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*Illustration: Elena Khavina*</center>

<div class="text-justify">A large team of physicists and engineers headed by AY Arasova, of the Panruso Experimental Physics Scientific Research Institute (VNIIEF), the main research center of Russia's military-industrial complex on nuclear weapons, has modeled the impact of a nuclear explosion on an asteroid that threatened the Earth. For this, miniature asteroids were made and bombarded with a laser. The modeling technique developed in this study is a way to experimentally evaluate the criteria for destroying asteroids, as well as the energy of the explosion necessary to eliminate a dangerous object in the process of collision with the Earth.

Asteroids are celestial bodies composed mainly of carbon, silicon, metals and, sometimes, ice. Usually, objects of more than 1 meter are classified as asteroids, although this lower limit is still under discussion. At the other end of the scale, the asteroids reach 900 kilometers in diameter. Traveling at 20 kilometers per second, these giants represent a credible threat to annihilate all life on Earth.

There are two basic options when it comes to protecting the planet from collision with an asteroid: either it is deflected or blown away in such a way that most do not impact the atmosphere and, those that do, burn in it. so that they reach the surface without causing catastrophic damage. The authors of this study have focused on the second option, creating an experimental model with which to check the effects of a powerful shock wave released by a nuclear explosion on the surface of the asteroid. The research team demonstrates that a brief laser pulse aimed at a miniature replica of an asteroid produces destructive effects similar to those of a nuclear explosion on a real space rock. The predicted heat and pressure distributions for the actual event generally coincide with the measurements in the laboratory experiment.

For the laser model to be accurate, the researchers made sure that the density and rigidity of the laboratory asteroid, and even its shape, mimicked the real ones. Thanks to this precise correspondence, the researchers obtained a way to directly calculate the required energy of a nuclear explosion on the real asteroid from the energy of a laser pulse that destroys its miniature replica.

The composition of the artificial asteroids for the tests corresponds to that of the chondritic meteorites (basically carbon), which represent approximately 90 percent of the asteroid remains that reach the surface of the Earth. The replicas were made using data on the meteorite recovered from the bottom of Lake Chebarkul. It is the largest fragment of the asteroid that entered the Earth's atmosphere in February 2013, exploding over the Oblast of Chelyabinsk, Russia. The asteroid material was made using a combination of sedimentation, compression and heating, mimicking the natural formation process. Spherical, ellipsoidal and cubic test asteroids were made from cylindrical samples.

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*Wake of the Chelyabinsk racing car photographed from Yekaterinburg*</center>

To estimate the asteroid destruction criteria, the researchers analyzed the available data from the Chelyabinsk meteorite. It entered the Earth's atmosphere as a 20-meter asteroid and fractured into small fragments that did not cause catastrophic damage. Therefore, it makes sense to state that a 200-meter asteroid has been eliminated if it breaks into pieces with diameters 10 times smaller and masses 1,000 times smaller than the initial rock. For obvious reasons, this conclusion is only true for a 200-meter asteroid that enters the atmosphere at a similar angle and for fragments that travel along trajectories similar to the Chelyabinsk meteor.

Experiments indicate that to eliminate an asteroid of 200 meters, the pump needs to release the energy equivalent to 3 megatons of TNT. This data was obtained because the researchers measured that a laser pulse of 500 joules is necessary to destroy a model of 8-10 millimeters in diameter. To put the data in perspective, 3 megatons would be equivalent to the energy of each and every one of the bombs detonated during World War II, including the atomic bombs that were dropped on Hiroshima and Nagasaki; on the other hand it is not so much either: the most powerful explosive ever detonated, the Tsar bomb, built by the Soviet Union in 1961, released 58.6 megatons (although the numbers vary somewhat according to sources, this is the unofficial Russian); American hydrogen bombs from the Cold War are estimated to release 25 megatons.

The research team now plans to expand the study by experimenting with replicas of asteroids of different compositions, including those containing iron, nickel and ice. They also intend to identify more precisely how the shape of the asteroid and the presence of cavities on its surface affect the criterion of general destruction.

At the moment there are no imminent threats of asteroids, so the team has time to perfect this technique to avoid a planetary disaster.</div>

##### Reference:

Aristova, E.Y., Aushev, A.A., Baranov, V.K. et al. (2018) J. Exp. Theor. Phys. Doi 10.1134 / S1063776118010132
https://link.springer.com/article/10.1134/S1063776118010132#citeas