Halley's Comet:
A Famous Visitor Once Every 76 Years
- It travels from the outermost solar system into the inner solar system:
- It crosses Earth's orbit.
- Then it loops around the sun at about Venus's distance.
- After which it returns to the outer solar system's depths.
- Once inside Jupiter's orbit:
- Sunlight heats the comet.
- Its ice sublimes furiously into gas.
- The gas creates a brught, glowing "coma" of gas and dust.
- A long, streaming tail trails away from the sun.
- The tail is a comet's most famous feature:
- It is created by an effect of the sun.
- Hydrogen gas streams away from the sun.
- Certain effects of sunlight itself carries gas and dust from the coma.
- The tail always points in a direction away form the sun.
- Therefore, the tail does not necessarily stream out behind the comet in its orbital motion.
- Halley's comet, as well as other bright comets consist of three parts:
- A tiny, solid nucleus
- Usually 1 to 20 kilometers (roughly .5 to 12 miles) across.
- It is too small to be seen in a telescope).
- The thin coma (larger than Jupiter).
- And a vast tail.
- It is long enough to stretch from one planet's orbit to another.
- It is among the most spectacular comets.
- Halley's comet has three claims to fame:
- Historically, it has been recorded during many periods of history.
- Much like a faithful friend who drops in on humanity every 76 years.
- It played a pivotal role in the progress of knwledge.
- English scientist Edmund Halley observed the comet in 1682.
- At age 26, he plotted its positions from night to night.
- Years later he conferred with the father of modern physics, Isaac Newton.
- Newton's theory of gravity proved that comet comets traveled in predictable, elliptical orbits around the sun.
- Halley ralized that comets seen in 1531, 1607, and 1682 were all the same body.
- A comet which moved around the sun with a period of 76 years.
- He correctly predicted its return in 1758.
- Thus it became the comet that bears his name.
- In this generation, Halley's comet became the first comet to be photographed at close range by a spacecraft.
- It was the first to reveal the secrets of the cometary nucleus.
- In 1986, when a flotilla of Japanese, Soviet, and European space probes flew past the comet.
- Close-up photos revealed a dark-colored, peanut-shaped nucleus.
- The nucleus was about 15 kilometers long and 7-10 km wide (about 9 x 5 miles).
- It was immersed in a vast glowing cloud.
- The comet's gas does not stream off the surface uniformly.
- It shoots out in bright jets, apparently from isolated vents tha may mark fractures.
- Most of the ice is apparently frozen water.
- The material of the comet is not bright like ice.
- It is as black as black velvet.
- Apparently it is rich in carbon-based organic compounds, such as those found in carbonaceous meteorites.
- Chemical analyzers on the space probes foudn that the black dust coming off the comet was rich in organic chemical compounds made of the same elements that form living matter on Earth, especially carbon, hydrogen, oxygen, and nitrogen.
- These elements that they came to be called CHON particles.
- On any single pass around the sun, at a distance inside Earth's orbit, any given comet may lose a meter (or a few feet) of ice, 'burned" of fits surface by solar heating.
- A periodic comet, like Halley's undergoes this loss every time it goes around the sun.
- Astronomers believe that a typical Halley's-sized comet may last only a few thousand trips around the sun.
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