Cometa are an icy, small Sol­ar Sys­tem bod­ies that, when passing close to the Sun, warm and begin to release gases, a pro­cess called out­gass­ing. This pro­duces a vis­ible atmo­sphere or coma, and some­times also a tail. These phe­nom­ena are due to the effects of sol­ar radi­ation and the sol­ar wind act­ing upon the nuc­le­us of the comet. Comet nuc­lei range from a few hun­dred meters to tens of kilo­met­ers across and are com­posed of loose col­lec­tions of ice, dust, and small rocky particles.

[descrip­tion from wiki­pe­dia]