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.

• Many scientists believe that a comet or asteroid struck Earth about 65 million years ago near Mexico's Yucatán Peninsula. The cataclysmic event may have caused widespread extinction of the dinosaurs and three-fourths of Earth's living organisms.

• Comets visible to the naked eye appear in the sky about once every five years but don't always make a dramatic scene. Distinctive, bright comets with long tails appear only every 10 to 12 years or so.

• We see comets because their dust reflects sunlight from the void of space and because some comet gases absorb ultraviolet light and glow, much like fluorescent lightbulbs.

• Ancient peoples of many cultures found religious and cultural signposts in the regular movements of celestial bodies such as the sun, moon, and planets. The seemingly random and surprising appearance of comets was often seen as a harbinger of momentous, sometimes ominous, events, such as death, war, or disaster.

Such portents appear in accounts of many legendary and historic events, including the assassination of Roman Emperor Julius Caesar, the onset of the Black Death plague during the Middle Ages, and the arrival of Francisco Pizarro, the Spanish conqueror of the Inca.

• Ancient Chinese astronomers carefully tracked and recorded comets as far back as the Han dynasty in the third century B.C. These records were important in allowing later astronomers to determine the true nature of the celestial bodies.

• Comets were considered random occurrences until the late 1600s and early 1700s. The famous Halley's comet was the first to be identified as a recurring event.

Edmund Halley studied the historical orbits of comets that appeared in 1531, 1607, and 1682 and concluded that they were the same body. He correctly predicted the 1758 return of the comet that now bears his name. But Halley died in 1742, before he could be proven correct.

• The most famous of comets, Halley's comet orbits the Earth every 76 years and next returns in 2061.

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