[meteorite-list] The Anatomy of Asteroid 25143 Itokawa

[Ron Baalke]

http://www.eso.org/public/news/eso1405/

The Anatomy of an Asteroid
European Southern Observatory
5 February 2014

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 Europe 
and the world's most productive ground-based astronomical observatory 
by far. It is supported by 15 countries: Austria, Belgium, Brazil, the 
Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, 
Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries 
out an ambitious programme focused on the design, construction and operation 
of powerful ground-based observing facilities enabling astronomers to 
make important scientific discoveries. ESO also plays a leading role in 
promoting and organising cooperation in astronomical research. ESO operates 
three unique world-class observing sites in Chile: La Silla, Paranal and 
Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s 
most advanced visible-light astronomical observatory and two survey telescopes. 
VISTA works in the infrared and is the world's largest survey telescope 
and the VLT Survey Telescope is the largest telescope designed to exclusively 
survey the skies in visible light. ESO is the European partner of a revolutionary 
astronomical telescope ALMA, the largest astronomical project in existence. 
ESO is currently planning the 39-metre European Extremely Large optical/near-infrared 
Telescope, the E-ELT, which will become "the world's biggest eye on the 
sky"

Contacts

Stephen C. Lowry
The University of Kent
Canterbury, United Kingdom
Tel: +44 1227 823584
Email: s.c.lowry at 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 at eso.org

Katie Scoggins
Press Officer, Corporate Communications Office, University of Kent
Canterbury, United Kingdom
Tel: +44 1227 823581
Email: K.Scoggins at kent.ac.uk