star constellations, delta constellations leave the Earth’s poles with little to no coverage. Delta constellations can more easily provide regional focus, covering a band of the Earth’s surface, centered on the equator. The Globalstar constellation – another network of commercial communication satellites – uses this type of design to cover 80 percent of the Earth’s surface (Globalstar 2018).
In the 2018 defense authorization act, the section outlining the plan for development of a space-based interceptor layer required the system to be “regionally focused” (Conference Report 2018). Since star constellations can only focus on the poles, the delta constellation would likely be a more appropriate choice for an SBI layer.
Modelling space-based interceptors
A computational model can be developed to study the effectiveness of a space-based missile interceptor system oriented in a delta constellation by using the assumptions outlined in a 2004 American Physical Society (APS) study on SBIs (Barton 2004). In the APS study, each hypothetical interceptor is capable of intercepting a missile inside a spherical region surrounding the interceptor. The size of the region – known as the kill radius – is dependent on the acceleration profile of the interceptor, its terminal velocity, and the response time between the interceptor firing and the moment it reaches its target.
For a missile to be intercepted in boost phase, it must pass within an interceptor’s kill radius before the booster burns out and the warhead is jettisoned. If multiple missiles are launched at once from the same location, then each warhead must pass within a unique interceptor’s kill radius in order to be destroyed.
Using the most generous values from the APS study for an interceptor’s maximum acceleration, terminal velocity, and response time, a kill radius of approximately 850 kilometers can be calculated (Visit aerospace.csis.org/sbi for more information about the parameters outlined by the American Physical Society, and the kill radius calculation.) With a kill radius of this size, a low-altitude, space-based missile defense system would need hundreds or thousands of interceptors in orbit to effectively defend against missile attacks from threat regions across the globe.
Due to the rotation of the Earth, delta constellations provide similar coverage at every location along an individual latitude. They also provide identical coverage to the northern or southern hemispheres. Thus, the “regional focus” requested in the 2018 NDAA is limited. A constellation specifically designed to defend against attacks from North Korea would also be well-equipped to defend against a missile launch from New York or Tasmania, regions where the United States would otherwise not invest in missile interceptors.
The figure above is a two-dimensional map
indicating the boost-phase missile defense provided by a
496-interceptor constellation, organized in a delta pattern with
45 degrees inclination. Such a constellation effectively defends
against missiles launched from North Korea, with a minimum of
three or four interceptors constantly within range. The
constellation provides less coverage to Iran – where a minimum of
one or two interceptors is always within range – and almost no
coverage of Russia. Learn more about the coverage provided by
space-based interceptor constellations at