The main asteroid belt, between Mars and Jupiter, is made up of millions of rocky bodies, from pebbles to worlds the size of dwarf planets. Most are very difficult to see even with the best telescopes because they are so small and so far away. This month (October 2021), astronomers at the European Southern Observatory (ESO) said they had obtained new images of 42 of the largest asteroids. The images show these asteroids in a level of detail never before seen by terrestrial telescopes.
Astronomers published the results of their imaging study in the peer-reviewed journal Astronomy & Astrophysics October 12.
A New Look at 42 of the Biggest Asteroids
Astronomers used the Very Large Telescope (VLT) in Chile for these new images. The images reveal a wide variety of shapes, from spherical shapes to those that resemble dog bones. Pierre Vernazza, from the Laboratoire d’Astrophysique de Marseille in France, and his colleagues conducted a comprehensive study of the largest bodies in the asteroid belt between 2017 and 2019. Only three of them have already been photographed in great detail by a spaceship, Vernazza said:
So far, only three large Main Belt asteroids, Ceres, Vesta, and Lutetia, have been photographed in high level of detail, as they have been visited by NASA’s Dawn and Rosetta space missions respectively and European Space Agency. Our ESO observations provided sharp images for many other targets, 42 in total.
Two families of asteroids
Asteroids are divided into two basic families: spherical and elongated. Ceres, for example, is almost perfectly spherical (and also classified as a dwarf planet), while the smaller Cleopatra resembles a dog bone.
Most of these 42 asteroids are over 62 miles (100 km) in length. The researchers also photographed 20 of the 23 asteroids larger than 124 miles (200 km).
Not only do asteroids vary in shape, but they also differ greatly in density. At the bottom of the scale, four of the asteroids have densities of about 1.3 grams per cubic centimeter (0.75 ounces per cubic inch). This is approximately the density of coal. This includes the Lamberta and Sylvia asteroids. The two densest asteroids are Psyche and Kalliope. They have densities of 3.9 and 4.4 grams per cubic centimeter, respectively (2.25 and 2.5 oz / cubic inch). To give a comparison, this is even higher than a diamond, which weighs 3.5 grams per cubic centimeter (2 oz / cubic inch).
The researchers said the different densities of the asteroids provide clues to their origin and composition. Co-author Josef Hanuš from Charles University in Prague, Czech Republic, explained:
Our observations strongly support a substantial migration of these bodies since their formation. In short, such a great variety in their composition can only be understood if the bodies originate from separate regions of the solar system.
The researchers used the SPHERE (Spectro-Polarimetric High Contrast Exoplanet REsearch) instrument on the VLT to better determine the shapes and other characteristics of asteroids. Co-author Laurent Jorda said:
Thanks to the improved capabilities of SPHERE and the fact that little was known about the shape of the larger asteroids in the Main Belt, we were able to make substantial progress in this area.
Then astronomers will be able to image these and other asteroids at even better resolution with the upcoming Extremely Large Telescope (ELT). The telescope is currently under construction and will begin operating later this decade. It will be a significant improvement over the VLT, as Vernazza explained:
ELT observations of main belt asteroids will allow us to study objects with a diameter of up to 35 to 80 kilometers [22 to 50 miles], depending on their location in the belt, and craters up to about 10 to 25 kilometers [6 to 16 miles] in size. Having a SPHERE-type instrument at the ELT would even allow us to image a similar sample of objects in the distant Kuiper Belt. This means that we will be able to characterize the geological history of a much larger sample of small soil bodies.
How big are asteroids?
To get a better idea of the size of asteroids, you can watch a video here, from MeatBallStudios. It shows the relative sizes of 22 different asteroids, from the very small 2008 TC3 to the huge Ceres.
Some of these asteroids are in the main asteroid belt, but 10 of them are so-called near-Earth objects. Some of them are also potentially dangerous, which means there is a chance they will hit Earth one day.
Conclusion: Astronomers at the European Southern Observatory (ESO) have obtained new, more detailed images of 42 of the largest asteroids in the main asteroid belt. They feature a wide variety of shapes, from spherical objects to dog bone shaped objects.
Source: VLT / SPHERE imaging survey of the largest asteroids in the main belt: final results and synthesis