The Trump Administration is pursuing a series of secret military space projects that collectively will cost tens of billions of dollars. At least one of those projects is too big to hide: a Space Sensor Layer that would place hundreds of satellites in low earth orbit (LEO) to track hostile hypersonic missiles, and many ballistic threats too.
The Pentagon has been working for decades on methods of intercepting ballistic missiles aimed at U.S. forward-deployed forces, overseas allies, and the American homeland. Longer-range missiles typically are harder to intercept than shorter-range missiles because they move faster, and the ones with intercontinental reach are usually equipped with penetration aids (like decoys) to elude defenders.
What all ballistic missiles have in common, though, is that they follow a stable trajectory that makes their approximate destination easy to estimate. Now a new danger has arisen that threatens to negate the huge investment Washington has made in missile defenses: hypersonic missiles that glide through the atmosphere and maneuver unpredictably at speeds of a mile per second or faster.
Because longer-range ballistic missiles often move faster than the hypersonic systems that Russia and China are developing, the Pentagon already has interceptor systems that potentially could destroy them. What it lacks is a sensor network for tracking the missiles and guiding interceptors to their vicinity.
That’s why the Space Sensor Layer is vital. The current approach to tracking ballistic missiles is for space-based infrared satellites in geosynchronous orbit to detect launches, and then cue terrestrial radar systems to follow them through their trajectories. But that won’t work with hypersonic glide weapons because they fly too low—generally within the atmosphere. Due to the curvature of the earth, surface radars won’t see them coming until they are only seconds away.
The only practical way to overcome this challenge is to look down on the low-flying hypersonic vehicles from above—in other words, from space. Geosynchronous sensors are too far away to precisely characterize hypersonic threats in the atmosphere or ballistic threats coasting through space, so the Pentagon needs a constellation of satellites in lower orbits to do the job.
This is not a new idea. As Global Security.org points out, “a spacebased sensor layer has been a feature of every missile defense architecture for the past five administrations.” But as long as the path of hostile missiles was ballistic, meaning predictable, Pentagon planners were able to convince themselves they didn’t absolutely require a low-orbit sensor network to complement their early warning satellites in geosynchronous orbit (additional early warning satellites are in elliptical orbits over the poles to fill gaps in coverage).
However, with the advent of hypersonic weapons, the need for additional space sensors is becoming urgent. Those weapons still remain largely in the future, but Russia and China are investing heavily in the technology with an eye to negating U.S. defenses. Pentagon planners at the Missile Defense Agency and the Space Development Agency have agreed in principle that low earth orbit (an altitude of 1200 miles or lower) is the optimum location from which to track hypersonic threats—and many ballistic threats too.
What they haven’t agreed on is the precise architecture for merging their respective efforts. The Vice Chairman of the Joint Chiefs of Staff, General John Hyten, has recently complained that too much time is being wasted sorting out who should do what, and how it should be done. Congressional appropriators have also noted a lack of consensus within the Pentagon on how to proceed, and expressed concern about duplication of efforts.
But building a Space Sensor Layer is challenging because any satellites orbited below geosynchronous positions will be traveling at thousands of miles per hour relative to the earth’s surface, rather than appearing to hover above one point. That means just to cover a specific area continuously, dozens of satellites might be needed in an orbital plane to assure that at least one can see what’s happening while the others are over the horizon.
If you want to cover the whole earth, that is going to require hundreds of LEO satellites in multiple orbital planes, plus a transport network for keeping the whole constellation connected and reporting tactical information to defenders in a timely fashion. The cumulative cost, at least until recently, would have been prohibitive.
The good news is that infrared sensors in low earth orbit can be used for other things, such as collecting tactical intelligence and characterizing terrestrial combat. The better news is that recent breakthroughs in satellite design and production might enable the hundreds of spacecraft needed to be built at relatively low cost. Under Secretary of Defense Research and Engineering Dr. Michael Griffin believes that “smallsat” technology, developed largely in the commercial world, could make a Space Sensor Layer affordable.
Once the hurdle of affordable production is overcome, there are distinct advantages to operating in low earth orbit. Launch costs would be modest, and with smallsats multiple satellites could be lofted into orbit on a single rocket. Spacecraft could be replaced frequently as technology advances, and might even be serviced while in orbit.
General Hyten believes the Pentagon should begin orbiting satellites as soon as possible and then proceed to building out the constellation. Dr. Griffin reportedly thinks a rudimentary Space Sensor Layer could be operational by mid-decade. Both agree the military needs to move fast, given the urgency of the threat.
If this were just the military equivalent of a moonshot, it might proceed with dispatch. But the Space Sensor Layer needs to be integrated with the other infrared sensing systems the Pentagon operates or has planned in space, and that architecture in turn needs to mesh with all the other military space activities currently underway, including the secret stuff. So it gets complicated.
This all would be subject to review if President Trump’s bid for reelection falters. Republicans generally have been more supportive of missile defense schemes than Democrats since the closing days of the Cold War. But there seems to be agreement among military leaders that concrete steps must be taken to counter the emergence of hypersonic threats, and the Space Sensor Layer looks essential to fielding a viable solution.