Late Heavy Bombardment
A period roughly 4 billion years ago when the inner solar system experienced an intense spike in asteroid and comet impacts, scarring the Moon and early Earth.
A hypothesized period approximately 4.1 to 3.8 billion years ago during which the Moon, Earth, and other inner planets endured a dramatic surge in collisions with asteroids and comets. Evidence comes primarily from lunar crater dating, suggesting a brief window of catastrophic impacts that may have delivered water and organic compounds to early Earth while also sterilizing its surface repeatedly.
What this means in real life
The Moon's heavily cratered highlands formed during this bombardment—massive impacts created basins like Imbrium and Orientale, which later filled with dark lava to become the 'seas' we see today.
What it isn’t
It is not the original formation of the solar system 4.6 billion years ago, nor a gradual decline in impacts—it describes a specific, intense resurgence of collisions half a billion years after planetary formation had largely settled.
Commonly misused online
Social media posts often conflate it with the asteroid that killed the dinosaurs 66 million years ago, or claim it 'created the Moon'—in reality, the Moon formed much earlier, and this bombardment only reshaped its surface.