specular trail—trail plasma measured when a meteoroid is traveling perpendicular to the line-of-sight of a radar.

sporadic meteoroid—meteoroid not found in a stream. Although such meteoroids do not have a clear common parentage, the sporadic background meteoroid population as detected at Earth shows strong directionality reflecting the general orbital properties of the parent body population.

SSP 30435—Space Station Program Natural Environment Definition for Design; includes the environmental models used to design the space station as they were defined in 1994.

state vector—provides the location, velocity, and future course that an object will take in space using a 3-dimen-sional frame of reference.

stream meteoroid—meteoroid with a common, recent (less than 100,000 years) parentage, resulting in all meteoroids associated with the stream following nearly identical heliocentric orbits.

Standard Breakup Model (SBM)—semi-empirical model that determines the number, mass, velocity, and ballistic coefficient distributions of fragments down to 1 mm produced from a breakup event.

terminal velocity—the constant, maximum speed that a falling object reaches when the downward force of gravity equals the upward force of drag.

trajectory—the path taken by a falling object.

two-line element set (TLE)—a data set made up of two 69-character lines that enables deducing the position and velocity of a satellite when used in collaboration with NORAD’s SGP4/SDP4 orbital model.

uncertainty—lack of knowledge and understanding of the structure of a risk and the connections between the stages of the evolution of a risk.

VAX computer—Virtual Address eXtension computer.

Whipple shield—a type of hypervelocity impact shield used to protect crewed and uncrewed spacecraft from collisions with small particles, typically consisting of two metallic or non-metallic panels placed a small distance apart from each other.

zodiacal light—a dim cone of light produced by scattering of sunlight off interplanetary dust. It can be seen in the night sky above the horizon just before or just after sunset.

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