Anatomy of asteroids revealed

Scientists have used ESO's New Technology Telescope (NTT) to find the first evidence that asteroids can have a highly varied internal structure.

By making exquisitely precise measurements astronomers have found that different parts of the asteroid Itokawa have different densities. As well as revealing secrets about the asteroid's formation, finding out what lies below the surface of asteroids may also shed light on what happens when bodies collide in the Solar System, and provide clues about how planets form.

Using very precise ground-based observations, Stephen Lowry (University of Kent, UK) and colleagues have measured the speed at which the near-Earth asteroid (25143) Itokawa spins and how that spin rate is changing over time. They have combined these delicate observations with new theoretical work on how asteroids radiate heat.

This small asteroid is an intriguing subject as it has a strange peanut shape, as revealed by the Japanese spacecraft Hayabusa in 2005. To probe its internal structure, Lowry's team used images gathered from 2001 to 2013, by ESO's New Technology Telescope (NTT) at the La Silla Observatory in Chile among others, to measure its brightness variation as it rotates.

Lowry's team measured that the YORP effect was slowly accelerating the rate at which Itokawa spins. The change in rotation period is tiny - a mere 0.045 seconds per year. But this was very different from what was expected and can only be explained if the two parts of the asteroid's peanut shape have different densities.