Key Takeaways:
What counts as “large” is a matter of opinion, but NASA’s Lunar Impact Monitoring program defines 1-pound meteoroids as “large.” Our estimates about impact frequencies are imprecise, particularly regarding such small objects; however, we think 2-pound rocks likely hit the Moon more than 260 times per year. Luckily, the impact rate diminishes rapidly with increasing rock size. For example, a body about a mile in diameter created the 13.5-mile-wide (22 kilometers) lunar crater Giordano Bruno. This is the most recent large lunar crater, and planetary scientists think it formed between 1 and 10 million years ago. Ejecta that splashed out due to such an impact on the Moon would have produced a massive meteor storm on Earth within about a week. — Teemu Öhman, Lunar and Planetary Institute, Houston, Texas