The Late Heavy Bombardment (LHB) is the name given to an intense period of impacts dated to between approximately 4.1 and 3.8 billion years ago — some 700 million years after the Solar System formed. During this interval, asteroids and comets struck the inner planets at an unusually high rate and intensity, targeting the Moon, Mars, Mercury, and Earth alike.

The majority of the Moon's large basins were formed in this period, including the South Pole-Aitken Basin at 2,500 km across and Mare Imbrium. Radiometric dating places basin rocks at 4.1–3.8 Gya, suggesting a sharp, concentrated pulse of bombardment. Some geologists, however, argue that what we see may represent the tail end of continuous background bombardment rather than a discrete spike — making the 'late heavy' characterisation itself a subject of ongoing debate.

For Earth the consequences are ambiguous. One reading is that the LHB repeatedly destroyed any nascent life and liquid-water environments that had formed, implying that life could only take permanent hold around 3.8 Gya when the bombardment waned. An alternative view, favoured by some astrobiologists, holds that comets delivered large quantities of water and organic molecules to Earth — meaning the LHB may have contributed one of the preconditions for life rather than merely threatening it. Which picture is closer to the truth remains unresolved.

On a planetary scale, the LHB fundamentally shaped the surface geology of the inner Solar System. Large impacts buried and remelted earlier craters, mixed the crust, and altered heat flux. Earth's active geology has erased most of the record, but the Moon preserves it in near-perfect stillness, providing researchers with a frozen archive of one of the Solar System's most violent chapters.