Ballistic missile penetration aids
- See also: Ballistic missile defense
Ballistic missile penetration aids increase the difficulty of identifying the true warheads and complicate the task of ballistic missile defense. They may be as simple as balloons that enclose the warhead and soon vaporize, or as complex as maneuvering electronic warfare decoys that deceive measurement and signature intelligence sensors. Some decoys release additional decoys, or the warhead itself may release decoys late in its reentry. Penetration aid technology is difficult, and harder for a nation to achieve that to acquire the missiles or technology limited by the Missile Technology Control Regime (MCTR).
The capabilities of penetration aids, their exact signatures that an enemy could use to discriminate them from warheads, and the ability of one side to detect the decoys are among the most highly classified secrets of any nation. U.S. policy guidance generally assumes any details will, minimally, be at the TOP SECRET-RESTRICTED DATA level, and often SENSITIVE COMPARTMENTED INFORMATION.
Penetration aids and the strategic balance
Whether or not to confuse the defense of a nation without BMD is more of a challenge. Especially if the nation has its own nuclear-armed ballistic missiles, it may be dangerously destabilizing for the target country not to understand the nature of the attack directed at it. For example, if an attack is limited, intended for psychological effect (i.e., "we are really serious") and perhaps not even targeting a populated area or military base, the target nation is more prone to wait to counterattack.
If, however, the attack may be a large-scale disarming counterforce attack, the target may decide, with respect to its own land-based ballistic missiles, it faces a choice of "use it or lose it", and may launch an even more destructive response to the attacker.
It sometimes is difficult to draw the line between penetration aids and warheads. Indeed, the real warheads may be configured to resemble a decoy, and vice versa.  The three basic ways in which penetration aids work are:
- Saturation: presenting a large number of (real) targets for defensive interceptors, as with conventional submunitions, or multiple reentry vehicles (MRV), whether the latter simply be separate, independently targetable from midcourse (MIRV) or maneuverable in the atmosphere (MARV);
- Concealment: obscuring a re-entry vehicle (RV), as with stealth or jamming, or mixing the RV with other objects, as with chaff or decoys;
- Evasion: maneuvering the RV to avoid interception, as with terminal guidance and aerodynamic controls. Again, this technology is blurred between warhead and penetration aid. Variations on evasion include techniques that would require significant space technology, such as (in violation of treaties) leaving reentry vehicles in orbit; fractional orbital bombardment systems (FOBS) which fly against the Earth's direction of rotation; or depressed trajectory ballistic missiles, typically fired from submarines, that do not rise to the altitude at which warning radar will detect or track them.
There are other possible aspects of countering ballistic missile defense, such as covert physical or electronic warfare attacks on radars, communications, command and control centers, and interceptors. During the Cold War, one of the major roles of Soviet Spetsnaz special operations forces was attacking offensive missiles and command centers; it is not hard to imagine they could engage defensive systems as well.
- "Missile Wars: Interview, Richard Garwin", PBS Frontline
- Speier, Richard (November 2007), "Missile Nonproliferation and Missile Defense: Fitting Them Together", Arms Control Today