BY DR EMILY BALDWIN
Posted: 11 February, 2009
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.
Artist's impression of asteroid (234) Barbara, which is best modelled as two bodies that may possibly be in contact. Image: ESO/L. Calcada.
Knowing the sizes and shapes of asteroids is vital for not only understanding the evolution of such bodies in the early Solar System, but for quantifying the size distribution of potentially hazardous asteroids.
"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 Cote d'Azur, France, who led the study.
Direct imaging, using ground- or space- based telescopes, or radar imaging, is the standard method of determining the properties of asteroids, but even with adaptive optics these techniques are usually limited to the largest objects or those that experience close counters with the Earth. Delbo and colleagues have devised a new method that uses interferometry to resolve asteroids as small as about 15 kilometres in diameter and located in the main asteroid belt 200 million kilometres away - the equivalent of being able to measure the size of a tennis ball from a distance of a thousand kilometres.
"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 technique will add significantly to the inventory of near Earth objects, bringing small asteroids into the light for the first time. This is especially important as their physical characteristics may vary from well-studied larger objects.
The researchers demonstrated their technique by observing main belt asteroid (234) Barbara, which was revealed to have a rather peculiar shape. The best fit model suggests that it is composed of two bodies with diameters of 37 and 21 kilometres respectively, and separated by at least 24 kilometres.
"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 will allow further properties to be deduced, since by combining the diameter measurements with the parameters of the orbits, the asteroid's density can be computed. "Barbara is clearly a high priority target for further observations," concludes Ligori.