Microgravity memory-test for granular materials suggests landing on asteroids may cause long-distance avalanches
Results from a microgravity experiment suggest 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. Dr Ben Rozitis of the Open University presented findings from the AstEx parabolic flight experiment at the National Astronomy Meeting in St Andrews on Thursday, 4 July.
"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 Supérieur de l'Aéronautique 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.”
As a postgraduate students back in 2009, Rozitis 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 manoeuvres 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 centre. During several free-fall phases of the parabolic flights, AstEx’s inner drum was spun for 10s 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 programme was then applied to the images, and the behaviour of the beads analysed.
The AstEx experiment was designed by Murdoch, Rozitis, and colleagues from The Open University, the Côte d’Azur Observatory and the University of Maryland. 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.
“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 behaviour of grains in conditions that are similar to those encountered on the surfaces of asteroids and comets.”
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.
“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.
A paper describing these results was published online on 27 May 2013 in the Monthly Notices of the Royal Astronomical Society: Simulating regoliths in microgravity N. Murdoch, B. Rozitis, S. F. Green, P. Michel, T.-L. de Lophem and W. Losert http://mnras.oxfordjournals.org/content/early/2013/05/27/mnras.stt742.abstract
Asteroid Experiment Parabolic Flight Experiment (AstEx) AstEx is an international collaboration between several universities (The Open University, UK; University of Nice Sophia-Antipolis, France; University of Maryland, USA) and between granular physicists, planetary scientists and astrophysicists. http://www.open.ac.uk/pssri/astex/index.html
Stacked photo of the grains in the Asteroid Experiment (AstEx). Credit: (AstEx team) http://www.ras.org.uk/images/stories/NAM2013/4July/murdoch1.jpg
Naomi Murdoch and Ben Rozitis in zero gravity alongside the experiment (Credit: A. Le Floc’h, ESA). http://www.ras.org.uk/images/stories/NAM2013/4July/murdoch2.jpg
Naomi Murdoch and Thomas-Louis de Lophem in zero gravity alongside the experiment (Credit: A. Le Floc’h, ESA). http://www.ras.org.uk/images/stories/NAM2013/4July/murdoch3.jpg
Dr Ben Rozitis
Milton Keynes, UK
Dr. Naomi Murdoch
Institut Supérieur de l'Aéronautique et de l'Espace
Phone: +33 5 61 33 95 45
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 / 4582 x214
Mob: +44 (0)794 124 8035
Ms Anita Heward
Royal Astronomical Society
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Ms Emma Shea
Head of Development Communications
University of St Andrews
Tel: +44 (0)1334 462 167
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Landline numbers in NAM 2013 press room (available from 9 a.m. to 5 p.m. from 1-4 July, 9 a.m. to 3 p.m. 5 July):
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NOTES FOR EDITORS
Bringing together more than 600 astronomers and space scientists, the RAS National Astronomy Meeting (NAM 2013) will take place from 1-5 July 2013 at the University of St Andrews, Scotland. The conference is held in conjunction with the UK Solar Physics (UKSP: www.uksolphys.org) and Magnetosphere Ionosphere Solar Terrestrial (MIST: www.mist.ac.uk) meetings. NAM 2013 is principally sponsored by the RAS, STFC and the University of St Andrews and will form part of the ongoing programme to celebrate the University’s 600th anniversary.
Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at http://www.nam2013.co.uk The Royal Astronomical Society (RAS: www.ras.org.uk, Twitter: @royalastrosoc), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
The Science and Technology Facilities Council (STFC: www.stfc.ac.uk, Twitter: @stfc_matters) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory.
Founded in the 15th century, St Andrews is Scotland’s first university and the third oldest in the English speaking world. Teaching began in the community of St Andrews in 1410 and the University was formally constituted by the issue of Papal Bull in 1413. The University is now one of Europe’s most research intensive seats of learning – over a quarter of its turnover comes from research grants and contracts. It is one of the top rated universities in Europe for research, teaching quality and student satisfaction and is consistently ranked among the UK’s top five in leading independent league tables produced by The Times, The Guardian and the Sunday Times. The University is currently celebrating its 600th anniversary and pursuing a £100 million fundraising campaign, launched by Patron and alumnus HRH Prince William Duke of Cambridge, including £4 million to fund the creation of an ‘Other Worlds’ Think Tank and Observatory. The new think tank and Observatory project will extend the University of St Andrews’ flagship work on extra-solar planets, and provide a creative environment for problem-focused research, education and continuing public engagement. For further information go to: www.st-andrews.ac.uk/600/