Comet Facts: From Black Death to Deep Impact

Brian Handwerk
for National Geographic News
Updated January 11, 2005

NASA scientists are set to launch their Deep Impact spacecraft tomorrow. The mission aims to slam a copper projectile the size of a trash can into a comet, Tempel 1, at 22,000 miles an hour (37,000 kilometers an hour).

The mission, the first to delve below a comet's surface, could answer basic questions about how the solar system was created. Scientists believe the material inside comets remains relatively unchanged from the time they were formed, billions of years ago.

In advance of the space launch, National Geographic News has compiled facts on what scientists do know about comets:

• Sometimes described as "dirty snowballs," comets are made of ice, gases, and dust—what's believed to be largely unchanged debris from the creation of the solar system some 4.5 billion years ago.

• A comet's solid nucleus can be several miles in diameter, about the size of a large city. As the sun melts the ice of a comet, a cloud of gas, water, and dust called the coma forms and encircles the nucleus.

• A comet's tail is composed of dust and ions driven from the comet's nucleus by escaping gasses and solar wind. The tail can stretch to 100 million kilometers (62 million miles) in length—two-thirds of the distance between the Earth and the sun.

• Comas and tails give comets a "hairy" appearance. In fact, the word "comet" comes from the Greek komEtEs, meaning "long haired."

• Comets spawn the annual meteor showers we enjoy on Earth. As orbiting comets pass near the sun, intense heat strips them of a layer of dust and ice. These tiny fragments remain in a trail along the path of the comet's orbit. Earth regularly passes through some of these orbits at the same time each year. When it does, the debris particles enter Earth's atmosphere at speeds of up 44 miles a second (71 kilometers a second), creating a meteor shower.

• Did life on Earth originate with comets? Some scientists believe that comets are the source of most water and organic material that was originally delivered to the planets—including Earth. The theory suggests that Earth experienced many comet impacts at the time of its formation about four billion years ago. Water and carbon-containing organic molecules, the building blocks of life, could have come from comets or comet debris.

• About 100 tons (90 metric tons) of space debris, including comet particles, falls to Earth each day. Some scientists believe that significant amounts of bacteria are part of that total and that disease-causing organisms could be evolving in space.

• Comets don't often approach within a few million miles of Earth. The Near-Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, California, watches for them (and meteors) just the same.

Today little could be done if a comet on an impact course with Earth was detected. While they work to identify all near-Earth objects and plot their paths, scientists currently rely on the statistical improbability of such an event. A catastrophic comet or meteor collision occurs, on average, only once or twice every million years.

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