Despite the vast size of its coma, Halley's nucleus is relatively small: barely 15 kilometres long, 8 kilometres wide and perhaps 8 kilometres thick. Its shape vaguely resembles that of a peanut shell. Its mass is relatively low (roughly 2.2 kg) and its average density is about 0.6 g/cm3, indicating that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile. Ground-based observations of coma brightness suggested that Halley's rotation period was about 7.4 days. Images taken by the various spacecraft, along with observations of the jets and shell, suggested a period of 52 hours. Given the irregular shape of the nucleus, Halley's rotation is likely to be complex. Although only 25% of Halley's surface was imaged in detail during the flyby missions, the images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.