OBITUARY Leonard Herzenberg, immunology technology pioneer, remembered p.34 HEALTH Microbe-rich life with the Hadzabe hunter- gatherers of Tanzania p.33 EDUCATION Weighing the case for gene streaming in schools p.32 PHYSICS Richard Feynman’s lectures, still loved 50 years on p.30 of new mechanisms to explain the observed characteristics of the Earth–Moon system. The main challenge is to simultane- ously account for the pair’s dynamics — in particular, the total angular momentum con- tained in the Moon’s orbit and Earth’s 24-hour day — while also reconciling their many com- positional similarities and few key differences. The collision of a large impactor with Earth can supply the needed angular momentum, but it also creates a disk of material derived largely from the impactor. If the infalling body had a different composition from Earth, as seems probable given that most objects in T he Moon is more than just a familiar sight in our skies. It dictates conditions on Earth. The Moon is large enough to stabilize our planet’s rotation, holding Earth’s polar axis steady to within a few degrees. Without it, the current Earth’s tilt would vary chaotically by tens of degrees. Such large vari- ations might not preclude life, but would lead to a vastly different climate. Knowing how the Moon was made is central to understanding Earth and the for- mation of other planets. Since the 1980s, work on lunar origins has focused on the ‘giant-impact’ theory. This proposes that the collision of another planet-sized body with the forming Earth generated a disk of debris that coalesced into the Moon. Such giant collisions were common in the Solar System during the final stages of Earth’s formation 4.5 billion years ago. But we still do not understand in detail how an impact could have produced our Earth and Moon. In the past few years, computer simula- tions, isotope analyses of rocks and data from lunar missions have raised the possibility Lunar conspiracies Current theories on the formation of the Moon owe too much to cosmic coincidences, says Robin Canup. She calls for better models and a mission to Venus. BRIAN A. VIKANDER/CORBIS NATURE.COM For more on the Moon’s origins, see: go.nature.com/5foh6i 5 DECEMBER 2013 | VOL 504 | NATURE | 27 COMMENT The Moon has a similar composition to the outer portions of Earth. © 2013 Macmillan Publishers Limited. All rights reserved