Because of the density of the asteroids ( 0.8 g\/cm Â ) is less than water and about one third that of rock , the team of researchers led by F. Marchis suggest that the Patroclus system , previously thought to be a pair of rocky asteroids , is more similar to a comet in make up . Because of the density of the components ( 0.8 g\/cm Â ) is less than water and about one third that of rock , the team of researchers led by F. Marchis suggest that the Patroclus system , previously thought to be a pair of rocky asteroids , is more similar to a comet in composition .