6th March 2019 Asteroids may be harder to destroy than previously thought A popular theme in the movies is that of an incoming asteroid that could extinguish life on planet Earth, and our heroes are launched into space to blow it up. But incoming asteroids may be harder to break apart than scientists previously thought, based on a study that revises our understanding of rock fracture and a new computer model to simulate asteroid collisions. The findings, to be published in the journal Icarus, could improve asteroid impact and deflection strategies, increase understanding of Solar System formation and help in the design of asteroid mining efforts. "We used to believe that the larger the object, the more easily it would break, because bigger objects are more likely to have flaws. Our findings, however, show that asteroids are stronger than we used to think and require more energy to be completely shattered," says Charles El Mir, a recent Ph.D graduate from the Johns Hopkins University's Department of Mechanical Engineering and the paper's first author. Researchers understand physical materials like rocks at a laboratory scale (about the size of your fist), but translating this knowledge to city-size objects like asteroids has been difficult. In the early 2000s, a different research team created a computer model into which they input various factors such as mass, temperature and material brittleness. They simulated an asteroid about a kilometre in diameter striking head-on into a 25-kilometre target asteroid at a velocity of five kilometres per second. Back then, their results suggested that the target asteroid would be completely destroyed by the impact.
In this latest study, El Mir and colleagues entered the same scenario into a new computer model called the Tonge-Ramesh model, which accounts for the more detailed, smaller-scale processes that occur during a collision. Previous models did not properly account for the limited speed of cracks in asteroids. "Our question was, how much energy does it take to actually destroy an asteroid and break it into pieces?" says El Mir. The simulation was separated into two phases: a short-timescale fragmentation phase, and a long-timescale gravitational reaccumulation phase. The first phase considered the processes that begin immediately after an impact, processes that occur within fractions of a second. The second, long-timescale phase considers the effect of gravity on the pieces that fly off the asteroid's surface after the impact, with gravitational reaccumulation occurring over many hours after impact. In the first phase, when the asteroid was hit, millions of cracks formed and rippled throughout the object, parts of the asteroid flowed like sand, and a crater was then created. This phase of the model examined the individual cracks and predicted overall patterns of how those cracks propagate. The new model showed that the entire asteroid is not broken by the impact, unlike what was previously thought. Instead, the impacted asteroid had a large damaged core that then exerted a strong gravitational pull on the fragments in the second phase of the simulation.
The end result of the impact was not just a "rubble pile" – a collection of weak fragments loosely held together by gravity. Instead, the impacted object retained significant strength, because it had not cracked completely. This indicates that more energy would be needed to destroy asteroids. Meanwhile, damaged pieces were now redistributed over the large core, providing guidance to those who might want to mine asteroids in future space ventures. "It may sound like science fiction, but a great deal of research considers asteroid collisions. For example – if there's an asteroid coming at earth, are we better off breaking it into smaller pieces, or nudging it to go a different direction? And if the latter, how much force should we hit it with to move it away without causing it to break? These are actual questions under consideration," adds El Mir. "We are impacted fairly often by small asteroids, such as in the Chelyabinsk event a few years ago," says co-author K.T. Ramesh, Director of the Hopkins Extreme Materials Institute. "It's only a matter of time before these questions go from being academic to defining our response to a major threat. We need to have a good idea of what we should do when that time comes – and scientific efforts like this one are critical to help us make those decisions."
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