THERE IS a link between the Beatles song and the Nasa mission “Lucy”, launched this past month. That song was playing when Donald Johanson and his team of anthropologists discovered the skeleton of Australopithecus afarensis, which they named Lucy in its honour. Those early hominid remains, three million years old, were an important link in the chain of human evolution. And that in turn inspired the name of the Nasa probe; Nasa’s Lucy hopes to explore a class of distant asteroids that may contain links to the origins of our solar system.

The idea is that as the planets were forming from collections of rock, ice and gas spinning around the young Sun, Jupiter formed first, in the sweet spot just far enough from the Sun that water would be frozen from the gas, but close enough that the gas would be dense and rich in such sticky ice crystals. As young Jupiter snowballed into its present size, its gravity could scatter away most of the material orbiting near it, thus clearing out the gap between Mars and Jupiter where today only a scattering of asteroids remain.