Comet 17P/Holmes underwent a massive outburst on October 23rd 2007, causing a million-fold increase in brightness. The comet ejected massive amounts of dust and gas from its nucleus. The ejected material can be seen as an expanding shell. The comet displayed similar activity 115 years ago when it was discovered in 1892 by Edwin Holmes.

While all comets undergo some mass loss in their lifetimes, 17P/Holmes is unique in the magnitude and nature of its outburst. Comets eject dust and gas as volatile substances sublimate from the surface of the nucleus, due to heating by sunlight. The gas and the smallest grains of dust form a coma around the nucleus, while larger dust grains will lag behind the nucleus as it orbits the Sun, forming a trail.