17P/Holmes is a periodic comet in our solar system discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. In late October 2007 the comet brightened significantly, by nearly a million times, from magnitude 17 to magnitude 2.8 over a few hours.Based on orbital computations and luminosity before the comet's 2007 outburst, its nuclear diameter is estimated at 3.4 km. In late October 2007 the diameter of its coma increased from 3.3 arcminutes to over 13 arcminutes, about half the diameter that the Moon subtends in the sky. At a distance of around 2 AU, this means that the true diameter of the coma swelled to over 1 million km, or about 70% of the real diameter of the Sun. By comparison, the Moon is 380,000 km from Earth. Therefore during this outburst, its coma would have been a sphere wider than the diameter of the Moon's orbit around Earth.

Text courtesy of Wikipedia; Solar System Model courtesy of Software Bisque's TheSky Ver 5 Level 4