Rosetta obtains 'light curve' of asteroid Steins (Forwarded)

ESA News
http://www.esa.int

20 March 2007

OSIRIS camera on Rosetta obtains 'light curve' of asteroid Steins

During the very first observations of Rosetta's flyby target 2867-Steins
in March 2006 the onboard camera OSIRIS obtained the most accurate 'light
curve' of this asteroid so far.

OSIRIS observed Steins from a distance of 159 million kilometres and, from
there, it obtained images and important clues about its characteristics.

Although ESA's Rosetta is still far away from its destination comet
67P/Churyumov-Gerasimenko and its flyby targets 2867-Steins and
21-Lutetia, scientists have already started to collect preliminary data
about these two largely unexplored asteroids. Advanced knowledge of the
asteroids' properties (like size and rotation period) is essential for the
preparation of the planned asteroid observation campaigns in September
2008 and July 2010, respectively.

"Lutetia was imaged by Rosetta on 2 and 3 January 2007, whereas Steins was
already observed during a 24-hour observation campaign on 11 March 2006.
Both observations were aimed at pre-characterizing the rotation direction
of the asteroid. This can be done by the study of the so-called 'light
curve' of the asteroid from different locations -- by analysing how the
light emitted by the observed object changes intensity for different
observers, one can deduce in what direction the object rotates," explains
Michael Küppers from the Max-Planck-Institute for Solar System Research.
The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) on
board Rosetta is up to this ambitious task. On its interplanetary cruise
it provided an observing geometry of Steins that cannot be obtained from
Earth.

Although the brightness of the Steins asteroid during the measurement
period was comparable to that of a candle seen at a distance of ~2000
kilometres, OSIRIS was able to measure brightness variations of the
asteroid with an accuracy of better than two percent of its total
brightness.

The observations show that Steins rotates with a spin period of slightly
more than six hours, in agreement with previous earth-based observations.
The asymmetry of the light curve suggests an irregular shape of Steins.
However, OSIRIS found no evidence for a 'tumbling' motion of the asteroid
or the presence of a satellite. Work is ongoing to construct the
orientation of the spin axis of Steins from a combination of the OSIRIS
observations with ground-based data.

Basically, asteroids are elemental components of the solar system,
orbiting the sun for thousand of millions of years. For scientists, these
minor bodies of rock or stone are much more than only boulders. Asteroids
as well as comets carry important information about the origin of the
Solar System -- a better understanding of which is one of the primary
goals of Rosetta. Rosetta will gather valuable data as it flies by these
primordial rocks.

Rosetta's instruments will provide information on the mass and density of
the asteroids, thus telling us more about their composition, and will also
measure their subsurface temperature and look for gas and dust around
them.

Note for editors:

Asteroid 2867-Steins will be visited again by Rosetta on 5 September 2008
from a distance of just over 1700 kilometres. This encounter will take
place at a relatively low speed of about 9 kilometres per second during
Rosetta's first excursion into the asteroid belt. On 10 July 2010 Rosetta
will pay its second visit to asteroid 21-Lutetia, passing within about
3000 kilometres of it, at a speed of about 15 kilometres per second.

This article is based on the paper "Determination of the light curve of
the Rosetta target asteroid (2867) Steins by the OSIRIS cameras onboard
Rosetta,² by M. Küppers et al published in: Astronomy & Astrophysics, Vol.
462-1 (January IV 2007, A&A 462, p. L13).

For more information:

Michael Küppers, Max-Planck-Institut für Sonnensystemforschung
Email: kueppers@xxxxxxxxxx

Uwe Keller, OSIRIS Principal Investigator, Max-Planck-Institut für
Sonnensystemforschung
Email: KELLER @ linmpi.mpg.de

Rita Schulz, ESA Rosetta Project Scientist
Email: rita.schulz @ esa.int

Gerhard Schwehm, ESA Rosetta Mission Manager
Email: gerhard.schwehm @ esa.int

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMCC2R08ZE_index_1.html ]


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