A collision between two bodies, often considered in planetary formation scenarios.An impact is a high-energy collision between two bodies, ranging from microscopic dust grains hitting a spacecraft to planet-sized objects smashing into each other. In planetary science, impacts are the primary mechanism for building planets from smaller bodies and for reshaping their surfaces after formation. The energy released in a large impact can melt rock, eject debris into space, and even strip a planet of its atmosphere.
The role of impacts in planetary formation was established in the 1960s–70s through the work of George Wetherill and others, who showed that the observed mass distribution of planets could be explained by a sequence of giant impacts.
When two bodies collide, kinetic energy is converted into shock waves that propagate through both objects. This energy can fracture, melt, or vaporize material, depending on the impact speed and composition. In the early solar system, repeated impacts accreted dust and pebbles into planetesimals and then into full planets. Later, giant impacts can knock planets off their orbits or strip their moons, as modeled in studies of planetary migration and satellite formation.
Impact physics is used to design spacecraft shielding against micrometeoroids and to interpret cratering records on planetary surfaces, which reveal the history of the solar system.
A single giant impact can eject enough water vapor and rock into orbit to form a temporary ring around a planet, which later coalesces into a moon.
