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New findings about the consequences of asteroids passing close to the Earth

Will substantial tidal effects have an impact on the asteroid Apophis as it passes within 36,000 km from the Earth and will they allow the asteroid to be seen with the naked eye on April 13, 2029? Yun Zhang and Patrick Michel from the Lagrange laboratory (UCA, OCA, CNRS) have carried out a new campaign of digital simulations of close encounters with asteroid planets represented as agglomerations of rocks held together by their gravity. Compared with previous studies, they have increased the realism of the simulations, in particular as concerns the arrangement of rocks within the asteroid and the contact forces between them as they pass near the planet. Applied to the comet Shoemaker-Levy 9 as it passed near Jupiter before the fragments produced from the close encounter collided with the planet in July 1994, their simulations predict a comet density value that is half that of previous estimates.

Publication : 29/07/2020
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When asteroids and comets pass close to planets or stars, their trajectories and physical properties can undergo substantial changes. The tidal effects produced by these encounters can even cause their destruction. This is what happened to the Shoemaker-Levy 9 comet, which broke into 21 pieces when it first passed near Jupiter. The pieces then collided with the planet during a second visit in July 1994. This collision was also the first major collision of an object with a planet observed in real time by humans using terrestrial telescopes.

The study of the tidal effects produced when small bodies pass close to a planet is essential to understand the evolution and characteristics of small body populations, and to predict the outcome of future encounters, such as the Apophis asteroid that will fly within 36,000 km of the Earth (within the path of geostationary satellites) on April 13, 2029. The outcome of such encounters and the key parameters need to be clarified.
The authors based their study of the processes involved on recent advances in the modeling of small bodies represented as agglomerations of rock held together by their gravity. This agglomeration representation is consistent with data provided by ground observations and space probes, which indicate that small asteroids have a relatively low density, which means that they do not to have a monolith structure but rather form an agglomeration of rocks containing internal voids. “We then carried out several thousand numerical simulations of the passage of these agglomerations near a planet, and determined the influence of the precise conditions of the passage and the properties of the agglomeration, and particularly its resistance, the rock configuration, the frictional properties of the rocks in contact with each other, and the number of rocks,” explains Yun Zhang.
The simulations performed using the pkdgrav tree code indicate three types of close pass results which depend on the conditions of the encounter and on the structure and resistance of the asteroids. These results range from minor changes in the properties of the asteroids to severe damage: deformation of the asteroid, loss of mass, or destruction.
The authors also showed that the speed of passage and the distance from the planet at which tidal effects cause asteroid destruction are much lower than predicted in previous studies. “In other words, an object has to pass nearer to a planet and at a lower speed for it to be destroyed, which reduces the number of small objects produced by this type of event,” explains Patrick Michel.
Also, some objects take on an extremely elongated form (length greater than the width by a factor of 6) when they encounter moderate tidal effects (see Fig. 1), confirming that the shape of some asteroids may be caused by these encounters.

Figure 1P Michel
Figure 1: from left to right: spherical agglomerate asteroid that loses a little mass and whose shape is modified by tidal effects during a close encounter with the Earth, producing an elongated shape (here at a distance of 1.6 earth radius).
Finally, simulations of the destruction of Comet Shoemaker-Levy 9 into 21 fragments when it first flew towards Jupiter before the fragments collided with the planet in July 1994 indicate that the comet's density must have been between 0.2 and 0.3 g/cc to disintegrate, which is about twice lower than previous predictions.
Yun Zhang was awarded a young researcher grant from the IDEX UCA JEDI program of Université Côte d'Azur. This study received financial support from CNES and from contract No. 870377 of the European Union's H2020 program (NEO-MAPP project). The simulations were carried out with the data centers: LICALLO at the Côte d'Azur Observatory, YORP administered by the Department of Astronomy and Deepthought2 administered by the Division of Computer Technology at the University of Maryland.

Press contact
Yun Zhang :
Patrick Michel: +33 6 88 21 28 33 -