(ESO)
Astronomers presented the results of a ground survey of the largest asteroids in the Main Belt, carried out using the VLT telescope.
They managed to obtain images of the surface of 42 bodies and determine their properties
In particular, they determined the dependence of the shape of asteroids on their mass and period of rotation around their own axis.
Until recently, only a few of the largest asteroids in the Main Belt (Ceres, Pallas or Vesta), became targets for observations using ground-based (VLT, Keck Observatory) or space (Hubble) telescopes.
Those telescopes made it possible to obtain images of their surface in the optical or near infrared range - in other cases, only space probes could get an image of the asteroid surface.
However, since 2014, the situation began to change, since the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch instrument) instrument was launched on the VLT telescope complex.
Despite the fact that its main task is to directly survey exoplanets around other stars, it is perfect for observing asteroids or dwarf planets in the solar system.
A group of astronomers led by Pierre Vernazza of the Astrophysical Laboratory in Marseille presented the results of an extensive observational program
It ran from May 2017 to September 2019, covering the 42 largest asteroids in the Main Belt, located between Mars and Jupiter.
Observations were carried out in the optical wavelength range using a ZIMPOL (Zurich IMaging POLarimeter) camera-polarimeter, part of SPHERE.
The objectives of the observations were 20 bodies with a diameter of more than two hundred kilometers, 19 asteroids with a diameter of one hundred to two hundred kilometers, and 3 objects with a diameter of 85 to one hundred kilometers, which belong to different spectral classes.
Scientists have determined that all observed asteroids (with the exception of Cleopatra) with a diameter of more than one hundred kilometers have a shape close to an ellipsoid, and the deviation from the ideal shape is the greater, the smaller the body size.
In addition, it has been observed that rapidly rotating bodies are more elongated than slowly rotating objects.
The density of the studied asteroids varied from 1.3 to 4.3 grams per cubic centimeter (m3), which, in combination with different albedos, indicates a variety of compositions of these bodies.
In particular, scientists divide the observed asteroids into two types: depleted in volatile substances (density greater than 2.7 m3) and rich in volatile substances (density less than 2.2 m3).
Finally, one more conclusion - the degree of macroporosity of asteroids decreases with an increase in their size and becomes minimal (less than 5%-10%) only in the case of the largest bodies weighing more than 1019 kilograms.
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