The first direct evidence has been found for a binary asteroid in the ‘near-Earth’ asteroid belt. The orbital characteristics of binary 2000 DP107 enabled Jean-Luc Margot of the California Institute of Technology and colleagues to estimate the size and density of its components more accurately than is possible for ordinary asteroids. Binary asteroids are valuable probes of the solar system because they can only form under certain conditions (J-L Margot et al 2002 Science to appear).
Since 1995, a handful of binary asteroids – which consist of two objects in orbit around their common centre of mass – have been discovered in the main asteroid belt between Mars and Jupiter. But pairs of impact craters found on Earth and the fluctuating light curves of several near-Earth asteroids had previously led astronomers to speculate that such binaries might exist closer to home. Asteroids in the near-Earth belt have well-defined orbits that cross those of the planets nearer the Sun.
Asteroid 2000 DP107 was not identified as a binary when it was first discovered in 2000 by astronomers at the Massachusetts Institute of Technology. But Margot’s team subsequently studied its reflectivity at wavelengths of 3.5 and 13 cm – using the Goldstone and Arecibo radars – and built up a picture of the asteroid with a resolution of about ten metres. This revealed a primary body about 800 metres in diameter and a smaller body about 300 metres in diameter.
Margot and colleagues found that the components of the binary are about 2.5 kilometres apart, and have an orbital period of 1.8 days. This allowed the researchers to calculate that the mass of the entire binary is about 5 x 1011 kilograms at the most. Assuming that the bodies have a composition similar to that of previously studied asteroids, they estimated the density to be about 1.7 grams per cubic centimetre.
Simulations have shown that binary asteroids are probably created when an asteroid passes through the gravitational field of a planet, which produces tidal forces in the asteroid that break it up. According to Margot, this suggests that such binaries are likely to be aggregates, loosely bound by their own gravity.
Margot’s team has since discovered four more binary asteroids in the near-Earth asteroid belt. Together with other studies, this suggests that binaries could account for around 16% of these asteroids, compared with around 2% of the asteroids in the belt between Mars and Jupiter. Margot and colleagues believe that they are more abundant in our region of the solar system because asteroids encounter planets more often. But the researchers also concede that it may just be harder to spot more distant binaries.
“Each discovery of an asteroid moon is a scientific bonanza as valuable as a spacecraft mission,” Margot told PhysicsWeb. “Because binary systems allow direct measurements of fundamental asteroid properties, they have a considerable impact on planetary science.”
