Do asteroids come in pairs?

Astronomy

Ron Cowen reports from Houston at the annual Lunar and Planetary

Science Conference

Do asteroids come in pairs?

A new study suggests that when they plow into a planet, a small but significant number of asteroids literally pack a double whammy -- because they come in pairs.

Several intriguing observations prompted William F. Bottke and H. Jay Melosh of the University of Arizona in Tucson to analyze paired asteroids. Recent radar studies have shown that some asteroids whose orbits cross Earth's, including Castalia and Toutatis, may be two or more objects barely touching one another. If the components of these "contact binary asteroids" pull apart but remain gravitationally bound, traveling together as they collide with a planet, they might produce the double craters detected on Earth, the moon, and, most recently, Venus.

To produce two distinct craters, the asteroids must break apart well in advance of a collision. Neither atmospheric friction nor the tidal force encountered by an asteroid just before it strikes a planet would draw the fragments far enough apart to make separate impacts.

But most asteroids pass by a planet one or more times before colliding with it. After a number of such near misses, tidal forces from the planet can pull apart a binary asteroid and create two or more well-separated fragments. Indeed, Jupiter's tidal force broke Comet Shoemaker-Levy 9 into more than 20 large chunks 2 years before the comet crashed into the giant planet. The breakup of a contact binary asteroid leads to one of three possible outcomes, says Bottke. The pieces may part company altogether, collide and merge, or pair off in orbit around each other. For example, the Galileo spacecraft recently discovered a tiny moon orbiting the asteroid Ida. Only in the third case will the fragments have the chance, should they eventually strike the planet, to create two neighboring craters.

In their computer simulation, Bottke and Melosh traced the evolution of thousands of contact binary asteroids that were initially far from Earth but came within 5 Earth diameters of our planet. Depending on their initial velocity, many of the binaries ended up as well-separated fragments orbiting each other.

Extending their model further, the Arizona scientists find that among binaries eventually striking Earth, 5 percent have the minimum separation required to form crater pairs. That's an intriguing number, Bottke notes, because it's close to the actual percentage of large, paired craters on Earth. The computer simulation began with the assumption that all asteroids big enough to make craters more than 20 kilometers across were contact binaries. This premise was only intended as a crude approximation; however, the close agreement between the model and the number of large crater pairs on Earth suggests that most large, near-Earth asteroids are binaries, Bottke speculates. If he's right, then asteroids such as Toutatis may prove commonplace rather than atypical oddballs. Moreover, many asteroids may have a moon orbiting them.

If double asteroids have struck Earth, it seems likely that such pairs would also have bombarded Venus, notes Cheryl M. Cook of the University of Arizona. Examining Magellan radar images of Venus, Cook, Melosh, and Bottke found that among large craters there, only 2.5 percent, or one-fourth the terrestrial percentage, come in pairs. Cook suggests that the smaller members of some asteroid binaries disintegrated in Venus' thick atmosphere instead of striking the surface.