The nucleus of a comet is a ‘dirty snowball’ just a few kilometres across and made of ice, rock and frozen gases. As the comet’s orbit approaches the Sun solar radiation heats the nucleus, evaporating the surface ices to produce a vast halo of gas and dust which streams off to form the distinctive tail. Comet Holmes (17P/Holmes) has an orbit between Mars and Jupiter and can be seen about every seven years as a very faint object in the sky.
