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Comets: Visitors From Deep Space

I am a retired engineer and small business owner who has authored over 70 books on history and various topics.

This color photograph of the comet Kohoutek (C/1973 E1) was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona, at the Catalina observatory with a 35mm camera on January 11, 1974.

This color photograph of the comet Kohoutek (C/1973 E1) was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona, at the Catalina observatory with a 35mm camera on January 11, 1974.

What Is a Comet?

Comets are small bodies made of rock, frozen gases, and dust and are members of the solar system. When their orbit brings them near the Sun, they warm and begin to release gases. This outgassing process caused by the Sun’s energy produces a visible atmosphere or coma that surrounds the core or nucleus of the comet. A visible “tail” is formed when the radiation from the Sun and the solar wind causes the gases surrounding the nucleus to stream away from the body of the comet. The comet’s nucleus varies from less than a mile to several miles in diameter. Though the nucleus may be small in astronomical terms, when the Sun warms the comet, the coma can grow to a size larger than the Earth, and the tail can stretch for millions of miles.

What Is the Composition of a Comet?

Comets are thought to be leftover material from the formation of the solar system. They are primarily composed of ice made up of elements such as water, methane, ammonia, carbon dioxide, carbon monoxide, and chunks of rock and dust. On the scale of the solar system, comets are small objects, with the central part of a comet, called the nucleus, as small as a few miles across to as large as tens of miles across.

During the 1950s, comets were described as “dirty snowballs,” a term given to them by astronomer Fred Whipple. Today the description has been updated to “snowy dirtball,” which more aptly describes the composition of a comet. Recent observations and encounters with space probes have determined that they’re covered in dark, dusty shells and only leak gases where holes expose the subterranean ices. Most are dark objects; thus, they have a low albedo (the ability to reflect light) of around four percent--less than a chunk of coal!

Since the nucleus is composed of frozen water and gases, comets tend to outgas when heated, forming a coma when they reach within 4 AU of the Sun (Astronomical Units are the Earth/Sun mean distance, about 93.5 million miles or 150,000,000 km). The force of the solar wind pushes the cometary tail away in a long curve so that it points away from the Sun. Active comets tend to have tenuous atmospheric comas tens of thousands of miles across and sometimes more than a million miles (1.6 million km) in diameter.

The nucleus of Halley's Comet taken in 1986 by the Giotto space probe. Shown is the dark coloration of the nucleus and jets of dust and gas erupting from its surface.

The nucleus of Halley's Comet taken in 1986 by the Giotto space probe. Shown is the dark coloration of the nucleus and jets of dust and gas erupting from its surface.

The Two Types of Tails of a Comet

The most spectacular feature of a bright comet is the tails, which can stretch for millions of miles and grandly sweep across the evening sky. Comets have two tails, one made from the dust ejected from the comet and the second from ionized particles blown off the comet. The ion tail is the less visible of the two, caused by ultraviolet radiation ionizing components of the coma. These ionized particles react with the charges of the solar wind to be pushed away from the Sun. The dust tail is made of surface material and sublimated frozen gases that get blown off the surface by the outgassing, generally forming a more irregular curved tail.

The bright 1997 comet Hale-Bopp showing the blue ion tail and the curved white dust tail.

The bright 1997 comet Hale-Bopp showing the blue ion tail and the curved white dust tail.

Sublimation

The process of sublimation is where a frozen substance changes directly to a gas or vapor. This is different from evaporation, where a liquid substance, such as water, changes to a gas or, in the case of water, steam.

The Oort Cloud

Well beyond the orbit of Pluto, there are vast clouds of material called the Oort Cloud, and it is believed this is the birthplace of comets. The Oort Cloud’s outer surface defines the edge of the Sun’s gravitational influence. The icy material at the cloud’s edge is held so tenuously that passing stars can knock chunks out of orbit to disappear forever into deep space or to be sent plummeting towards the inner solar system to orbit around the Sun as a long period comet.

