Astronomers find out when asteroid Bennu hit near-earth space

(ORDO NEWS) — Analysis of photographs of craters on the surface of asteroid Bennu showed that it entered near-earth space about 1.75 million years ago. The conclusions of the planetary scientists were published in the scientific journal Nature.

“We calculated how many small craters with a diameter of 50 cm or less on Bennu’s surface. They arose relatively recently, when this asteroid left the main belt and entered near-Earth space. The study of these craters shows that this happened no later than 1.75 million years back, “the researchers write.

The OSIRIS-REx mission is studying the asteroid Bennu, a small celestial body from the Apollo group, discovered in 2013. The interplanetary station was sent to the asteroid in September 2016, and it entered Bennu orbit at the end of December 2018.

The goal of OSIRIS-REx is to compile a geological map of a celestial body and deliver soil samples from its surface to Earth. Studying it will allow astronomers to learn more about the structure of the primordial matter of the universe, of which Bennu is supposedly composed.

The first observations showed that Bennu’s surface is dotted with large cobblestones, and its rocks contain a lot of water. This made him especially interesting in terms of studying how the solar system formed.

Further data showed that the density of the asteroid’s substance is quite low, and inside its depths there are many voids, which are rather unevenly distributed throughout the celestial body. Moreover, scientists have found hints of the existence of rocks that arose as a result of the flow of streams of water over its surface. This made the matter samples obtained by OSIRIS-REx last week particularly interesting to analyze.

Determining the origin of these samples and studying them requires a clear understanding of how and when Bennu entered near-earth space and how his collisions with other asteroids affected the composition and structure of its surface.

The history of the wanderings of the asteroid

To find out, NASA scientists analyzed images from the OCAMS camera, which is installed on board OSIRIS-REx. Thanks to the high resolution of these photographs, 1–3 cm per pixel, scientists have identified all the small craters that exist both on the asteroid itself and inside the cobblestones scattered on its surface.

As a rule, the larger the crater, the more time has passed since its formation. Such structures gradually fill in debris and regolith, as a result of which they become invisible to observers. Therefore, they can be used to estimate the age of celestial bodies and the time of their arrival in the inner regions of the solar system, where collisions of large asteroids are vanishingly rare.

As experts of the OSIRIS-REx mission note, the same craters can be used to estimate the force of the asteroid impacts, due to which they were formed. In addition, they can be used to find out with what speed the objects that collided with the asteroid were moving, and how strong the rocks of its interior were. Based on similar considerations, NASA specialists rewrote all craters with a diameter of 50 cm and less.

Analysis of these data showed that Bennu entered the near-earth space relatively recently. In addition, scientists have found that for the entire time that the asteroid is in near-Earth space, it collides with about the same set of micro asteroids as the Earth and the Moon, and also did not experience serious collisions with large objects.

This suggests that Bennu still retains traces of collisions that he experienced inside the main asteroid belt, and that the asteroid has changed little since then. On the other hand, collisions with micrometeorites have an unexpectedly strong effect on its surface, which suggests that under the action of this form of “cosmic erosion”, near-Earth asteroids can rapidly collapse. This is important to understand for assessing the age of other near-Earth asteroids, the scientists concluded.

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