60 Comets-High

Comets FOCUS Question What are comets, and how do they move through the solar system?

What Is a Comet? What is the difference between a comet and a big rock? Not much! Comets are icy rocks flying through space. They are best known for their long tails visible in the night sky. Comets have been around since the solar system formed 4.6 billion years ago. They are leftovers from materials that formed the planets. Their long orbits take them to the farthest reaches of the solar system and also bring them dangerously close to the Sun. Some comets orbit in the Asteroid Belt between Mars and Jupiter. But most come from much farther away. Beyond Neptune is the Kuiper (KY-pur) Belt. It contains billions of comets. Even farther out is the Oort Cloud. It is home to trillions of comets!

Trips Around Sun The time it takes a comet to orbit the Sun is called a period. Short-period comets orbit the Sun in 200 or fewer years. They come from the Kuiper Belt. Long- period comets usually come from the Oort Cloud. Why do comets orbit the Sun? As they fly, comets are pulled toward the Sun because of its powerful gravity. But instead of crashing into the Sun, comets usually curve around it. Their speed propels them back out to distant places in the solar system. Comet Type Period (years) Year of next pass around the Sun Halley's short-period 75 2061 Swift-Tuttle short-period 133 2126 Hale-Bopp long-period 2,530 4527 Hyakutake long-period 17,000-70,000 72,000+

Parts of a Comet What happens when a comet comes close to the Sun? To answer this question, it helps to know the parts of a comet. The main part is the nucleus, which is made up of rock, dust, and ice. The nucleus may be 1 to 40 kilometers (0.6 to 25 mi.) across. When a comet nears the Sun, the intense heat melts some of the ice and burns off tiny bits of the rock and dust. The water vapor and burning bits combine to form an atmosphere called a coma (KO-muh). Even closer to the Sun, the comet develops a tail— a trail of dust and gas. The tail always points away from the Sun. This is due to solar wind, a nonstop flow of particles speeding away from the Sun.

It's a Comet's Life Each time a comet nears the Sun, it loses some of its ice and dust. Once all the ice is gone, the comet becomes more like an asteroid. Without the ice to hold it together, it may break into smaller pieces. Some comets meet a more dramatic end. They may get pulled into the gravity of a planet and burn up in its atmosphere. Others may survive to impact the surface. Still other comets are completely destroyed when they travel too close to the Sun. Life Cycle of a Comet 1. A comet begins one of many orbits around the Sun. 2. As the comet nears the Sun, its ice begins to melt, creating the tail. 3. The comet's tail grows longer as it nears the Sun. 4. Due to solar wind, the comet's tail always faces away from the Sun. 5. The comet loses its tail as it moves away from the Sun.

Famous Comets In 1994, astronomers observed a comet's doom— and power! Comet Shoemaker-Levy 9 was in close orbit around Jupiter. That July, it began to break up. Pieces crashed into the massive planet. The impacts formed dark spots on the planet's surface. Comet Hale-Bopp was a bright spot in Earth's skies in 1997. Its orbit takes over 2,500 years. The last people who saw it may have been ancient Egyptians! But the real comet superstar is Halley's Comet. It is well known because about every 75 years, right on schedule, it streaks past Earth.