Missile defense categories
Missile defense can be divided into categories based on various characteristics: type/range of missile intercepted, the trajectory phase where the intercept occurs, and whether intercepted inside or outside the Earth's atmosphere:
Type/range of missile intercepted
The types/ranges are strategic, theater and tactical. Each entails unique requirements for intercept, and a defensive system capable of intercepting one missile type frequently cannot intercept others. However, there is sometimes overlap in capability.
Targets long-range ICBMs, which travel at about 7 km/s (15,700 mph). Examples of currently active systems: Russian A-135 system which defends Moscow, and the U.S. Ground-Based Midcourse Defense system that defends the United States from missiles launched from Asia. Geographic range of strategic defense can be regional (Russian system) or national (U.S. system).
Targets medium-range missiles, which travel at about 3 km/s (6,700 mph) or less. In this context, the term "theater" means the entire localized region for military operations, typically a radius of several hundred kilometers. Defense range of theater defensive systems is usually on this order. Examples of deployed theater missile defenses: Israeli Arrow missile, American THAAD, and Russian S-400.
Targets short-range tactical ballistic missiles, which usually travel at less than 1.5 km/s (3,400 mph). Tactical anti-ballistic missiles (ABMs) have short ranges, typically 20–80 km (12–50 miles). Examples of currently-deployed tactical ABMs: American MIM-104 Patriot and Russian S-300V.
Ballistic missiles can be intercepted in three regions of their trajectory: boost phase, midcourse phase, or terminal phase.
Intercepting the missile while its rocket motors are firing, usually over the launch territory (e.g., American aircraft-mounted laser weapon Boeing YAL-1 [program canceled]).
- Bright, hot rocket exhaust makes detection and targeting easier.
- Decoys cannot be used during boost phase.
- At this stage, the missile is full of flammable propellant, which makes it very vulnerable to explosive warheads.
- Difficult to geographically position interceptors to intercept missiles in boost phase (not always possible without flying over hostile territory).
- Short time for intercept (typically about 180 seconds).
Intercepting the missile in space after the rocket burns out (example: American Ground-Based Midcourse Defense (GMD), Chinese SC-19 & DN-series missiles, Israeli Arrow 3 missile).
- Extended decision/intercept time (the coast period through space before reentering the atmosphere can be several minutes, up to 20 minutes for an ICBM).
- Very large geographic defensive coverage; potentially continental.
- Requires large, heavy anti-ballistic missiles and sophisticated powerful radar which must often be augmented by space-based sensors.
- Must handle potential space-based decoys.
Intercepting the missile after it reenters the atmosphere (examples: American Aegis Ballistic Missile Defense System, Chinese HQ-29, American THAAD, American Sprint, Russian ABM-3 Gazelle)
- Smaller, lighter anti-ballistic missile is sufficient.
- Balloon decoys do not work during reentry.
- Smaller, less sophisticated radar required.
- Very short intercept time, possibly less than 30 seconds.
- Less defended geographic coverage.
- Possible blanketing of target area with hazardous materials in the case of detonation of nuclear warhead(s).
Intercept location relative to the atmosphere
Missile defense can take place either inside (endoatmospheric) or outside (exoatmospheric) the Earth's atmosphere. The trajectory of most ballistic missiles takes them inside and outside the Earth's atmosphere, and they can be intercepted in either place. There are advantages and disadvantages to either intercept technique.
Some missiles such as THAAD can intercept both inside and outside the Earth's atmosphere, giving two intercept opportunities.
Endoatmospheric anti-ballistic missiles are usually shorter ranged (e.g., American MIM-104 Patriot Indian Advanced Air Defence).
- Physically smaller and lighter
- Easier to move and deploy
- Endoatmospheric intercept means balloon-type decoys won't work
- Limited range and defended area
- Limited decision and tracking time for the incoming warhead
Exoatmospheric anti-ballistic missiles are usually longer-ranged (e.g., American GMD, Ground-Based Midcourse Defense).
- More decision and tracking time
- Fewer missiles required for defense of a larger area
- Larger/heavier missiles required
- More difficult to transport and emplace than smaller missiles
- Must handle decoys