The US long-range probe OSIRIS-REx, launched in 2016, reached its target – near-Earth asteroid (101955) Bennu – in 2018. Soon, in October 2020, it should collect samples from its surface to deliver it to Earth in the fall of 2023. Meanwhile, the spacecraft is carefully studying the surface of the asteroid and recently made one very unexpected finding. Scientists write about it in a paper published in the journal Nature Astronomy.

“We found six fragments ranging in size from 1.5 to 4.3 meters, scattered in the southern hemisphere of Bennu and near the equator. These fragments are much brighter than the rest of Bennu’s material, and are similar in composition of Vesta,” explains D. N. DellaGiustina from the University of Arizona, one of the authors of the new work. “Our main hypothesis is that Bennu “inherited” them from its mother’s body, which was smashed by the impact of a “vestoid” – a fragment of Vesta.” Recall that Vesta is the largest and most massive asteroid in the main belt, and its fragments make up their own asteroid family.

Alien debris on Bennu literally catches the eye thanks to its bright shine against the overall dark background of its surface. The OVIRS spectrometer, operating onboard the OSIRIS-REx spacecraft, showed that they are composed mainly of pyroxenes – silicate minerals that are widespread both on Earth and in space, including the Moon and asteroids. The formation of pyroxenes occurs at high temperatures, which, according to scientists, Bennu did not survive, as the surrounding rocks consist of hydrated minerals, preserving the remnants of water.

The necessary heat could have been generated locally as the result of the impact, but calculations show that Bennu would not have survived the collision and would have simply collapsed on its own. All this led scientists to believe that the pyroxene fragments on its surface are brought from another asteroid – and the most likely candidate is Vesta rich with these minerals: more precisely, small asteroids that broke away from it. Such metabolism is not uncommon for these celestial bodies.

In fact, it is believed that Bennu arrived in near-Earth orbit from the outer region of the main asteroid belt. Two large bodies are known here – Eulalia and new Polana – with the same dark carbon surface as its own. One of these asteroids could act as the mother body for Bennu. The fragment, having survived several collisions with smaller fragments of Vesta, could capture several of their “samples” and fly out of the belt in the direction of the Sun, carrying them with it.