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Probing the anatomy of asteroid Itokawa

Jenny Winder
Feb 7, 2014, 0:00 UTC

Sen—Astronomers using the European Southern Observatory's (ESO) New Technology Telescope (NTT) have found that different parts of the asteroid Itokawa have different densities, providing first evidence that asteroids can have a highly varied internal structure.

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 and combined these observations with new theoretical work on how asteroids radiate heat.

This small asteroid, which has a strange peanut shape, was the first asteroid to be the target of a sample return mission, 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 NTT at the La Silla Observatory in Chile, among others, to measure its brightness variation as it rotates. This data was then used to deduce the asteroid's spin period very accurately and determine how it is changing over time. When combined with knowledge of the asteroid's shape this allowed them to explore its interior, revealing the complexity within its core for the first time. The density of the interior was found to vary from 1.75 to 2.85 grammes per cubic centimetre. The two densities refer to Itokawa's two distinct parts.

A schematic view of the strange peanut-shaped asteroid Itokawa. Image credit: ESO/JAXA.

"This is the first time we have ever been able to to determine what it is like inside an asteroid," explains Lowry. "We can see that Itokawa has a highly varied structure - this finding is a significant step forward in our understanding of rocky bodies in the Solar System."

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.

The spin of an asteroid and other small bodies in space can be affected by sunlight. Known as the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, this occurs when absorbed light from the Sun is re-emitted from the surface of the object in the form of heat. When the shape of the asteroid is very irregular the heat is not radiated evenly and this creates a tiny, but continuous, torque on the body and changes its spin rate.

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.

Up until now, the properties of asteroid interiors could only be inferred using rough overall density measurements. This rare glimpse into the diverse innards of Itokawa has led to much speculation regarding its formation. One possibility is that it formed from the two components of a double asteroid after they bumped together and merged.

Lowry added, "Finding that asteroids don't have homogeneous interiors has far-reaching implications, particularly for models of binary asteroid formation. It could also help with work on reducing the danger of asteroid collisions with Earth, or with plans for future trips to these rocky bodies."

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