Comet Holmes (17P/Holmes) in the constellation of Perseus, optical image. This image was obtained on 1 November 2007, and shows the bright spherical coma, a dusty cloud surrounding the comet and shining by reflected sunlight. By late-November the spherical coma seen here had swollen to a diameter of over 1.4 million kilometres (larger than the Sun), due to an outburst that took place in late October. The central comet is only 3.4 kilometres across, and it is thought that the coma expansion was caused when a fragment broke away from the comet and disintegrated. Comet Holmes is a short-period comet, with an orbit of only 6.9 years. It was discovered in 1892 by the British astronomer Edwin Holmes.