DECAY — 

natural loss of altitude of a space object culminating in reentry into the Earth's atmosphere. At low altitudes the rate of decay is determined largely by atmospheric density and the object's area-to-mass ratio, but for space objects in highly elliptical orbits, solar-lunar gravitational forces usually drive the rate of decay. Decay may be accelerated by lowering the perigee of an object's orbit.

DEORBIT — 

deliberate, forced reentry of a space object into the Earth's atmosphere by applying a retarding force, usually via a propulsion system.

FRAGMENTATION — 

process by which an orbiting space object disassociates and produces debris. Fragmentation includes such processes as breakup and physical deterioration due to exposure and aging. The planned, controlled, and intentional release of objects (see ''Mission-related Object") is not considered fragmentation.

GEOSYNCHRONOUS EARTH ORBIT (GEO) — 

see entry under "Orbital Regions."

HYDROCODE — 

numerical computer capability to simulate hypervelocity impacts and the structural deformation, changes of state, fragmentation, etc., that result from such impacts.

HYPERVELOCITY — 

relative velocity of two objects that, in general, exceeds the speed of sound in solid materials (about 5 km/s) and results in an impact response that is not dominated by material strength effects.

INCLINATION — 

angle between the orbital plane of a space object and the plane of the Earth's equator.

LIGHT GAS GUN — 

two-stage gun device that uses a highly compressed light gas (such as hydrogen) to accelerate projectiles to typical speeds of 5-10 km/s under well-controlled conditions.

LOW EARTH ORBIT (LEO) — 

see entry under "Orbital Regions."

MISSION-RELATED OBJECT — 

object intentionally released from a spacecraft or rocket body during the course of a mission. These objects normally perform no useful service after release and are sometimes referred to as "operational debris." Examples of mission-related debris include spacecraft-launch vehicle separation and stabilization devices,



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