- guardian.co.uk, Thursday June 8 2000 01.46 BST
Astronomers often speak of the cosmic zoo, reflecting the exotic variety of phenomena to be seen in the deep space. Pulsars, colliding galaxies and superluminous quasars are just a few of the bizarre images their telescopes pick up. But right here in the solar system, we still have a few surprises in store. Such as an asteroid shaped like a dog's bone.
Cartoonists tend to draw asteroids as rocky spheres pockmarked by craters. They're halfway correct. Asteroids not only slam into the moon and planets producing craters, but they're also holed themselves by smaller projectiles.
But spherical? Only the very largest asteroids - bigger than a couple of hundred miles across - have sufficient gravity to pull themselves into a completely rounded shape. Smaller bodies are expected to be irregular in profile. But just how irregular?
Our vantage point is so far from the main belt between Mars and Jupiter that only the largest asteroids can be resolved. Using the Hubble Space Telescope, we find that the behemoths such as Ceres (at 600 miles diameter the largest asteroid), Pallas and Vesta are close to being 7938 6285 spherical, but we know little about the smaller rocks.
While on its trip to Jupiter Nasa's Galileo spacecraft returned close-up photos of Gaspra and Ida, showing these to be angular and elongated, like pebbles. No surprises there. Similarly, since the NEAR-Shoemaker space probe entered orbit around asteroid Eros in February, it has sent back vivid photographs of that body indicating the anticipated rounded but non-spherical shape, scattered with craters.
Another way of profiling an asteroid's shape is to use radar. Over the past few years Nasa researchers led by Steve Ostro at the Jet Propulsion Laboratory in California have taken the briefly available opportunities to get radar bounces from various small asteroids whizzing close by the Earth, revealing the expected rocky shapes. Some look like peanuts, some like potatoes, others like footballs in need of a pumping up.
But the huge numbers of asteroids in the main belt, more than 100 million miles away, were too distant, out of the range of their radar equipment. Too far away, that is, until the recent multi- million dollar upgrade of the vast radar located at Arecibo in Puerto Rico was completed, funded by the US National Science Foundation. This has dramatically improved its sensitivity, making observations of more distant objects feasible.
The Arecibo radar employs a dish 1,000 feet across, fixed in place in a natural bowl-shaped valley to the north of the island. It is not steerable in the way that most radio telescopes, such as that at Jodrell Bank, can be directed to any point in the sky. So astronomers using Arecibo need to choose targets that happen to pass overhead during their observation runs.
The first main belt asteroid to be selected was Kleopatra. This object, discovered in 1880, was not thought to be remarkable in any way. It was just a suitably located target on which to test the new radar system. Data from optical telescopes indicated a size of about 100 miles, and its colour indicated a metallic composition. Because metals are good reflectors of radio waves, a strong echo seemed likely.
This was a complicated experiment. Kleopatra was so far away - further away than the sun from the earth - it took the radar pulse 19 minutes to make the round trip. A strong echo was obtained, but it varied in a way which initially perplexed the astronomers. As more data were collected, it became possible to build up a picture of the overall shape of the asteroid. It looks somewhat like a dumb-bell, 135 miles long and 58 miles wide. "With its dog bone shape, Kleopatra has the most unusual shape we've seen in the solar system," commented Ostro.
The best guess is that the surprising shape may be the result of some phenomenal inter-asteroid collision many aeons ago. The lumps thrown off at high speed in that collision may then have made their way to our planet, some of them ending up in museum collections of meteorites. Many meteorites have nickel-iron compositions similar to the presumed make-up of Kleopatra, making it a candidate parent body for at least some of these samples. So maybe we've got some of the meat off the dog's bone available for study in our laboratories.
Alternatively, rather than Kleopatra originally having been more rounded, and disfigured by a cataclysmic collision, perhaps it gradually accumulated debris to attain this form.
Another member of the team, Scott Hudson of Washington State University, suggests that the asteroid "may once have been two separate lobes in orbit around each other with empty space between, subsequent impacts filling in the area between the lobes with debris." This idea has appeal because the radar echoes also indicate Kleopatra to be somewhat porous, rather than one solid lump of metal.
The shape of Kleopatra remains a mystery for the time being. If it tells us anything, it's that we still have much to learn about asteroid origins and collisions. We can expect nothing less than the unexpected as more members of the asteroid zoo are unveiled in the coming years.
• Duncan Steel researches asteroids and comets at the University of Salford.
