Impact Origin of the Moon?

Over forty years ago, in the aftermath of the Apollo landings, scientists began to paint a picture of Moon formation featuring a giant impact, where a wayward planet crashed into the Earth and spun off the Moon 4.5 billion years ago. The first to publish this idea were Tucson scientists Bill Hartmann and Don Davis. Today there is consensus that a giant impact happened, but was it the canonical collision of a Mars-sized body? Or was the Earth already spinning close to breakup, then hit by a Pluto-like projectile? Or was it the merger of two semi-Earths? Was it a succession of giant impacts? Every year brings novel scenarios. The challenges of Moon formation theory puts the creativity of scientists on display, aided by the increasing use of 3D simulations and supported by convincing movies. I will present an accessible overview of the scientific problem, focusing on the dynamics, and on the stringent cosmochemical constraint that lunar rocks are almost indistinguishable from Earth rocks in isotopic composition. I will outline several prevailing models and focus on our own latest attempts to solve the problem. While the formation of the Moon remains elusive, decades of detailed modeling have shown us that every giant impact is unique -- they are factories for planetary diversity that can explain why Mercury, Venus, Mars and the Earth and Moon are each so different.