FIGURE 3-1 Number of objects in LEO as estimated from various measurements.

SOURCE: National Aeronautics and Space Administration.

These concentrations of higher spatial density are due to large numbers of objects in near-circular orbits at or near these altitudes. The lower background level of spatial density visible in Figure 3-3 at altitudes up to 40,000 km is due to objects in highly elliptical orbits with perigees in LEO and apogees up to 40,000 km. This background spatial density also exists in LEO, where most highly elliptical orbits have their perigee. Most objects in highly elliptical orbits are either rocket bodies that placed spacecraft in semisynchronous orbit or GEO or objects in Molniya-type orbits. Few objects are cataloged in orbits higher than 40,000 km.

Figures 3-4 and 3-5 indicate the distribution of different types of cataloged space objects by mean altitude. At less than 2,000 km, the majority of cataloged objects are fragmentation debris, but at altitudes between 2,000 and 16,000 km, mission-related debris represents the largest frac-

TABLE 3-1 Approximate Orbital Debris Population by Size

Orbital Debris Size Range

Number of Objects

Percentage of Objects >1 mm

Percentage of Total Mass

Large (>10 cm)

>10,000

<0.5

>99.95

Medium (1 mm-10 cm)

Perhaps tens of millions

>99.5

<0.05

Small (<1 mm)

Trillions

<0.01



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