2020-01-11 13:38:04
Today we begin a round trip to the asteroid Bennu, a rocky body barely half a kilometer in diameter that belongs to the group of asteroids called Apollo, because they follow orbits close to Earth’s. There you are right now OSIRIS-Rex, a space probe of the NASA orbiting it and observing the asteroid in detail before coming close enough to its surface to collect samples of material. A material that it will keep as a treasure to bring it to Earth, where human beings will study its content looking for keys that allow a better understanding of these minor bodies that store pieces of history of the formation of the Solar System.
At the probe OSIRIS.Rex first came into sight of Bennu in August 2018 and in December of that year performed the maneuvers that placed it in orbit around it. Since then, the probe has obtained countless images with the instruments OCAMS, OVIRS y OTES.
The first surprise revealed by the images was the type of terrain that Bennu presented. Instead of offering a more or less clean and smooth surface, composed of regolith and dust, as exists in other similar bodies, the asteroid is strewn with a multitude of rocks and rubble of different sizes. It is estimated that there are more than 200 rocks with dimensions greater than 10 meters and many of them smaller. In fact, the analysis of the images suggests that the entire surface is full of rocky pieces with sizes between 0.5 and 5 cm. Given such an abundance of rocks on the surface, the operation to collect samples by the mission’s robotic arm poses risks that the team of engineers from the NASA try to solve by choosing a suitable place.
The biggest surprise that the asteroid provided arose when, in a routine study of the images, particles were discovered emerging from a specific place on Bennu and dispersing through space. Nobody expected the asteroid to be active, in fact, as Julia de León comments during the interview, when studying the different bodies that can be the objective of the mission, great care is taken to choose one that lacks activity, because the ejection of material into space, something normal in comets, can compromise the safety of the mission.
Bennu’s activity was evidenced by observing a total of three particle ejection events, on January 6 and 19 and February 11, 2019, an emission that also originated from different places on Bennu’s surface. The ejected particles described orbits closer and closer to the surface until they fell back again.
After evaluating different options, the research team came to the conclusion that the ejection of material could be due to three main causes. The first would be the impact of meteorites on the surface of Bennu. However, an impact would create a crater that would reveal the interior of the asteroid, something that has not been verified. Another possibility is that the ejection is due to sudden fractures in the surface rocks due to the enormous difference in temperatures to which they are subjected as the asteroid rotates. Bennu’s rotation has a period of 4.3 hours and the change from light to dark produces temperature changes that exceed 150ºC difference. This change could produce fractures in the rocks and, given the low gravity, when broken, expel particles into space. A third possibility is that certain detected minerals, which were produced by contact with liquid water in the remote past, may release the trapped water when heated by the sun, creating pressure that escapes when the rocks that contain it fracture.
OSIRIS-Rex continues his mission, the next and most delicate moment will take place when the ship approaches Bannu and extends its robotic arm to capture the particles that it will previously blow up by blowing pressurized gas onto the surface. Then, if the operation is successful, he will return to Earth, drop the precious trove of captured particles into an airtight capsule, and continue on his way.
OSIRIS-Rex is not the first mission to approach an asteroid, many of the ships that have traveled to the outer planets, such as Galileo, Cassini or New Horizons crossed the Asteroid Belt and passed close to some of them providing images that spoke of the enormous diversity of bodies that inhabit those places. Those were transit ships, others targeted different bodies and remained attached to them. Only two Japanese missions, named Hayabusa I and II and OSIRIS-Rex were designed to descend to the surface of different asteroids, extract samples of their material and bring it back to Earth.
Julia de Leonresearcher at the Institute of Astrophysics of the Canary Islandsdiscusses in this Talking with Scientists program some of the discoveries of OSIRIS-Rex and the missions Hayabusa I and II. We invite you to listen to it.
More information in:
Lauretta et al., Episodes of particle ejection from the surface of the active asteroid (101955) Bennu Science 366, eaay3544 (2019) 6 December 2019
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