Why Craters Are Far Larger Than the Objects That Make Them

A crater is typically 10 to 20 times wider than the body that formed it, and the ratio grows with impact energy. The reason is that nothing about crater formation resembles a projectile pushing material aside. On contact, shock pressures reach hundreds of gigapascals — well beyond the strength of any material — and both the impactor and a comparable mass of target rock vaporise within microseconds. The cavity is then excavated by the outward flow field the shock wave drives. A 50 metre stony asteroid at 17 km/s carves roughly a kilometre of ground, which is why meteoritic fragments are rarely recovered at large craters: the projectile no longer exists in recognisable form.

Pi-Group Scaling and What It Rests On

The estimates here follow pi-group scaling relationships, principally from the work of Holsapple and Schmidt, which express crater size through dimensionless combinations of impactor size, velocity, density, target properties, and surface gravity. The approach works because cratering is governed by a small number of dimensionless ratios rather than absolute magnitudes, which is what permits laboratory experiments, explosive tests, and numerical hydrocodes to be extrapolated across many orders of magnitude to planetary scales. Gravity enters with a negative exponent — stronger gravity resists excavation and yields smaller craters for identical impacts.

Simple Bowls, Complex Basins, and the Transition Between

The transient crater is the cavity at maximum excavation, before gravity acts on unstable walls. Below a threshold diameter the walls slump only modestly and the final crater ends up about 25% wider, retaining a clean bowl shape with a depth-to-diameter ratio near one to five. Above the threshold the cavity cannot support itself: the floor rebounds into a central uplift, the rim collapses inward along terraces, and the result is dramatically wider and shallower. That threshold is about 3.2 km on Earth, 7 km on Mars, and 15 km on the Moon — the differences track surface gravity, since stronger gravity destabilises a shallower cavity.

Angle, Frequency, and the Atmosphere

Impact angle matters less than intuition suggests. Diameter scales with roughly the cube root of the sine of the angle from horizontal, so a 45° impact — the statistically most probable case — produces about 89% of the vertical diameter, and craters stay essentially circular down to about 15°. What angle strongly affects is ejecta asymmetry. Impact frequency falls steeply with size: metre-scale objects arrive several times a year, Chelyabinsk-scale bodies every 60 to 100 years, kilometre-scale impactors every few hundred thousand years, and Chicxulub-scale events on hundred-million-year timescales. One important limit on these results is that atmospheric entry is not modelled here — stony objects below roughly 25 metres typically airburst rather than reaching the ground, so results for small stony impactors should be read as an upper bound.