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New method to measure asteroids

Artist's impression of the asteroid (234) Barbara. ESO image

PARIS (BNS): A team of French and Italian astronomers have invented a new method called the inter-ferometry technique for measuring the size and shape of asteroids that are too small or at a distance 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 European Space Organisation's Very Large Telescope Interferometer (VLTI).

In a paper published in the recent issue of the Astrophysical Journal, describing the VLTI-MIDI direct determinations of asteroid sizes, Marco Delbo from the Observatoire de la Côte d'Azur, France, who led the study said that the knowledge of the sizes and shapes of asteroids is crucial to understanding how, in the early days of the Solar System, dust and pebbles collected together to form larger bodies and how collisions and re-accumulation have since modified them.

Delbo and his colleagues developed a new method that uses inter-ferometry to resolve asteroids as small as about 15 km in diameter located in the main asteroid belt, 200 million km away. This is equivalent to being able to measure the size of a tennis ball at a distance of 1000 km. 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 inter-ferometric 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.

Co-author Sebastiano Ligori, from INAF-Torino, Italy, said that 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. 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 had a peculiar shape. The best fit model is composed of two bodies each the size of a major city – with diameters of 37 and 21 km – separated by at least 24 km. “The two parts appear to overlap so the object could be shaped like a gigantic peanut or, it could be two separate bodies orbiting each other,” Delbo said.

“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,” Ligori said.

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