Planetary Bombardments, Past and Future

Is Mercury the long-lost moon of Venus? What’s our risk for a massive asteroid impact? A third dispatch from the annual planets meeting.

FAJARDO, Puerto Rico—A fascinating idea came up in an informal chat I had yesterday with asteroid expert Erik Asphaug of the University of California, Santa Cruz. The early solar system was a veritable shooting gallery. Our moon is thought to have formed when a Mars-size body hit Earth and threw out a cloud of debris […]

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