The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa
Abstract
During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, mass, and surface topography as well as its mineralogic and elemental abundances. The asteroid's orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 × 1010 kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.
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The gravitational plus rotational potential is defined by the Jacobi integral of the system: J = V 2/2 – U(r), where U is the gravitational plus centripetal force potential, r defines a location on the surface of the body, and V is the speed of a particle relative to the rotating body frame (14). The gravitational plus rotational potential is defined as the value of J evaluated for zero speed or the value of –U(r) over the surface of the body. The speed necessary to have sufficient energy to travel from the lowest point of the potential [largest value of U(r) at location r*] to any other point r on the surface of the body is then \( \(\sqrt{2[U(r^{{\ast}})-U(r)]}\) \), which is plotted in the figure.
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We thank the mission operation and spacecraft team of the Hayabusa project at ISAS/JAXA for their efforts that resulted in making Hayabusa the first Japanese spacecraft rendezvoused and landed at the asteroid. This research was supported by ISAS/JAXA, NASA, Kobe University through the 21st Century COE Program of the Origin and Evolution of Planetary Systems, and University of Aizu. A portion of the research reported in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
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Science
Volume 312 | Issue 5778
2 June 2006
2 June 2006
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American Association for the Advancement of Science.
Submission history
Received: 6 February 2006
Accepted: 6 April 2006
Published in print: 2 June 2006
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