The energy of impacts and airbursts

Impact energies range from a firecracker to the end of the dinosaurs. This lesson quantifies them, compares airbursts with cratering impacts, and introduces how crater size scales with energy.

From size to energy

For a roughly spherical impactor of diameter d and density ρ, the mass is m = ρ × (π/6)d³ and the kinetic energy is E = ½mv². Because mass depends on d³ and energy on v², a body twice as large carries eight times the energy at the same speed.

Impact energies are quoted in TNT equivalents: 1 kilotonne (kt) = 4.184 × 10¹² J and 1 megatonne (Mt) = 4.184 × 10¹⁵ J.

Airburst or crater?

Whether energy is released in the air or at the ground depends on size, strength and speed. Stony bodies up to a few tens of metres usually fragment and explode in the atmosphere: Chelyabinsk (2013) released about 0.5 Mt at around 30 km altitude, and Tunguska (1908) about 10 to 15 Mt, flattening some 2,000 km² of forest. Iron bodies are much stronger and can reach the ground at high speed even when tens of metres across, as at Meteor Crater. Bodies larger than a few hundred metres pass through the atmosphere with little loss of speed and always crater.

Crater scaling

Crater diameter does not grow in proportion to energy. Experiments and models show that for large craters, diameter scales roughly as energy to a power of about one quarter to one third, depending on how gravity and target strength limit excavation. A thousand-fold increase in energy therefore produces only a five- to ten-fold increase in diameter. Above a few kilometres on Earth, craters collapse under gravity into complex forms with central peaks or peak rings.

Explore on Howardites