Its diameter is estimated 300 kilometers , which is approximately one-third the diameter of Salacia; thus, Salacia and Actaea are viewed by some astronomers to be a binary system.date=November 2019 Assuming that the following size estimates are correct, Actaea is about the sixth-biggest known moon of a trans-Neptunian object, after Charon (1212 km), Dysnomia (700 km), Vanth (443 km), Ilmarë (326 km) and Hiokinaiaka (320 km), but possibly also Hiisi (250 km).