September 29, 2006

Astronomers Muse About Orbiting Asteroids

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Only seven of them have ever been photographed, potato-like and a porosity reminiscent of Swiss cheese. What are they? Asteroids are small rocky bodies surviving from when the solar system first formed. They are very small compared to the planets and difficult to see. In fact, only seven have ever been photographed with enough resolution to make out their shapes.
Unlike the earth or even the moon, most of them are too small for their own gravity to pull them into a spherical shape. Of those seven seen, they resemble large crater-ridden potatoes. Since they have not been well observed, very little is known about the majority of the asteroid population. However, a small group of asteroids with some special characteristics are giving astronomers new insight.
Binary asteroid systems are two asteroids with a common center of rotation. If one is much more massive than the other, then the smaller asteroid will orbit the larger one. If they are of near equal mass, they will both orbit some common point in between them. Since the periods of their orbits around each other are much less than the time it takes for them to orbit the sun, astronomers are able to measure their mass much more easily than if they weren’t binaries. This combined with knowledge of their size can be used to calculate a density and make some assumptions about composition and formation. Surprisingly asteroids have very low densities, much less than regular rock. This suggests that asteroids are loose piles held together only by gravity with no internal structure to provide strength. Some even suggest that 85 percent of the volume of some asteroids is empty space.
A decade ago, very few binary systems were known and many believed they were a gravitational oddity, but today, approximately 70 systems have been observed. Even one triple system and a quadruple system; the quadruple system includes Pluto and its moon Charon along with two other orbiting bodies. Asteroids are divided into three classes: Near Earth Asteroids, Main Belt Asteroids, and Trans-Neptunian Objects. Trans-Neptunian Objects are asteroids furthest from the sun and like the name suggests, they exist near Neptune. These are the least well-understood and the hardest to observe. About 23 binaries of this type are known to exist including Pluto’s and Eris’ system. Eris is a body larger than Pluto but much farther away.
Inside our solar system is the asteroid belt between Mars and Jupiter. About two to three percent of these are binary systems. The nearest and potentially hazardous group of asteroids is the Near Earth Asteroids. In fact, the government maintains a close eye on all of these in order to predict any collisions with Earth. The next close call is in 2029 when an asteroid will pass within six Earth radii — that’s closer than some man-made satellites. Of these asteroids, 15 percent are binary systems. Numerous theories have been proposed to try and explain how these small bodies start orbiting around each other. Some involve larger bodies capturing small ones as they pass by. Others involve impacts or the presence of third bodies. Certain formation methods may be more prevalent for the different types of asteroids. Derek Richardson, University of Maryland, spoke about the formation of Near Earth Asteroid binaries at yesterday’s astronomy colloquium. His group explored the gravitational influence of Earth on asteroids.
When the gravity of a large body interacts with a smaller body and the rotation of either body is involved, the forces exerted on both bodies are referred to as tidal forces. The tidal forces on the smaller body can be strong enough to squish and stretch it or even rip it apart entirely. Since asteroids are essentially loose piles of rubble, Richardson modeled them as such. His conclusion after testing over 100,000 model asteroids with varying size, shape, rotation, mass and distance to the Earth was that tidal forces could create the binary systems observed.