Not Just Drifting Rocks – Asteroids Have Hidden Depths

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Knowing what asteroids are made of and how dense they are reveals secrets about the asteroid’s formation, what lies below their surface, sheds light on what happens when bodies collide in the Solar System, provide clues about how planets form and the big question – how best to attempt to prevent an incoming asteroid from hitting Earth.

Using very precise ground-based observations, Stephen Lowry and colleagues from the University of Kent in the United Kingdom 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 measured it’s brightness variation as the asteroid rotated. This data was obtained from images gathered from 2001 to 2013 by the European Southern Observatory’s New Technology Telescope (NTT) at the La Silla Observatory in Chile, The 60 inch Palomar Observatory’s telescope in California, the 2-metre Liverpool Telescope in La Palma, and many more of the world’s ground based telescopes.

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This rotation timing data was then used to deduce the asteroid’s spin period very accurately and determine how it changes 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.

“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.”

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 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.

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This new insight into the varying density at each side of the asteroid leads to the possibility 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.”

Dr. Lowry is also a member of the science team for the OSIRIS optical camera instrument on board ESA's Rosetta spacecraft, en route to comet 67P/Churyumov-Gerasimenko, and is Principal Investigator on a new Large Programme at the European Southern Observatory to study near-Earth asteroids.