VLT Measures Main-Belt Asteroid Euphrosyne and Its Moon

Using the high-angular resolution observations from ESO’s Very Large Telescope (VLT), astronomers have determined the 3D shape, diameter and density of the asteroid (31) Euphrosyne and the diameter of its moon.

This image, taken with the ZIMPOL instrument on ESO’s Very Large Telescope (VLT), shows the asteroid Euphrosyne and its moon S/2019 (31) 1 (right). Image credit: Yang et al, arXiv: 2007.08059.

Euphrosyne, one of the biggest objects in the Solar Systrem’s main asteroid belt, was discovered by James Ferguson on September 1, 1854, the first asteroid found from North America.

This asteroid is the namesake of an asteroid family that occupies a highly inclined region in the outer main belt and contains a remarkably large number of members.

Euphrosyne orbits the Sun every 5.61 years and has a small satellite, S/2019 (31) 1, discovered in 2019.

It is a C-type asteroid with a primitive surface possibly covered by ejection blanket in the same collision which created its moon and other Euphrosyne asteroids.

“The main asteroid belt is a dynamically living relic, with the shapes, sizes, and surfaces of most asteroids being altered by ongoing collisional fragmentation and cratering events,” said ESO astronomer Bin Yang and her colleagues.

“Space probes and ground-based observations revealed a fascinating variety among asteroid shapes, where large asteroids are nearly spherical and small asteroids are irregularly shaped.”

Full set of VLT/SPHERE/ZIMPOL images of (31) Euphrosyne. Image credit: Yang et al, arXiv: 2007.08059.

“Most asteroids with diameters greater than 100 km (62 miles) have likely kept their internal structure intact since their time of formation because the dynamical lifetime of those asteroids is estimated to be comparable to the age of the Solar System.”

Dr. Yang and co-authors used the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) and the ZIMPOL (Zurich Imaging Polarimeter) instruments on VLT to observe Euphrosyne and its tiny moon.

Their observations show that the asteroid has a nearly spherical shape with the sphericity index of 0.9888 and its surface lacks large impact craters.

“Euphrosyne is the third most spherical body among the main belt asteroids with known shapes after Ceres and Hygiea,” they said.

“Its round shape is consistent with a re-accumulation event following the giant impact at the origin of the Euphrosyne family.”

According to the astronomers, Euphrosyne’s diameter is 268 km (166.5 miles), making it one of the top ten largest main belt asteroids.

“The bulk density of Euphrosyne is 1,665 kg/m3, which is the first high precision density measurement via ground-based observations for a Cb-type asteroid,” they said.

“Such density implies that a large amount of water (at least 50% in volume) must be present in Euphrosyne.”

“The surface of Euphrosyne is nearly featureless with no large craters detected, which is consistent with its young age and ice-rich composition.”

The researchers also estimated the diameter of Euphrosyne’s satellite to be 4 km (2.5 miles).

“The orbit of S/2019 (31) 1 is circular, prograde, and equatorial, similar to most known satellites around large main belt asteroids,” they said.

The findings will be published in the journal Astronomy & Astrophysics.