Astronomers Capture Images of 42 Large Main-Belt Asteroids

Using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument mounted on ESO’s Very Large Telescope (VLT), astronomers have conducted a high-angular-resolution imaging survey of 42 large asteroids in the main asteroid belt, located between Mars and Jupiter. Their results appear in the journal Astronomy & Astrophysics.

This image depicts 42 of the largest objects in the asteroid belt, located between Mars and Jupiter. The images of the asteroids were captured with the SPHERE instrument on ESO’s Very Large Telescope. Image credit: ESO / M. Kornmesser / Vernazza et al. / MISTRAL algorithm / ONERA / CNRS.

This image depicts 42 of the largest objects in the asteroid belt, located between Mars and Jupiter. The images of the asteroids were captured with the SPHERE instrument on ESO’s Very Large Telescope. Image credit: ESO / M. Kornmesser / Vernazza et al. / MISTRAL algorithm / ONERA / CNRS.

“Only three large main belt asteroids, Ceres, Vesta and Lutetia, have been imaged with a high level of detail so far, as they were visited by NASA’s Dawn and ESO’s Rosetta space missions, respectively,” said Dr. Pierre Vernazza, an astronomer at the Laboratoire d’Astrophysique de Marseille.

“Our ESO observations have provided sharp images for many more targets, 42 in total.”

Most of the 42 objects are larger than 100 km (62 miles) in size; in particular, the astronomers imaged nearly all of the belt asteroids larger than 200 km (124 miles), 20 out of 23 asteroids.

The two biggest objects they probed were Ceres and Vesta, which are around 940 km (584 miles) and 520 km (323 miles) in diameter, whereas the two smallest asteroids are Urania and Ausonia, each only about 90 km (56 miles).

By reconstructing the objects’ shapes, the team realized that the observed asteroids are mainly divided into two families.

Some are almost perfectly spherical, such as Hygiea and Ceres, while others have a more peculiar, ‘elongated’ shape, their undisputed queen being the ‘dog-bone’ asteroid Kleopatra.

This poster shows 42 of the largest objects in the asteroid belt, located between Mars and Jupiter (orbits not to scale). Image credit: ESO / M. Kornmesser / Vernazza et al. / MISTRAL algorithm / ONERA / CNRS.

This poster shows 42 of the largest objects in the asteroid belt, located between Mars and Jupiter (orbits not to scale). Image credit: ESO / M. Kornmesser / Vernazza et al. / MISTRAL algorithm / ONERA / CNRS.

By combining the asteroids’ shapes with information on their masses, Dr. Vernazza and colleagues found that the densities change significantly across the sample.

The four least dense asteroids studied, including Lamberta and Sylvia, have densities of about 1.3 g/cm3, approximately the density of coal.

The highest, Psyche and Kalliope, have densities of 3.9 and 4.4 g/cm3, respectively, which is higher than the density of diamond (3.5 g/cm3).

This large difference in density suggests the asteroids’ composition varies significantly, giving astronomers important clues about their origin.

“Our observations provide strong support for substantial migration of these bodies since their formation,” said Dr. Josef Hanuš, an astronomer at the Charles University.

“In short, such tremendous variety in their composition can only be understood if the bodies originated across distinct regions in the Solar System.”

In particular, the results support the theory that the least dense asteroids formed in the remote regions beyond the orbit of Neptune and migrated to their current location.

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P. Vernazza et al. 2021. VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. A&A 654, A56; doi: 10.1051/0004-6361/202141781

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