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.

Scientists divide asteroids into two groups, based on how they appear in infrared images: light and dark. The lightest-looking asteroids are rocky bodies with lots of iron and nickel, and they resemble lunar rocks. The darkest asteroids have high quantities of hydrated minerals and carbon.

In the early days of the solar system (some 4.6 billion years ago) asteroids had metallic cores, middle regions of stone and iron, and surfaces of stone. Over time, many of them collided with others and broke apart. The fragments, which became many of today's asteroids, are therefore classified as irons, stony-irons or stony.

When an asteroid, or a part of it, crashes into Earth, it's called a meteorite.

Origin

There are two hypotheses about how most of the asteroids formed. One says they broke off of a mother planet that existed between Mars and Jupiter. More likely, however, they represent what space was like before the planets formed, and they are the remnants of that process -- bits and pieces that never quite joined together.

The threat of impact

Since the Earth was formed more than four billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA.

An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say.

Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis.

More than 160 asteroids have been classified as "potentially hazardous" by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet.

Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it.

For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening. NASA puts the odds at 1 in 10,000 of discovering an asteroid that is within 10 years of impact.

Two programs have been set up to actively search for Near-Earth Objects (NEO's): NASA's Near Earth Asteroid Tracking (NEAT) program, and Spacewatch at the University of Arizona.

Also, the Spaceguard Foundation was established in 1996 in Rome. The international organization's goal is to protect Earth from the impacts by promoting and coordinating discovery programs and studies of NEOs. A January report shows that