The Tail of a Comet

Momentum is not always equal to mv. Like many comets, Halley's comet has a very elongated elliptical orbit. About once per century, its orbit brings it close to the sun.The comet's head, or nucleus, is composed of dirty ice, so the energy deposited by the intense sunlight boils off steam and other gases

Steam and other gases boiling off of the nucleus of Halley's comet. This close-up photo was taken by the European Giotto space probe, which passed within 596 km of the nucleus on March 13, 1986.

The sunlight does not just carry energy, however - it also carries momentum. Once the gas boils off, the momentum of the sunlight impacting on it pushes it away from the sun, forming a tail.

Halley's comet, in a much less magnified view from a ground-based telescope.

By analogy with matter, for which momentum equals mv, you would expect that massless light would have zero momentum, but the equation p = mv is not the correct one for light, and light does have momentum. (Some comets also have a second tail, which is propelled by electrical forces rather than by the momentum of sunlight.)