BOX 4-2 Determining Collision Velocities

Orbital velocities are directly related to altitude—objects in lower-altitude orbits move faster than objects in higher orbits. Orbital velocity for circular LEO orbits varies from almost 8 km/s (skimming the top of the atmosphere) to about 7 km/s (at 2,000 km). In GEO, orbital velocity is about 3 km/s. The velocity of objects in elliptical orbits varies throughout their orbits. At their perigee, they are moving faster than the local circular orbital velocity, and at apogee, they are moving slower than the local circular orbital velocity.

Impact velocities for objects in circular orbits can vary from nearly 0 km/s for an object striking another object in virtually the same orbit to twice the orbital velocity for a head-on collision. (Collisions with objects in elliptical orbits can occur at even higher velocities.) As the angle at which the two objects' orbits intersect increases toward 180 degrees, so does the collision velocity. If the orbits of the two objects intersect with an angle of greater than 60 degrees, the relative collision velocity will be larger than the objects' orbital velocity.

The impact velocity distribution of the debris flux on a space object is thus influenced by its orbital altitude, eccentricity, and inclination, as well as the eccentricity and inclination distribution of objects in intersecting orbits.

FIGURE 4-9 Calculated collision velocity distribution versus inclination for cataloged objects in LEO (averaged over all LEO altitudes).

SOURCE: Calculated from Kessler et al., 1989.

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