Washington: A new study suggests that the rubble and dust covering asteroids and comets can feel changes in force-chains between particles over much larger distances than on Earth, making these surfaces less stable than previously imagined.
"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," Dr. Naomi Murdoch, a researcher at the Institut Superieur de l`Aeronautique et de l`Espace in Toulouse, said.
"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," she said.
As postgraduate students back in 2009, Dr. Ben Rozitis of the Open University and Murdoch took part in the European Space Agency`s `Fly your Thesis` campaign.
They conducted experiments inside an Airbus A300, which flew in a series of parabolic maneuvers that gave the students 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 several free-fall phases of the parabolic flights, AstEx`s inner drum was spun for 10 secs and then the direction of rotation reversed.
High-speed cameras imaged the top and bottom layers of beads through glass plates. After the flight, a particle-tracking program was then applied to the images, and the behavior of the beads analyzed.
AstEx data 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, both in microgravity and on Earth.
The team found that although particles close to the rotating surface of the drum were affected less in microgravity by the change in direction, those at the edge of the cylinder moved more than in the experiment on Earth.
This implies that any changes to force-chains in low-gravity environments could be felt over much larger distances.
During their orbits, comets and asteroids may experience forces in a certain direction over long periods, for example during a planetary encounter or due to the rotation of the body.
The AstEx findings suggest that an event like a meteorite impact or a spacecraft landing may have long-distance effects on the stability of regolith.