Looking Up – Bob Eklund

eso1405aPeanuts, anyone? It turns out there’s a big one in the sky—a strange peanut-shaped asteroid named 25143 Itokawa, about one-third of a mile long and roughly half that wide.

By making exquisitely precise measurements using the European Southern Observatory’s New Technology Telescope (NTT) in Chile, astronomers have found that different parts of this asteroid have different densities. This is the first evidence that asteroids can have a highly varied internal structure.

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 (http://en.wikipedia.org/wiki/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 (http://www.isas.jaxa.jp/e/enterp/missions/hayabusa) in 2005. To probe its internal structure, Lowry’s team used images gathered from 2001 to 2013, by ESO’s New Technology Telescope at the La Silla Observatory in Chile among others, to measure its brightness variation as it rotates. This timing 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 grams per cubic centimeter (the two densities refer to Itokawa’s two distinct parts).

“This is the first time we have ever been able 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.”

The spin of an asteroid and other small bodies in space can be affected by sunlight. This phenomenon, known as the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, 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.

As a simple and rough analogy for the YORP effect, if one were to shine an intense enough light beam on a propeller it would slowly start spinning due to a similar effect.

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 second 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.

This is the first time that astronomers have found evidence for the highly varied internal structure of asteroids. 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.

 

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