22 Kalliope

22 Kalliope is a large asteroid in the main belt, discovered by J. R. Hind on 16 November 1852. Its name comes from Calliope, the Greek Muse of epic poetry. The asteroid has a small moon called Linus.

Characteristics

Kalliope is somewhat elongated and slightly asymmetric, with a diameter of about 166 km. This measurement, taken from observations of mutual eclipses between Kalliope and Linus using the VLT at the European Southern Observatory, is 8% smaller than earlier estimates from IRAS. Its spectrum is M-type, suggesting a surface that may partly consist of iron–nickel metal. The asteroid’s density is roughly 3.4 g/cm³.

Because Kalliope is likely a rubble pile with a possible porosity of 20–40%, the material density would be between 4.2 and 5.8 g/cm³, indicating a mix of metal and silicates. However, spectroscopic studies show evidence of hydrated minerals and silicates, pointing to a stony surface composition. Its low radar albedo also argues against a purely metallic surface.

Lightcurve analysis places Kalliope’s pole near ecliptic coordinates (β, λ) = (−23°, 20°) with a 10° uncertainty, giving it an axial tilt of 103° and a slightly retrograde rotation. Between 2004 and 2021, the asteroid was observed to occult fifteen stars. In 2022, it was found to belong to an asteroid family that formed about 900 million years ago; its moon Linus may have formed at the same time.

Satellite

Kalliope’s only known natural satellite, Linus, is about 28 km in diameter—large enough to be a notable asteroid on its own. It orbits roughly 1100 km from Kalliope’s center, or about 13.2 Kalliope radii. Linus was discovered on 29 August 2001 by Jean-Luc Margot and Michael E. Brown; another team led by William Merline independently detected the moon three days later.

First stellar occultation

On 7 November 2006, the first stellar occultation by an asteroid’s satellite (Linus) was successfully observed by a group of Japanese observers. The prediction had been made just one day earlier by Berthier et al., based on over five years of regular observations of the Kalliope binary system using adaptive optics on ground-based telescopes. The observed chords of Linus allowed an estimate of its size at 20–28 km.