A kinetic energy penetrator, or KE penetrator, is a projectile weapon that does not explode, but rather damages its target by slamming into it at high speeds. (Through kinetic energy alone.) The primary modern usage of a KE penetrator is to penetrate tank armor. In the military, the term armor-piercing fin-stabilized discarding sabot (APFSDS) is used for the most common form of KE penetrator. The APFSDS is essentially a superfast (1.4 - 1.9 km/sec) arrow with a tip made of depleted uranium alloy. Besides slamming into a target at tremendous speeds, depleted uranium ignites from the energy of the impact and subsequent contact with air, making this KE penetrator a very effective weapon for taking out tanks.
Other kinetic energy penetrators have been researched and developed by the military. The Special Purpose Individual Weapon program was a KE penetrator project pursued by US Army researchers and engineers for a couple decades. The goal of this program was to create a workable flechette-based rifle for soldiers to use in the field. There are several military motivations for developing small arms KE penetrators. First, small darts at high velocity (flechettes) have higher penetrating power than traditional bullets. Second, they have less recoil than bullets, allow for better accuracy in automatic fire. Finally, flechettes weigh less, allowing an infantryman to carry more rounds. Unfortunately, though many billions was spent on the project, a handheld KE penetrator never performed well enough to see the light of day. Ammunition jamming was a frequent issue, as well as overheating.
Recently, the US military unveiled a massive kinetic energy weapon - a Trident II missile with a payload of tungsten rods designed to reenter the atmosphere at extremely high speeds, scattering kinetic buckshot everywhere and doing damage similar to the very largest conventional explosives. This is part of the "Prompt Global Strike" program, launched to find non-nuclear alternatives for strategic first strike capability. Because a nuclear-tipped Trident II and a KE penetrator Trident II both look the same from the outside, its hypothetical use may actually increase the chance of nuclear confrontation rather than decrease it, its intended purpose. Russian President Vladimir Putin mentioned this in a speech in May 2006, saying, "The launch of such a missile could... provoke a full-scale counterattack using strategic nuclear forces."
KE penetrating weapons are frequently seen in science fiction, both in small and large form. Small arms frequently known as "needleguns" make appearances in cyberpunk fiction. Larger KE penetrators, such as space rocks dropped from orbit, feature in some sci-fi stories, and could truly be developed in the longer term future, despite treaties against the militarization of space. The ultimate form of KE penetrator would be a relativistic projectile, an object accelerated into a target at close to the speed of light. Fortunately, accelerating an object like this would require a tremendous amount of space and energy.