Astronomy Picture of the Day . A Cosmic Snowball . ... Representing a previously unknown class of Solar System objects, these proposed small, icy comets disintegrate in the upper atmosphere at altitudes of 600 to 15,000 miles and so do not pose an impact threat to the Earth's surface or even to spacecraft in low Earth orbit . ... The suspected trail of one such cosmic snowball vaporizing over the Atlantic Ocean and Western Europe at an altitude of 5,000 to 15,000 miles is seen above. ...