42 asteroids photographed by ESO – Sciences et Avenir

Not one nod to Douglas Adams and his Galactic Traveler’s Guide (H2G2), but two! The cult book was first published on October 12, 1979 and today, October 12, 2021, the European Southern Observatory (ESO) publishes images of these asteroids from the main belt (located between the orbits of Mars and Jupiter) numbering … 42! If your observation will undoubtedly not answer the “big question about life, the universe and the rest”, you will still contribute to a better understanding of the arrangement of the small bodies of the solar system.

The contribution of the SPHERE instrument

Normally dedicated to the search for exoplanets, the SPHERE instrument installed at the VLT in Chile has deviated for once from its main object of study to scan the asteroids of the solar system. The tool uses extreme adaptive optics: it is a mirror that corrects more than 1,200 times per second and on a nanometric scale for the effects of atmospheric turbulence.

Differential spectroscopy and imaging techniques are also used to reveal differences in color or polarization in the light of a star. Few of the main belt asteroids have been photographed so far, except those that have been flown by probes such as Ceres, Vesta or Lutetia. Therefore, Pierre Vernazza’s team from the Marseille Astrophysics Laboratory strove to fill this gap using SPHERE between 2017 and 2019 to observe the largest asteroids, including 20 of the 23 that are more than 200 km in diameter. The others are more than 100 km, except for two: Urania and Ausonia, which are approximately 90 km in diameter.

Representation of the 42 asteroids in our Solar System and their orbits (on a black background). Credit: ESO / M. Kornmesser / Vernazza et al./ MISTRAL Algorithm (ONERA / CNRS).

Two families of asteroids

The observations made by the VLT made it possible to obtain very detailed images of all these asteroids and, therefore, to reconstruct their shapes. It turns out that they can be classified into two families: spherical ones like Ceres or Hygiea and those with much more elongated or even “dog bone” shapes like Kleopatra. In a study published in the journal Astronomy & Astrophysics, the researchers, by combining the shapes of asteroids with information about their masses, were able to analyze their density, which varies considerably from star to star. So the ones that are less dense have a density of about 1.3 grams per cubic centimeter, or about the density of coal. In contrast, Psyche and Kalliope, for example, have respective densities of 3.9 and 4.4 grams per cubic centimeter, which is greater than the density of diamond (3.5 grams per cubic centimeter).

These different densities imply that the composition of the asteroids is very different depending on the star considered. This suggests that its current position, between Mars and Jupiter, is not that of its origin. In fact, the solar system has not always had its current configuration. In their youth, some planets like Uranus or Neptune were much closer to the Sun than today and many other stars have also changed direction, moving towards the center or on the contrary towards the outer solar system. Clues recovered by the Dawn probe, which flew over Ceres in 2015, attest that the dwarf planet formed much further away, beyond the orbit of Neptune. This would also be the case for the less dense asteroids identified in this research. Then they would have migrated secondarily inward when the two giants Uranus and Neptune reached the suburbs of the solar system, more than four billion years ago.

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