Asteroid Impact Calculator: Energy & Crater Size

Impact energy is ordinary kinetic energy, one half times mass times velocity squared, with mass taken as a sphere of the given diameter and bulk density. Density is the main compositional lever: cometary ice sits near 1,000 kg per cubic meter, stony chondritic material near 3,000, and iron-nickel bodies near 7,800. Velocity is bounded from below by Earth's escape speed of 11.2 km/s, averages roughly 17 to 20 km/s for near-Earth asteroids, and can reach about 72 km/s for retrograde long-period comets. Because energy scales with the square of speed but only linearly with mass, a doubling of impact velocity delivers the same energy gain as a fourfold increase in mass, or roughly a 60 percent increase in diameter. Results are conventionally quoted in megatons of TNT, where one megaton is defined as 4.184 x 10^15 joules. Crater size uses pi-group scaling laws fitted to explosion and laboratory impact experiments; final craters in competent rock typically end up around 15 to 20 times the impactor diameter, and on Earth transient bowls wider than about 3.2 km collapse into shallow complex craters with central peaks. Several caveats matter. The sine of the impact angle is only a crude proxy for energy coupling. The seismic magnitude conversion assumes all energy becomes seismic waves, whereas real coupling efficiency is closer to 10^-4. Atmospheric entry is not modeled at all, yet stony bodies smaller than roughly 50 meters usually disrupt as airbursts rather than reaching the ground intact.

Impact cratering was not accepted as a terrestrial geological process for most of the twentieth century. Grove Karl Gilbert studied the Arizona crater in 1891 and concluded it was volcanic. Daniel Barringer argued from 1903 that it was an impact scar and spent decades drilling for a buried iron mass that had in fact vaporized. The question was settled in 1960 when Eugene Shoemaker and Edward Chao identified coesite and stishovite, high-pressure silica polymorphs that form only under shock, in the crater rocks. Shoemaker went on to found the modern discipline of impact geology and to link crater counts on the Moon to solar system chronology. Quantitative impact scaling followed from nuclear explosion cratering work, formalized by Keith Holsapple and Robert Schmidt in the early 1980s and synthesized in H. Jay Melosh's 1989 book Impact Cratering: A Geologic Process. Public awareness surged after Luis and Walter Alvarez reported an iridium anomaly at the Cretaceous-Paleogene boundary in 1980, and after Alan Hildebrand and colleagues in 1991 identified the buried Chicxulub structure that Antonio Camargo and Glen Penfield had detected in Pemex survey data. Richard Binzel introduced the Torino Scale at a 1999 conference in Turin. Congress directed NASA to catalog kilometer-scale objects in 1998 and 140-meter objects in the 2005 George E. Brown Jr. Act, and the DART mission demonstrated kinetic deflection in September 2022.