||This article relies largely or entirely upon a single source. (January 2010)|
The term hypervelocity usually refers to a very high velocity, approximately 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 so high that the strength of materials upon impact 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 impacts.
Hypervelocity refers to velocities in the range from a few kilometers per second to some tens of kilometers per second. This is especially relevant in 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 Stardust Sample Return Capsule (SRC) reentries as well as the recent 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.
NASA studies the effects of simulated micrometeoroids propelled at speeds in excess of 7.5 km/s at the White Sands Test Facility Remote Hypervelocity Test Laboratory (RHTL). These RHTL micrometeoroid and orbital debris impacts on spacecraft, so that improved protection measures can be taken for NASA, DOD, and commercial space missions. Four Two-stage Light Gas Guns propel single 0.05 mm to 22.2 mm diameter projectiles to velocities in excess of 7.5 km/s.
Hypervelocity reentry events 
|8 September 2004||Genesis SRC||11.04||Crashed (drogue chute failure)|
|15 January 2006||Stardust SRC||12.79||Fastest man-made reentry on record|
|13 June 2010||Hayabusa SRC||12.2||Leading main Hayabusa spacecraft by 6,500 feet (destructive reentry)|