How the Ananke group formed

One captured asteroid, one impact, and a family of small backwards moons — including Harpalyke.

Published 2026-10-23

Harpalyke is one of the pieces of a broken body.^[2][4]

Clues in the orbits

The core Ananke-group moons have very similar mean orbits. The spread is small enough to be explained by a velocity kick of 15–80 m/s, compatible with a single collision.^[4]

The parent body

From the sizes of the moons, the original asteroid may have been about 28 km across — close to the size of Ananke itself, so the parent was probably not heavily disrupted. The largest fragment still holds 96% of the original mass.^[4]

The impactor

The collision required an impactor only about 0.53 km across — a size that was very common early in the Solar System.^[4]

A puzzle

Harpalyke, Praxidike and Iocaste share similar grey colours, while Ananke is between grey and light red. This has led to suggestions that the Ananke and Pasiphae groups have been mixed by secular resonance.^[4]

Numbered citations link to the sources list on the Harpalyke overview page.