In LEO, the impact of medium-sized debris can severely damage or destroy smaller spacecraft or major systems of large spacecraft. Box 4-4 illustrates the destructive force of medium-sized debris traveling at typical LEO collision velocities. In GEO, typical collision velocities are much lower—they are comparable to speeds involved in a midair aircraft collision—so only the largest medium-sized GEO particles are probably capable of causing serious damage.
Hypervelocity impact can cause various modes of damage to spacecraft, including craters, spallations, perforations, and petaled holes and cracks, depending on impact conditions and the configuration of the impacted spacecraft; this damage may result in different failure modes depending on the nature of the spacecraft and the location of the impact. When a piece of medium-sized debris strikes a spacecraft, it can either penetrate the spacecraft's skin or leave a crater on the surface. The impact can cause damage even if it does not penetrate the spacecraft's skin; reflection of the impact's shock wave can cause small particles to spall from the back of the impacted wall. These particles can travel at nearly the velocity of the impacting object, potentially causing serious damage to components inside the spacecraft.
If the impacting debris penetrates the spacecraft's outer skin, its often fragmented or liquefied remnants will travel into the spacecraft and deposit over an area typically significantly larger than the impact hole. The momentum of the impact can cause impulsive damage including buckling and bending of structural components and the transmission of a traveling shock wave through the spacecraft's structure and components. Table 4-1 shows NASA's 1970 assessment of the vulnerability of a spacecraft's subsystems to various modes of hypervelocity impact damage.
The effects of the impact of a 1-cm-diameter aluminum sphere on a
BOX 4-4 Energy of High Velocity Objects
The kinetic energy of an object increases with the square of its velocity. The energy of an object moving at 13 km/s (a typical impact velocity in LEO) is roughly equivalent to the energy released by the explosion of 40 times its mass of TNT. For example, a 1-cm-diameter aluminum sphere (which has a mass of about 1.4 grams) moving at 13 km/s has a kinetic energy equivalent to the energy released by the explosion of 56 grams of TNT (about 0.24 MJ); for a 10-cm aluminum sphere, the equivalent is 56 kg of TNT (about 236 MJ).