A brief glossary of terms used in space-based ballistic missile defense systems

Please refer to a page one story today on ''War in Space, Part 2.'' AIRBORNE OPTICAL ADJUNCT: A set of sensors designed to detect, track, and discriminate an incoming warhead. The sensors are typically optical or infrared devices flown in aircraft stationed above clouds.

BOOST PHASE: The portion of a missile's flight during which the payload is accelerated by large rocket motors.

DIRECTED ENERGY: Energy in the form of particle or laser beams that can be sent long distances at nearly the speed of light.

ELECTROMAGNETIC GUN: A gun based on the idea of an open solenoid. The projectile is accelerated by electromagnetic forces rather than by an explosion.

HYPERVELOCITY GUN: A gun that can accelerate projectiles to 5 kilometers per second (1,118 miles per hour) or more; for example, an electromagnetic (or rail) gun.

INFRARED SENSOR: A sensor to detect the infrared radiation from objects such as a missile reentry vehicle.

KINETIC ENERGY: The energy from the momentum of an object.

LASER: A device that amplifies light, and emits it in a narrow, highly concentrated beam.

LEAKAGE: The percentage of warheads that get through a defensive system intact and operational.

MIDCOURSE PHASE: The period of a warhead's flight after it has been dispensed from the post-boost vehicle (''bus'') until it reenters the atmosphere.

PARTICLE BEAM: A type of directed energy device that emits a stream of atoms or subatomic particles (electrons, protons, or neutrons) accelerated to nearly the speed of light.

PENETRATION AIDS: Methods to defeat defenses by camouflage, deception, decoys , and countermeasures.

SIGNAL PROCESSING: A computer system's capability to organize the raw data received from many different sources.

TERMINAL PHASE: The final phase of a ballistic missile trajectory, during which warheads and penetration aids reenter the atmosphere.

THREAT CLOUDS: Dense concentrations of both threatening and nonthreatening objects. The defense must distinguish between them.

We want to hear, did we miss an angle we should have covered? Should we come back to this topic? Or just give us a rating for this story. We want to hear from you.