July 5, 2013
Asteroid Surfaces May Be More Unstable Than Previously Thought
Lee Rannals for redOrbit.com - Your Universe Online
An experiment in microgravity has revealed future missions seeking to land on an asteroid may have to watch out for an avalanche.Researchers discovered the rubble and dust covering asteroids and comets could feel changes between particles over much larger distances than on Earth, making these surfaces much less stable than previously thought.
"We see examples of force-chains everywhere. When you pick an orange from a pile in a supermarket, some come away easily but others bring the whole lot crashing down. Those weight-bearing oranges are part of a force-chain in the pile," said Dr. Naomi Murdoch, a researcher at the Institut Superieur de l'Aeronautique et de l'Espace in Toulouse. "One important aspect of such chains is that they give a granular material a 'memory' of forces that they have been exposed to. Reversing the direction of a force can effectively break the chain, making the pile less stable."
Murdoch and her colleague Dr. Ben Rozitis of Open University were invited to conduct their experiment inside an Airbus 300, which flew in a series of parabolic maneuvers to give the researchers around 30 minutes of microgravity conditions.
The Asteroid Experiment (AstEx) consisted of a cylinder filled with glass beads with a rotating drum at the center. During the microgravity phase, the inner drum was spun for ten seconds and then the direction of rotation was reversed. After the flight, a particle-tracking program was applied to the images, and the behavior of the beads was analyzed.
Data from the experiment showed how chains of influence built up between beads when exposed to a force in one direction, and how those chains were disrupted when the force was reversed.
"Many smaller asteroids are thought to be entirely granular in nature - piles of rock and gravel held together by gravity. Understanding the physics of granular materials is important for interpreting spacecraft images of these small bodies, to understand their evolution, and also to help design space missions that will interact with their granular surfaces," said Murdoch. "AstEx allowed us to study the behavior of grains in conditions that are similar to those encountered on the surfaces of asteroids and comets."
They found although particles close to the rotating surface of the drum were affected less in microgravity by the change in direction, those found at the edge moved more than a control experiment on Earth.
The findings suggest a spacecraft landing on an asteroid may have long-distance effects on the stability of the surface.
"A lander touching down on the surface on one side of a small, rubble-pile asteroid could perhaps cause an avalanche on the other side, by long-range transmission of forces through chains. It would, however, depend on the angle and location of the impact, as well as the history of the surface - what kind of memories the regolith holds," said Murdoch.