The United States announced Friday it has successfully tested an unarmed prototype of a hypersonic missile, a nuclear-capable weapon that could accelerate the arms race between superpowers. The Pentagon said a test glide vehicle flew at hypersonic speeds -- more than five times the speed of sound, or Mach 5 -- to a designated impact point. Hypersonic weapons can travel much faster than current nuclear-capable ballistic and cruise missiles at low altitudes, can switch direction in flight and do not follow a predictable arc like conventional missiles, making them much harder to track and intercept. In December, Russia declared it had placed into service its first Avangard hypersonic missile, which it claimed reached speeds of up to Mach 27, making it the first country to claim an operable hypersonic weapon. Last October China displayed its DF-17 hypersonic glide vehicle in its national day military parade.