A penetration aid (or "penaid") is a device or tactic used to increase an aircraft's capability of reaching its target without detection, and in particular intercontinental ballistic missile (ICBM) warhead's chances of penetrating a target's defenses.^[1]

These can consist of both physical devices carried within the ICBM (as part of its payload), as well as tactics that accompany its launch or flight path, operate as either passive or active counters, and may include one or more of the following concepts:

• The missile booster can have a short burn time, and/or (if existing) the MIRV bus carrying the nuclear warheads can have some form of stealth technology, thereby hindering detection before the warhead reentry vehicles are released.
• MIRV and MRV (instead of single warhead missiles) themselves largely improve penetration since there are many more warheads to destroy than missiles, which may saturate the defensive system's stock of weapons. However, these technologies are very demanding since they require the ability to highly miniaturize both the physics package inside, such as the Teller–Ulam design used in most US and NATO "staged" fission–fusion (thermonuclear) weapons, as well as including the warheads themselves and, for MIRVed warheads, to master the art of accurately dispensing each warhead and possibly other payload elements (penaids, etc.) in what is often designated as the post-boost phase or payload deployment phase.
• Incidental or deliberate fragmentation of the final-stage rocket booster can cloud the enemy's radar by projecting a radar cross-section much larger than the actual missile and/or creating a large number of false tracks.^[2]
• Chaff may be deployed over a large area of space, creating a large, radar-reflecting object that will obscure incoming warheads from defensive radar.
• Radar jammers are active radio transmitters that can be deployed on the decoys and the warhead to jam the frequencies used by defensive radars or literally blind them from seeing any warheads around.
• Decoys such as mylar balloons that can be inflated in space and are designed to have the same radar characteristics as the warhead. As the warhead and the decoy balloons may be at different temperatures, the warhead and the balloons may both be surrounded by heated shrouds that put them all at the same temperature. This defeats attempts to discriminate between decoys and warheads on the basis of temperature, which can confuse an enemy's missile defense systems.
• Reentry decoys, consisting of very small reentry bodies that mimic the decelerating trajectory and radar signature of a warhead during atmospheric reentry, force the defense system to spend many interceptor weapons on fake targets instead of warheads.
• Nuclear radar blackout over the target area can be created: in a first-use decapitation strike, one thermonuclear device may be deliberately exploded in space by the attacker, in order to provide a total radar (and partial or total communications) blackout lasting several minutes per detonation (length of time varies by weapon yield) that will allow subsequent warheads and delivery vehicles to pass through the enemy's defenses undetected.
• Maneuvering reentry vehicles MARV (instead of symmetrically shaped warheads) induce lateral drag during reentry and hence strongly bend the trajectory, thus deceiving lower altitude interceptor systems that generally assume a straight decelerating trajectory and which have a limited terminal guidance maneuverability and course correction capability (especially hit-to-kill or conventional warhead interceptors). This has some penalty in terms of decreased attacking warhead accuracy on the target (unless the reentry vehicle has an active guidance and control system on board, which is quite complex to master).

Carrying such devices has a price in terms of payload weight and volume, which requires a compromise versus warhead size and numbers on board, as well as missile range.

• Chevaline
• Stealth aircraft