Solar system

Pluto's satellite, Charon, is larger in proportion to its planet than any other satellite in the solar system.

Pluto has one moon, Charon, which was discovered in 1978. The satellite may be a chunk that broke off Pluto in a collision with another large object.

PLUTO: HADES IN ANCIENT MYTH, ROMAN GOD OF THE UNDERWORLD

Historical notes

Pluto was not discovered until 1930, by amateur American astronomer Clyde

Tombaugh. Since Tombaugh's death in 1997, many astronomers have increasingly urged the International Astronomical Union, which names celestial objects, to strip Pluto of its status as a planet.After a news report generated a flurry of irate e-mails about the possible change, officials assured the world that Pluto would remain a planet. But it will also likely become the first in a new class of celestial object known as a TNO, or Trans-Neptunian Object. It seems Pluto may then have a sort of dual citizenship.

Comets

Made of dust, ice, carbon dioxide, ammonia and methane, comets resemble dirty snowballs. You may remember them as blurry smudges in the sky. Comets orbit the Sun, but most are believed to inhabit in an area known as the

Oort Cloud, far beyond the orbit of Pluto. Occasionally a comet streaks through the inner solar system; some do so regularly, some only once every few centuries.

Heads and tails

As a comet nears the Sun, its icy core boils off, forming a cloud of dust and gas called a head, or coma. Comets become visible when sunlight reflects off this cloud. As the comet gets closer to the sun, more gas is produced.

The gas and dust is pushed away by charged particles known as the solar wind, forming two tails. Dust particles form a yellowish tail, and ionized gas makes a bluish ion tail. A comet's tails, like these on comet Halley, always points away from the Sun.

Meteor showers

When Earth crosses the path of a comet, even if the comet hasn't been around for a few years, leftover dust and ice can create increased numbers of meteors.

Asteroids

Quick quiz: How many planets orbit our Sun? If you said nine, you're shy by several thousand. Scientists consider asteroids to be minor planets - some are hundreds of miles wide (and seldom round).

Orbits

Most, but not all, orbit the sun in an asteroid belt between Mars and

Jupiter. The huge gravitational pull of Jupiter accelerated these asteroids to more than three miles per second -- too fast to prevent violent collisions. Otherwise, they might have joined up to form "real" planets.

When asteroids collide, fragments sometimes are sent on a collision course with Earth and become meteors.

Size and makeup

The vast majority of asteroids are small, compared with a large one like

Ida, this 32-mile-long chunk of stone and iron that was photographed in

1993 by the Galileo spacecraft. Though we normally think of asteroids as crater-makers, they are typically pockmarked with their own impact craters.