Space is the place for a variety of missile defense tasks — including launch detection, tracking, discrimination, intercept, and kill assessment.
Ballistic missiles travel in space, and the missile defense task is by definition largely a challenge in and through the space domain. For all but very short range missiles, a considerable part of the ballistic trajectory is spent in space, after the motors burn out and before the warhead re-enters the atmosphere. Exoatmospheric midcourse intercept is the exclusive realm for two of the four currently deployed U.S. missile defense programs, Aegis Standard Missile-3 and Ground-based Midcourse Defense.
Space is thus the place for a variety of missile defense tasks — including launch detection, tracking, discrimination, intercept, and kill assessment. Space-based sensor concepts have been underway since the beginning of the missile age, from the early Missile Defense Alarm System, to Brilliant Eyes, to more recent efforts such as the Space Tracking and Surveillance System demonstrators. It is therefore unfortunate that U.S. funding for space- and near-space missile defense assets is at an all-time low. It may be time to reverse that trend and renew efforts for a space sensor layer. The concept of a “layered” defense applies, after all, not just to interceptors, but also to sensors.
To intercept a missile in its midcourse phase, one must detect its launch, track its flight, and then differentiate or “discriminate” the threatening warhead target from any countermeasures and from the flying junk pile of debris created from launching it. For launch detection and warning, the United States relies on the 1970s-era Defense Support Program, two Space-Based Infrared System-GEO satellites, and two highly elliptical SBIRS payloads. Tracking tells the interceptor and other sensors where to look, and discrimination determines what interceptors need to kill. In the 1960s and 1970s, the way to compensate for discrimination shortcomings was with nuclear-armed interceptors, which besides frying satellites within line of sight would also damage the defenders’ own radars. Nobody wants to go back to that, and missile defense efforts have for decades focused on hit-to-kill.