Box 1-4 Degradation Debris From LDEF

A variety of small and medium-sized debris is known to have been created by the degradation of surfaces on the LDEF spacecraft. Several multilayer insulation (MLI) blankets on the space-facing end came partially loose when the Kapton tape holding them to the spacecraft became brittle in the ultraviolet light exposure. Subsequent shrinking of the top face sheets of the MLI blankets stressed the embrittled Kapton tape and caused it to crack, partially releasing the MLI blankets (See et al., 1990; Adams et al., 1991). Several solar array specimens (each of which was approximately 5 cm by 10 cm) also came loose from LDEF. These specimens were mounted on Kapton substrates that were eroded by atomic oxygen exposure. (Whitaker and Young, 1991). The astronauts on board the shuttle visually identified (and filmed) one of the released solar array specimens as they approached LDEF during its retrieval mission. The films from this mission also show a cloud of small particles surrounding LDEF.

an orbiter window was apparently caused by a paint chip smaller than a millimeter in diameter. Subsequent analyses of spacecraft components returned from LEO have confirmed the presence of a large population of paint particles, even though the orbits of individual particles decay quite rapidly.

PERTURBATION FORCES AFFECTING SPACE OBJECTS

Once in orbit, debris is affected by perturbing forces that can alter its trajectory and even remove it completely from orbit. Other than the gravitational attraction of the Earth, the primary forces acting on a space object in lower orbits (below about 800 km) are atmospheric drag and gravitational perturbations from the Earth. These gravitational perturbations, however, although affecting some orbital parameters, do not generally strongly affect orbital lifetime. For space objects in higher orbits, solar and lunar gravitational influences become more important factors. Small debris can also be affected by solar radiation pressure, plasma drag, and electrodynamic forces, although the effects of plasma drag and electrodynamic forces are typically dwarfed by the effects of solar radiation pressure.

The rate at which a space object loses altitude is a function of its mass, its average cross-sectional area impinging on the atmosphere, and the atmospheric density. Although the Earth's atmosphere technically extends to great heights, its retarding effect on space objects falls off rapidly with increasing altitude. Atmospheric density at a given altitude,



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