http://www.eso.org/public/outreach/press-rel/pr-2009/pr-04-09.html

ESO 04/09 - Science Release
4 February 2009
For Immediate Release

Powerful New Technique to Measure Asteroids' Sizes and Shapes

A team of French and Italian astronomers have devised a new method for
measuring the size and shape of asteroids that are too small or too
far
away for traditional techniques, increasing the number of asteroids
that
can be measured by a factor of several hundred. This method takes
advantage of the unique capabilities of ESO's Very Large Telescope
Interferometer (VLTI).

"Knowledge of the sizes and shapes of asteroids is crucial to
understanding how, in the early days of our Solar System, dust and
pebbles collected together to form larger bodies and how collisions
and
re-accumulation have since modified them," says Marco Delbo from the
Observatoire de la Côte d'Azur, France, who led the study.

Direct imaging with adaptive optics on the largest ground-based
telescopes such as the Very Large Telescope (VLT) in Chile (see ESO
21/05
http://www.eso.org/public/outreach/press-rel/pr-2005/pr-21-05.html>
and
18/07
http://www.eso.org/public/outreach/press-rel/pr-2007/pr-18-07.html>),
and space telescopes, or radar measurements (ESO 11/07
http://www.eso.org/public/outreach/press-rel/pr-2007/pr-11-07.html>)
are the currently favoured methods of asteroid measurement. However,
direct imaging, even with adaptive optics, is generally limited to the
one hundred largest asteroids of the main belt, while radar
measurements
are mostly constrained to observations of near-Earth asteroids that
experience close encounters with our planet.

Delbo and his colleagues have devised a new method that uses
interferometry to resolve asteroids as small as about 15 km in
diameter
located in the main asteroid belt, 200 million kilometres away. This
is
equivalent to being able to measure the size of a tennis ball a
distance
of a thousand kilometres. This technique will not only increase the
number of objects that can be measured dramatically, but, more
importantly, bring small asteroids that are physically very different
from the well studied larger ones into reach.

The interferometric technique combines the light from two or more
telescopes. Astronomers proved their method using ESO's VLTI,
combining
the light of two of the VLT's 8.2-metre Unit Telescopes. "This is
equivalent to having vision as sharp as that of a telescope with a
diameter equal to the separation between the two VLT Unit Telescopes
used, in this case, 47 metres," says co-author Sebastiano Ligori, from
INAF-Torino, Italy. The researchers applied their technique to the
main
belt asteroid (234) Barbara, which was earlier found, by co-author
Alberto Cellino, to have rather unusual properties. Although it is so
far away, the VLTI observations also revealed that this object has a
peculiar shape. The best fit model is composed of two bodies each the
size of a major city â with diameters of 37 and 21 km â separated by
at
least 24 km. "The two parts appear to overlap," says Delbo, "so the
object could be shaped like a gigantic peanut or, it could be two
separate bodies orbiting each other."

If Barbara proves to be a double asteroid, this is even more
significant: by combining the diameter measurements with the
parameters
of the orbits, astronomers can then compute the density of these
objects. "Barbara is clearly a high priority target for further
observations," concludes Ligori.

Having proven the validity of their new and powerful technique, the
team
can now start a large observing campaign to study small asteroids.

More information

These observations are presented in a paper by Delbo M. et al., First
VLTI-MIDI direct determinations of asteroid sizes, in
press in the Astrophysical Journal.

The team is composed of Marco Delbo and Alexis Matter (OCA, France),
Sebastiano Ligori and Alberto Cellino (INAF-Torino, Italy), and Jerome
Berthier (IMCCE, Observatoire de Paris, France).

The Italian version of this release published by INAF is available on
this link
http://www.inaf.it/ufficio-stampa/comunicati-stampa-del-2009/
cs_02_040209>

Contacts

Marco Delbo
Observatoire de la Côte d'Azur, Nice, France
E-mail: delbo{at}obs-nice.fr
Phone: +33 (0)4 9200 1944

Sebastiano Ligori
INAF- Osservatorio Astronomico di Torino, Italy
E-mail: ligori{at}oato.inaf.it
Phone: +39 011 8101 964

ESO La Silla - Paranal - ELT Press Officer: Dr. Henri Boffin - +49 89
3200 6222 - hboffin{at}eso.org
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vrodrigu{at}eso.org

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