‘Avalanche’ Risk Higher Than Thought For Asteroid Landings: Study

Imagine plunking your spacecraft down on an asteroid. The gravity would be small. The surface would be uneven. The space rock might be noticeably spinning, complicating your maneuvering.

Humans have done it with robotic spacecraft before. The first time was in 2001, when NASA made a stunning landing with the NEAR Shoemaker spacecraft on Eros — using a craft that was not even designed to reach the surface. A new study, however, portrays getting close to these space rocks as perhaps even more hazardous than previously thought.

An experiment done aboard a “Vomit-Comet” like airplane, which simulates weightlessness, suggests that dust particles on comets and asteroids may be able to feel changes in their respective positions across far larger distances than on Earth.

“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,” stated Naomi Murdoch, a researcher at the Higher Institute of Aeronautics and Space (Institut Supérieur de l’Aéronautique et de l’Espace) in Toulouse, France.

“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.”

The Asteroid Experiment Parabolic Flight Experiment (AstEx) experiment was designed by Murdoch, Open University’s Ben Rozitis, and several collaborators from The Open University, the Côte d’Azur Observatory and the University of Maryland. It had a cylinder with glass beads inside of it, as well as a rotating drum at the heart.

In 2009, when they were postgraduate students, Murdoch and Rozitis took their contraption on board an Airbus A300, which flew parabolas to simulate microgravity while the aircraft falls from its greatest height.

During this time, the inner drum spun up for 10 seconds and then the rotational direction was reversed. What happened was tracked by high-speed cameras. Later, the researchers analyzed the movement of the beads with a particle-tracking program.

The researchers found that particles at the edge of the cylinder (the closest analog to low-gravity environments) moved more than those in similar environments on Earth. Those closer to the center, however, were not as greatly affected.

“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.

Check out more details of the experiment in the June 2013 issue of the Monthly Notices of the Royal Astronomical Society. It’s some interesting food for thought as NASA ponders an asteroid retrieval mission that so far has met with skeptical Congress representatives.

Source: Royal Astronomical Society