<text><span class="style10">he Sun and the Solar System (6 of 6)</span><span class="style7">COMETS Comets can best be described as `dirty snowballs'. They are thought to originate in a region known as the Oort Cloud, about one light year from the Sun.Sometimes comets are perturbed from the Oort Cloud and swing in towards the Sun. The gravitational attraction of a planet may trap a comet into a closed but highly elliptical orbit, which will periodically bring it close to the Sun. The best-known example is </span><span class="style26">Halley's Comet</span><span class="style7">, which has a period of 76 years. Halley's Comet is named in honor of the English astronomer Edmond Halley (1656-1742), who successfully predicted the comet's return in 1758.Other comets may reach open parabolic or hyperbolic orbits. These will swing past the Sun just once and then be lost from the Solar System for ever.Comets may have on occasion collided with the Earth. On 30 June 1908 a great explosion occurred in the sparsely populated Tunguska area of Siberia, flattening trees over a wide area. The object that caused this enormous devastation may have been the nucleus of a small comet, rather than a meteorite, which would have left a crater.</span></text>
</content>
<content>
<layer>background</layer>
<id>26</id>
<text><span class="style10">alley's Comet</span><span class="style7">, photographed in 1910. Comets probably consist of a small nucleus of ice and dust. When a comet approaches the Sun, part of the nucleus vaporizes to form a luminous cloud (the coma) and the tail, which always points away from the Sun. Although the nucleus may only be a few km in diameter, comas may have diameters up to 1 million km (620 000 mi).</span></text>
</content>
<content>
<layer>background</layer>
<id>23</id>
<text>ΓÇó STARS AND GALAXIESΓÇó THE INNER PLANETSΓÇó THE OUTER PLANETS</text>
