On March 18, nearly one month into the invasion of Ukraine, a Russian Air Force MiG-31B took to the skies bearing a 26-foot white rocket against its belly, bound for an ammunition depot in western Ukraine. This type of rocket, dubbed the Kh-47M2 Kinzhal—“dagger” in Russian—had only been fired twice in history, both times for tests; this was its first appearance in active conflict. According to the Russian Defense Ministry, the Kinzhal destroyed the depot, pushing the war in Ukraine into a troubling new phase of uncertainty and escalation.

With a reported range of more than 1,200 miles and a claimed top speed of Mach 10, the Kinzhal is an air-launched weapon based on Russia’s 9K720 Iskander short-range ballistic missile. Like all ballistic missiles, the Kinzhal achieves hypersonic velocities using a rocket engine and an arcing ballistic flight path. But Russia claims the Kinzhal can perform evasive maneuvers at every leg of its journey. It’s a weapon described by President Joe Biden as “almost impossible to stop.”

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“Hypersonic” describes vehicles that can travel faster than Mach 5, or 3,836 miles per hour, a new frontier in the world of military technology. Hypersonic weapons so far take two main forms: boost-glide vehicles and hypersonic cruise missiles. The former type, a successor to the ballistic missile, takes slightly flatter flight paths and adjusts course during gliding descents. The latter is powered by a new propulsion system called a scramjet. Boost-glide vehicles offer maneuverability and faster top speeds. They can reach Mach 20, while hypersonic cruise missiles max out around Mach 5.

Read more: Popular Mechanics