They are normally defined as fast, low-flying, and highly maneuverable weapons designed to be too quick and agile for traditional missile defense systems to detect in time. Unlike ballistic missiles, hypersonic weapons don’t follow a predetermined, arched trajectory and can maneuver on the way to their destination, according to the Congressional Research Service. The term “hypersonic” describes any speed faster than five times that of sound, which is roughly 760 miles (1,220 kilometers) per hour at sea level, meaning these weapons can travel at least 3,800 miles per hour. At hypersonic speeds, the air molecules around the flight vehicle start to change, breaking apart or gaining a charge in a process called ionization. This subjects the hypersonic vehicle to “tremendous” stresses as it pushes through the atmosphere, according to a 2018 U.S. Army paper.