Asteroid (25143) Itokawa seen in close-up
Asteroid (25143) Itokawa seen in close-up
Asteroid (25143) Itokawa seen in close-up
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ESO’s New Technology Telescope (NTT) has
been used 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 [1],
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 [2].
“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 [3], [4].
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.
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.
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.”
This new ability to probe the interior of an asteroid is a
significant step forward, and may help to unlock many secrets of these
mysterious objects.
Notes
[1] As well as the NTT, brightness
measurements from the following telescopes were also used in this work:
Palomar Observatory 60-inch Telescope (California, USA), Table Mountain
Observatory (California, USA), Steward Observatory 60-inch Telescope
(Arizona, USA), Steward Observatory 90-inch Bok Telescope (Arizona,
USA), 2-metre Liverpool Telescope (La Palma, Spain), 2.5-metre Isaac
Newton Telescope (La Palma, Spain) and the Palomar Observatory 5-metre
Hale Telescope (California, USA).
[2] 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.
[3] 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
.
[4] Lowry and colleagues were the first to observe
the effect in action on a small asteroid known as 2000 PH5 (now known as
54509 YORP, see eso0711). ESO facilities also played a crucial role in this earlier study.
More information
This research was presented in a paper “The
Internal Structure of Asteroid (25143) Itokawa as Revealed by Detection
of YORP Spin-up”, by Lowry et al., to appear in the journal Astronomy & Astrophysics.
The team is composed of S.C Lowry (Centre for Astrophysics and
Planetary Science, School of Physical Sciences (SEPnet), The University
of Kent, UK), P.R. Weissman (Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, USA [JPL]), S.R. Duddy (Centre for
Astrophysics and Planetary Science, School of Physical Sciences
(SEPnet), The University of Kent, UK), B.Rozitis (Planetary and Space
Sciences, Department of Physical Sciences, The Open University, Milton
Keynes, UK), A. Fitzsimmons (Astrophysics Research Centre, University
Belfast, Belfast, UK), S.F. Green (Planetary and Space Sciences,
Department of Physical Sciences, The Open University, Milton Keynes,
UK), M.D. Hicks (Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, USA), C. Snodgrass (Max Planck Institute for Solar
System Research, Katlenburg-Lindau, Germany), S.D. Wolters (JPL), S.R.
Chesley (JPL), J. Pittichová (JPL) and P. van Oers (Isaac Newton Group
of Telescopes, Canary Islands, Spain).
ESO is the foremost intergovernmental astronomy organisation in
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Links
Contacts
Stephen C. LowryThe University of Kent
Canterbury, United Kingdom
Tel: +44 1227 823584
Email: s.c.lowry@kent.ac.uk
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Katie Scoggins
Press Officer, Corporate Communications Office, University of Kent
Canterbury, United Kingdom
Tel: +44 1227 823581
Email: K.Scoggins@kent.ac.uk
