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Meteoroids and orbital debris impact spacecraft at hypervelocities.
A particle strikes a spacecraft at hypervelocity if the magnitude of the impact
velocity is greater than the speed of sound in the impacted material.
Since most spacecraft structures are aluminum, the lower limit of hypervelocity
is generally considered to be 3 km/s.
A quick overview of the basic physics associated with these types of impacts will
illustrate our need to protect spacecraft from this threat.
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Just like those impacts on the moon, hypervelocity often leaves craters on
the surface of the impacted object. Deep craters in a thin plate can often cause perforations (holes).
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Reflected shock waves can cause internal cracking, or they can propel
detached material from the back of an impacted object at potentially lethal speeds to any astronaut in harm's way.
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Impacts at hypervelocities between metal objects usually cause the
metals to behave like fluids for a short period of time. This phenomena is called hydrodynamic flow.
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One reason metals behave like fluids is that stress waves move
through the metal at supersonic speeds.
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When a projectile impacts and perforates a thin plate, a debris cloud is
is propelled out the rear of the plate, while an ejecta cloud is ejected back out the front surface.
Both clouds contain fragments of both projectile and plate material.
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Meteoroid and debris shields are rated based on the size projectile they can
stop at a given velocity. Ballistic limit curves functionally describe the shielding efficiency
of a material based on its ability to withstand the physical phenomena described above.
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