TABLE 2–1 Predicted Number of Impacts on Orbiter (1997 environment, 400 km altitude, 51.6 degree inclination orbit)

Diameter of Meteoroid or Debris

10-day Mission

400 10-day Missions

>0.04 mm

700

300,000

>0.1 mm

100

40,000

>0.5 mm

1

400

>1 mm

0.09

35

>2 mm

0.008

3

>3 mm

0.002

0.8

>5 mm

0.0005

0.2

>10 centimeters

0.000004

0.002

The flux of meteoroids at shuttle altitudes is comparable to the flux of debris for particles between 0.01 mm and 1 mm in diameter. Above and below this size range, debris are normally more populous than meteoroids. Figure 2–2 compares the modeled flux of meteoroids and debris at the altitude at which the orbiter will visit the International Space Station (ISS).

Once a meteoroid or piece of debris has struck the orbiter, the amount of damage it does depends in large part on the impactor’s composition and velocity, as well as on the composition and thickness of the components that were struck. Meteoroids are typically silica-based, with mass densities on the order of 0.5 grams per cubic centimeter (g/cm3), although meteoroids less than 1 mm in diameter are generally considered to have average densities on the order of 1 to 2 g/cm3. Meteoroids are believed to impact Earth-orbiting objects at average velocities of 19 km/s, although impact velocities can be as high as 70 km/s. Orbital debris can be composed of a variety of materials, such as paint, aluminum, steel, and composites. Steel fragments may have densities of 8 g/cm3, but the densities of paint and composites are more comparable to meteoroids. Aluminum, which is the most common material used in spacecraft, has a density of 2.7 g/cm3. The collision velocities of orbiting objects average about 10 km/s at the shuttle’s altitude and inclinations.

Because of the different characteristics of meteoroids and orbital debris, they will cause different amounts of damage. On average, the impact velocity of meteoroids is twice the impact velocity of debris, but meteoroids are less dense. Meteoroids typically also have lower yield strengths, and the speed of sound in meteoroids is lower than in typical debris. Orbital debris typically causes significantly more damage to a given surface or component than similar-sized meteoroids, primarily because denser objects that are less dispersed by a high-velocity initial impact are better able to punch through spacecraft surfaces and components.



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