The term hypervelocity usually refers to a very high velocity, typically over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, it refers to velocities sufficiently high that the strength of materials is very small compared to inertial stresses. Thus, even metals behave like fluids under hypervelocity impact. Extreme hypervelocity results in vaporization of the impactor and target. For structural metals, hypervelocity is generally considered to be over 2,500 m/s (5,600 mph, 9,000 km/h, 8,200 ft/s, or Mach 7.3). Meteorite craters are also examples of hypervelocity impact.
Hypervelocity tends to refer to velocities in the range of a few kilometers per second to some tens of kilometers per second. It is especially relevant to the field of space exploration and military use of space, where hypervelocity impacts (e.g. by space debris or an attacking projectile) can result in anything from minor component degradation to the complete destruction of a spacecraft or missile. The impactor, as well as the surface it hits, can undergo temporary liquefaction. The impact process can generate plasma discharges, which can interfere with spacecraft electronics.
Hypervelocity usually occurs during meteor showers and deep space reentries, as carried out during the Zond, Apollo and Luna programs. Given the intrinsic unpredictability of the timing and trajectories of meteors, space capsules are prime data gathering opportunities for the study of thermal protection materials at hypervelocity (in this context, hypervelocity is defined as greater than escape velocity). Given the rarity of such observation opportunities since the 1970s, the Genesis and the recent Stardust Sample Return Capsule (SRC) reentries as well as the upcoming Hayabusa SRC reentry have spawned observation campaigns, most notably at NASA Ames Research Center.
Hypervelocity collisions can be studied by examining the results of naturally-occurring collisions (between micrometeorites and spacecraft, or between meteorites and planetary bodies), or they may be performed in laboratories. Currently the primary tool for laboratory experiments is a light gas gun, but some experiments have used linear motors to accelerate projectiles to hypervelocity.
The properties of metals under hypervelocity have been integrated with weapons, such as explosively formed penetrator. The vaporization upon impact and liquefaction of surfaces allow metal projectiles formed under hypervelocity forces to penetrate vehicle armor better than conventional bullets.
The White Sands Test Facility is a NASA facility that performs, among other tests, hypervelocity impact experiments to study the effect of micrometeorite and orbital junk impacts on spacecraft, so that improved protection measures can be taken for its space missions. The facility uses two-stage light gas guns to accelerate projectiles to extreme velocities.