How a 4 km moon weighed Euphrosyne
The 2019 discovery of a tiny companion gave the first accurate mass — and a density that points to lots of ice.
Published 2026-10-23
Euphrosyne’s moon is tiny compared with the asteroid itself, but it solved one of the main questions about it.[2]
The discovery
In 2019 a small satellite was found next to Euphrosyne. VLT images indicate a diameter of about 4 km, if it has the same albedo as its parent. Preliminary orbit calculations gave a period of about 1.2 days and a distance of about 670 km.[2]
Orbits reveal mass
A moon’s period and distance depend on the mass of the body it orbits. With the moon’s orbit measured, astronomers derived Euphrosyne’s mass in 2020: (1.7 ± 0.3) × 1019 kg.[2]
Density and ice
Mass and size together give a bulk density of 1.7 ± 0.2 g/cm³. If the interior has about 20% porosity, that low density suggests Euphrosyne could be half water ice.[2]
A collisional origin
The moon probably formed in the same collision that created the Euphrosyne family.[2] Euphrosyne’s rounded shape is also thought to come from re-accreting after being disrupted — essentially rebuilding itself as a rubble pile.[2][6]
Numbered citations link to the sources list on the Euphrosyne overview page.