Using very precise ground-based observations with ESO’s New Technology Telescope (NTT), an international team of planetary scientists has found that different parts of the bizarre peanut-shaped asteroid (25143) Itokawa have different densities.
This is a schematic view of the peanut-shaped asteroid Itokawa. Image credit: ESO / JAXA.
Discovered in 1998 by the LINEAR project, this intriguing asteroid was given the provisional designation 1998 SF36 and was officially named after the Japanese rocket scientist Hideo Itokawa.
Itokawa measures 630 m long and 250 m wide and has a peanut shape, as revealed by the Japanese space probe Hayabusa in 2005.
To study its internal structure, Dr Stephen Lowry from the University of Kent and his colleagues measured the speed at which the asteroid spins and how that spin rate is changing over time.
When combined with knowledge of the asteroid’s shape, these data allowed the scientists 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 determine what it is like inside an asteroid,” said Dr Lowry, who is the lead author of a paper published in the journal Astronomy & Astrophysics (full paper in .pdf).
“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.
The researchers 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.
This is the first time that scientists 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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S. C. Lowry et al. 2014. The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up. A&A vol. 562, A48; doi: 10.1051/0004-6361/201322602
