Imaging asteroids is quite difficult for astronomers – direct imaging, even with adaptive optics, is generally limited to the largest asteroids, while radar measurements are constrained to near-Earth asteroids. Marco Delbo from the Observatoir de la Cote d’Azur, France and his team have devised a new technique using interferometry that can resolve a 15km diameter asteroid in the main asteroid belt, 200 million kilometres away. This is the equivalent to being able to measure the size of a tennis ball at a distance of a 100km.

Delbo explains, “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.”

By using the interferometry (combing the light from two or more telescopes) the team observed the main belt asteroid (234) called Barbara. It seems that Barbara is composed of two bodies, each the size of a large city. The asteroid could be shaped like a giant peanut or it could be two separate bodies orbiting each but if it is a double asteroid it would be possible to calculate its density from diameter and orbit measurements.

“Barbara is clearly a high priority target for further observations,“ concludes colleague Ligori.

By Leila Sattary