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A Comet in the outer parts of the Solar System is too small to be seen from Earth. You can think of a comet as a large snowball filled with or covered by dust, metal, and debris. They have an icy core, covered by a layer of black dust. The center is mainly composed of water and gases, frozen and mixed together with bits of rock and metal. The nickname for a comet can be a “dirty snowball”.
When a comet gets closer to the Sun, the ice melts and changes into a gas. Dust particles spread out around the nucleus in a cloud called a coma, and the Sun causes them to glow. The coma of an average comet is sixty thousand miles across, but it is also very thin. Radiation and the solar wind drive the gases of the coma away causing it to form a straight tail. The tail may grow to ninety-million miles in length.
Comets
Celestial Wanderers of our Solar System
Comets, asteroids, and meteors are all fascinating celestial objects that have captured human imagination for centuries. While they may appear similar at first glance, these cosmic travelers have distinct characteristics and origins.
Where do comets come from?
Where do comets come from?
Comets are mostly found way out in the solar system. Some exist in a wide disk beyond the orbit of Neptune called the Kuiper Belt. We call these short-period comets. They take less than 200 years to orbit the Sun.
Other comets live in the Oort Cloud, the sphere-shaped, outer edge of the solar system that is about 50 times farther away from the Sun than the Kuiper Belt. These are called long-period comets because they take much longer to orbit the Sun. The comet with the longest known orbit takes more than 250,000 years to make just one trip around the Sun!
What brings comets near Earth so we can see them?
What brings comets near Earth so we can see them?
The gravity of a planet or star can pull comets from their homes in the Kuiper Belt or Oort Cloud. This tug can redirect a comet toward the Sun. The paths of these redirected comets look like long, stretched ovals.
As the comet is pulled faster and faster toward the Sun, it swings around behind the Sun, then heads back toward where it came from. Some comets dive right into the Sun, never to be seen again. When the comet is in the inner solar system, either coming or going, that's when we may see it in our skies.
What are the parts of a comet?
What are the parts of a comet?
At the heart of every comet is a solid, frozen core called the nucleus. This ball of dust and ice is usually less than 10 miles (16 kilometers) across – about the size of a small town. When comets are out in the Kuiper Belt or Oort Cloud, scientists believe that’s pretty much all there is to them – just frozen nuclei.
But when a comet gets close to the Sun, it starts heating up. Eventually, the ice begins to turn to gas. This can also cause jets of gas to burst out of the comet, bringing dust with it. The gas and dust create a huge, fuzzy cloud around the nucleus called the coma.
Why do comets have tails?
Why do comets have tails?
As dust and gases stream away from the nucleus, sunlight and particles coming from the Sun push them into a bright tail that stretches behind the comet for millions of miles.
When astronomers look closely, they find that comets actually have two separate tails. One looks white and is made of dust. This dust tail traces a broad, gently curving path behind the comet. The other tail is bluish and is made up of electrically charged gas molecules, or ions. The ion tail always points directly away from the Sun.
How do we learn about comets?
How do we learn about comets?
People have been interested in comets for thousands of years. But it wasn't possible to get a good view of a comet nucleus from Earth since it is shrouded by the gas and dust of the coma. In recent years, though, several spacecraft have had the chance to study comets up close.
NASA’s Stardust mission collected samples from Comet Wild 2 (prounounced like “Vilt two”) and brought them back to Earth. Scientists found those particles to be rich in hydrocarbons, which are chemicals we consider the “building blocks” of life.
Rosetta, a mission of the European Space Agency that had several NASA instruments onboard, studied Comet 67P Churyumov-Gerasimenko. Rosetta dropped a lander on the nucleus, then orbited the comet for two years. Rosetta detected building blocks of life on this comet, too. And images showed Comet 67P to be a rugged object with lots of activity shaping its surface.
Comets:
Composition
Appearance
Famous examples
Comets are often called "dirty snowballs" because they are composed of ice, dust, and rocky material. The ice is a mixture of frozen gases like water, methane, and ammonia.
Comets have two defining features: a coma and a tail. The coma is a hazy cloud of gas and dust that surrounds the comet's nucleus (solid core) as it heats up approaching the Sun. The tail, made of dust and gas, stretches millions of kilometers away from the Sun-facing side of the comet, appearing like a luminous streak.
Famous examples: Halley's Comet, probably the most famous comet, is visible from Earth every 75-76 years. Comet NEOWISE, discovered in 2020, was a recent spectacle with its bright tail visible even to the naked eye.
Famous People
Frederick Douglass (c.1818–1895) was an African American who escaped from slavery as a young man and dedicated the rest of his life to campaigning for equal rights for all. Some have called him the most influential African American of the 19th Century.
Frederick was the first African American to hold a high position in the US government. In 1872, he also became the first African American nominated for Vice President of the United States, without his knowledge!
Did You Know?
Venus is the hottest planet. Although Mercury is the planet closest to the sun, Venus has the highest temperature due to the gases in its atmosphere. A Greenhouse Effect maintains its constant temperature of 430°C.
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