They are made up of rubble

Feb 16, 2010 14:05 GMT  ·  By
Smaller asteroids are actually made up of loosly-bound dust and pebbles, new data indicates
   Smaller asteroids are actually made up of loosly-bound dust and pebbles, new data indicates

Astronomers have known for a long time that some of the smaller asteroids flying through space are not actually pieces of rock, but loosely-bound piles of rubble and debris. According to the basic laws of physics, these components should not stick together, but fly apart. However, this is not the case, and scientists now believe they know why. They hypothesize that van der Waals forces actually hold these clumps of matter together, but in very specific configuration. Researchers mention that this is the reason why smaller asteroids usually tend to be a maximum of 150 meters in diameter.

According to a paper published online in the journal arXiv, it may be that, as asteroids clump together from boulders and dust, they tend to fall out of equilibrium. That is to say, they are too small to exert a gravitational force large enough on themselves to keep all boulders and rocks together. As a result, they shed some of the smaller rocks into space, until the van der Waals force becomes sufficiently strong to cancel out the centripetal force that seeks to disintegrate the spinning asteroid. This research is based on data collected by the Japanese Hayabusa space probe, which surveyed the small asteroids Ikotawa.

Researchers at the University of Colorado, led by expert Daniel Scheeres, add that this reasoning only makes sense when applied to bodies that travel in the microgravity environment of outer space. What the UC team attempts to do is to “perform a survey of the known relevant forces that act on grains and particles, state their analytical form and relevant constants for the space environment, and consider how these forces scale relative to each other,” the experts write. The team also investigated other possible factors that may keep the space rocks together, such as friction and electrostatic forces between ionized dust, as well as radiation pressure from the Sun.

They found that none of these factors played a large enough part in the process to be considered relevant. But moving past asteroids, what is impressive about the new research is that it also provides a new way of looking at the forming process behind the rings of matter surrounding gas giants. Planets such as Saturn and Jupiter feature rings around them, which are only formed out of very fine and small materials, with no large rocks inside. Maybe the van der Waals forces also played a role in their formation, the UC group says. But this idea needs further investigation, they add.