[meteorite-list] Rosetta's OSIRIS Cameras Reveal the Nature of Asteroid Steins

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 12 Jan 2010 14:52:55 -0800 (PST)
Message-ID: <201001122252.o0CMqt5h009568_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=46254

Rosetta's OSIRIS cameras reveal the nature of asteroid Steins
European Space Agency
08 Jan 2010

Close-up images of asteroid (2867) Steins, obtained with the OSIRIS
cameras on Rosetta, have provided extensive new measurements of the
physical properties of this main-belt asteroid. Steins is revealed to be
a loosely-bound 'rubble pile' whose diamond shape has been honed by the
YORP effect. This is the first time this effect has been seen in a
main-belt asteroid. The results are reported by H. Uwe Keller and
colleagues in the 8 January issue of Science magazine.

Most models of Solar System formation posit that the planets formed from
the collision and eventual coalescence of planetesimals. Beyond the
orbit of Mars, the gravitational perturbation of the giant planet
Jupiter prevented the formation of a planet-sized body by disrupting the
orbits of many of these planetesimals. The remaining bodies, some of
them several hundred kilometres in size, have undergone frequent
collisions since this time and today mostly occupy the asteroid belt
between the orbits of Mars and Jupiter. Asteroids are of interest for
understanding the formation process of our Solar System because they
carry information about the nature and composition of the Solar System
at an early stage of its formation. Asteroid (2867) Steins, an E-type
asteroid, is a rare type of solar system body. Only a few tens of these
asteroids have been detected.

In the results published in the 8 January issue of Science, H. Uwe
Keller and colleagues report on observations of asteroid Steins obtained
during the Rosetta flyby of 5 September 2008. This is the first time
that a close-up view of an E-type asteroid has been obtained. The
closest approach to the asteroid was at 18:38:20 UTC at a distance of
803 km. About 60 per cent of the surface was resolved during the flyby
providing a unique set of images from which a number of important
physical properties can be inferred.

Little was known about asteroid (2867) Steins when it was chosen early
in 2004 as one of the targets for a close flyby during the Rosetta
mission. At the time, it was classified as an E-type asteroid on the
basis of its visual and near-infrared spectrum and its high albedo.
Later, ground-based observations estimated a diameter of approximately
4.6 km and determined a rotation period of about 6 hours.

The new OSIRIS images show Steins to be an oblate body, resembling a
brilliant cut diamond, with dimensions of 6.67x 5.81 x 4.47 km??. Its
surface is mostly covered with shallow craters with some of the larger
craters being pitted with smaller ones. Analysis of the impact craters
reveals a deficit of small craters (those with diameter less than 0.5
km) which Keller and his colleagues attribute to surface reshaping as a
result of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect (see
footnote). The effect would have caused landslides which filled-in the
smaller craters. This is the first time that the YORP effect has been
seen in a main-belt asteroid.

Two remarkable features are clearly visible in the images obtained near
closest approach: a large, 2.1 km diameter crater located at the south
pole, and a chain of pits which extend northwards from this crater.
Taken together these features suggest that Steins was subject to a big
impact which created the large crater and caused fracturing of the
asteroid body resulting in it having a rubble pile structure. This type
of loosely-bound structure is also consistent with the YORP effect
hypothesis.

Detailed study of the OSIRIS images has also allowed Keller and his
colleagues to confirm the nature of Steins as an E-type asteroid - the
albedo and spectral characteristics are consistent with this
classification - and to demonstrate that there is no measurable surface
colour variation, which points to a homogeneous composition.

The view of Steins obtained by OSIRIS during the September 2008 flyby
with Rosetta has provided scientists with the first detailed
characterisation of this rare type of solar system body.

The results are reported in "E-type asteroid (2867) Steins as Imaged by
OSIRIS on board Rosetta" by H. U. Keller, C. Barbieri, D. Koschny, P.
Lamy, H. Rickman, R. Rodrigo, H. Sierks, M. F. A???Hearn, F. Angrilli, M.
A. Barucci, J.-L. Bertaux, G. Cremonese, V. Da Deppo, B. Davidsson, M.
De Cecco, S. Debei, S. Fornasier, M. Fulle, O. Groussin, P. J.
Gutierrez, S. F. Hviid, W.-H. Ip, L. Jorda, J. Knollenberg, J. R. Kramm,
E. K??hrt, M. K??ppers, L.-M. Lara, M. Lazzarin, J. Lopez Moreno, F.
Marzari, H. Michalik, G. Naletto, L. Sabau, N. Thomas, K.-P. Wenzel, I.
Bertini, S. Besse, F. Ferri, M. Kaasalainen, S. Lowry, S. Marchi, S.
Mottola, W. Sabolo, S. E. Schr??der, S. Spjuth, and P. Vernazza, Science,
Vol. 327. no. 5962, pp. 190 ??? 193, 8 January 2010. DOI:
10.1126/science.1179559

Footnote: The YORP effect is a phenomenon that occurs when photons from
the Sun are absorbed by a body and reradiated as infrared emission which
carries off momentum as well as heat. The loss of momentum causes a
change in the rotation rate of a small body such as an asteroid. The
resulting high spin rate of asteroid Steins could have caused material
to migrate towards the equator of the asteroid resulting in the
distinctive conical shape.

For further information:

H. Uwe Keller, OSIRIS Principal Investigator
Max-Planck-Institut f??r Sonnensystemforschung
Email: KELLERlinmpi.mpg.de

Michael K??ppers, Scientist in the ESA Rosetta Science Operations Team
and OSIRIS Co-Investigator
ESA Directorate of Science and Robotic Exploration
Email: Michael.Kuepperssciops.esa.int

Detlef Koschny, OSIRIS Co-Investigator
ESA Directorate of Science and Robotic Exploration
Email: Detlef.Koschnyesa.int
Received on Tue 12 Jan 2010 05:52:55 PM PST