Comet nuclei with radii of up to 30 kilometers (19 mi) have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only 100–200 meters (330–660 ft) in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 meters (330 ft) across. Known comets have been estimated to have an average density of 0.6 g/cm (0.35 oz/cu in). Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.