Published online 7 October 1999 | Nature | doi:10.1038/news991007-10

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The moons of the asteroids

Asteroids are lumps of rock orbiting - in the main - in a swarm between the planets Mars and Jupiter. As débris left over from collisions between larger bodies, they may have interesting things to tell us about the violent early history of our Solar System. The problem is getting close enough to have a look. As yet, only four of the thousands of bodies have been examined at close quarters by spacecraft. Because asteroids are very small (the largest ones are only a few hundred kilometres in diameter), it is hard to make very much of them from the surface of the Earth.

However, there is a clever way of getting to know asteroids from a distance: a method that William J. Merline of the Southwest Research Institute in Boulder, Colorado and colleagues have used to remarkable effect. As they report in Nature1(7 October 1999), they have used a telescope system with so-called 'adaptive optics' to reveal that the asteroid Eugenia (which is 215 kilometres in diameter) has a tiny speck of a moon: a 13-kilometre lump of rock orbiting Eugenia every four and a half days at a distance of just under 1,200 kilometre.

This composite image shows Eugenia (centre) and its tiny moon at several points around its orbit.This composite image shows Eugenia (centre) and its tiny moon at several points around its orbit.

Thisamazing technical feat of acuity would not be possible with a conventional telescope. However, the researchers used a system installed at the Canada-France-Hawaii Telescope in Hawaii in which an 'intelligent' mirror constantly adjusts itself to compensate for the image-degrading flicker of the Earth's atmosphere. By using adaptive optics, Earthbound telescopes can get the kind of image quality usually reserved for space-based instruments - but because they can have much bigger mirrors than, say, the Hubble Space Telescope, they can be used to image remarkably tiny objects indeed.

At first sight, there seems nothing interesting in the fact that a bigger lump of lifeless rock is orbited by a smaller lump of lifeless rock. However, the very fact that one asteroid orbits another has the potential to tell us a great deal about how asteroids are made, and shed light on the early history of the Solar System.

Plotting the orbit of the moon - provisionally known as S/1998(45)1 - and plugging in details about the force of gravity allow us to get a fix on the density of Eugenia. Eugenia is one of the largest 25 asteroids, but it is also extremely dark, and all the signs suggest that it is richer in carbon compounds than other asteroids that have a more stony or metallic countenance. The orbit of its moon implies that Eugenia has an extremely low density for a solid body, perhaps as little as 1.2 grams per cubic centimetre: hardly more than pure water. This could mean that Eugenia has a very fragile structure, perhaps more of a 'rubble pile' than a solid body.

Another surprising fact about asteroid moons is their rarity. Given that there are so many asteroids, one might expect there to be many cases of smaller ones being gravitationally bound to larger ones. However, this is only the second known instance of such a pairing: the first was the case of the 31-kilometre-diameter asteroid Ida, discovered (by the Galileo spacecraft, which was passing by) in 1993 to have a small companion, now called Dactyl.

To address this question, Merline and colleagues started a systematic program of observing 200 asteroids, to see if any companions turned up. They hit the jackpot with Eugenia on their very first observing night, but have yet to spot another example. "Although our discovery of Eugenia's satellite demonstrates that Dactyl was not a fluke," the researchers say, "there are growing indications that asteroidal satellites are not common." 

  • References

    1. Merline,W. J., Close, L. M., Dumas, C., Chapman, C. R., Roddier, F., Ménard, F., Slater, D. C., Duvert, G., Shelton, C. & Morgan, T. Discovery of a moon orbiting the asteroid 45 Eugenia. Nature 401, 565 1999. | Article | ISI | ChemPort |