THE history of planet Earth is a fascinating story, involving catastrophic collisions with other small planets and a veritable plethora of asteroid impacts. The prevailing theory about the formation of the moon is called the giant impact hypothesis: the theory goes that a Mars-sized object, known as Theia, crashed in to the young Earth. What left was Earth, and its moon. A new computer model suggests, however, that the Moon may not have been the only reminder of that big collision. Jack J. Lissauera of the Space Science and Astrobiology Division, NASA Ames Research Centre, and John E. Chambers of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, have suggested that moonlets called Trojans may have been left behind in the collision. The giant impact that likely led to the formation of the Moon launched a lot of material into Earth orbit, and some could well have been caught in the Langrangian points, -points in space where the gravity between two objects cancels the other out, said says study team member Lissauer. Their theory places small moonlets, or Trojans, in Earths orbit, for up to 100 million years. Over time, gravitational tugs from other planets would have eventually altered Earths orbit, even if it was only slightly. Thus, the Langrangian points would have altered, leaving the Trojans once again susceptible to gravity. From there, they could be anywhere by now, or destroyed entirely. DG The perturbations from the other planets are very, very tiny, said Lissauer. But they change the shape of Earths orbit, which subsequently changes the effect that the Suns gravity has on the moons, that what ultimately destabilizes the Trojans. A separate yet similar model created by Matija Cuk, an astrophysicist at the University of British Columbia in Canada has suggested that smaller, asteroid sized objects, only a few tens of kilometers in width, could have lasted longest in those stationary positions. She believes that they could have lasted a lot longer too, up to a billion years or more. However she noted that they would have looked more like Jupiter or Venus in the sky than a satellite. They would have resembled very bright stars. DG