NASA - National Aeronautics and Space Administration

Stardust and Wild 2

Comets are big balls of ice and dust. As they get near the sun, they warm up and the ice sublimes, going from a solid ice to gas. As the gas is ejected from the comet, it blows off like a wind, releasing dust particles and producing a comet's tail. In the case of comet Wild 2, there were at least 20 jets operating. The whole comet measured nearly three miles (3 km) in diameter.

Credit: NASA