Dutch astronomer Jan Oort first described the Oort Cloud in 1950. This cloud of icy primordial planetesimals that surrounds the Sun at a distance ranging from 2,000 to 200,000 AU is a vast storehouse of comets. Astronomers believe that the material composing the Oort cloud formed closer to the Sun and was scattered farther into space by gravitational effects from the giant planets early in the solar system’s history. Although direct observations of the Oort cloud have not been made, it is assumed that this is the domain of future long-period comets just waiting for a gravitational nudge to send them racing toward the Sun.

Drawing showing the relative size of the solar system when compared to the Kuiper Belt and the Oort Cloud.

Drawing showing the relative size of the solar system when compared to the Kuiper Belt and the Oort Cloud.

Comet Shoemaker-Levy 9 Crashes Into Jupiter

Astronomers Carolyn and Eugene M. Shoemaker, and David Levy, spotted a new comet on March 24, 1993. Once the orbit of the comet was calculated, they quickly realized it would collide with Jupiter. The comet, named Shoemaker-Levy 9, had become hopelessly caught in the gravitational pull of Jupiter about 20 years before. It was circling the planet in a highly elliptical orbit that was slowly deteriorating.

The unusual comet caught the attention of professional and amateur astronomers all over the world. As a result, many telescopes, including the Hubble Space Telescope, were focused upon it at the appointed time to record the historic collision. The impacts of the comet fragments into the giant gas planet Jupiter were phenomenal. The first comet fragment went screaming through the atmosphere at 134,000 miles per hour (216,000 km/h) on July 16, 1994. Upon impact it threw up a plume 1,900 miles (3,000 km) high and left a huge dark spot 3,700 miles (6,000 km) wide, or about half the width of Earth.

The impacts continued regularly over the next six days. The largest impact was from Fragment G, which left a dark spot big enough to contain the entire Earth. Fragment G was estimated to be only 1.2 miles (2 km) wide. The energy released by the collision was estimated to be 6x1012 tons of TNT, or 600 times greater that the entire nuclear arsenal capability of every atomic bomb on Earth. Two more of similar sized impact fragment followed over the next few days. The locations where the fragments entered the atmosphere of Jupiter were visible in telescopes for several months after the event.

Ultraviolet image of Jupiter taken by the Wide Field Camera of the Hubble Space Telescope. The image shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9.

Ultraviolet image of Jupiter taken by the Wide Field Camera of the Hubble Space Telescope. The image shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9.

Periodic Comets

Depending on the nature of a comet’s orbit around the Sun, they can be one-time visitors to the inner solar system, or they can orbit the Sun on a recurrent basis. Comets with orbital periods of less than 200 years are called short-period comets. Probably the best known short-period comet is Halley’s comet, which becomes visible about every 75-76 years. The last time it passed by the Earth was in 1986. Due to the position of the comet, the Earth, and the Sun, the 1986 apparition of the comet was less than spectacular.

Observations of Halley’s comet go back to 240 BC. It has been recorded by the Chinese, Babylonians, and numerous medieval European astronomers/astrologers; however, none of them recognized it as the same comet appearing repeatedly. In 1705, English astronomer Edmond Halley discovered that this comet was returning to be visible on Earth approximately every 76 years, making it the first periodic comet known. Since Halley’s discovery that some comets are periodic, many more have been discovered.

Video on Comets

Halley’s Comet in March 1986.

Halley’s Comet in March 1986.

Recent Bright Comets

Every few years a new comet is found in the night sky that grows very bright and is easily observed from many parts of the Earth; however, these are the exceptions as most are dim patches of light only visible through telescopes. On those rare occasions when a comet becomes exceedingly bright, as was the case in the 1997 appearance of comet Hale-Bopp, the people of the world were given a special threat in the night sky.

Another bright comet in recent memory was McNaught, also known as the Great Comet of 2007. It was discovered on August 7, 2006, by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope. It was the brightest comet in over 40 years and was easily visible to the naked eye for observers in the southern hemisphere in January and February 2007. With an estimated peak magnitude of −5.5 (brighter than Venus), the comet was the second brightest since 1935. At its peak brightness, it was visible worldwide in broad daylight with a tail that measured an estimated 35 degrees in length. Really bright comets normally only appear one or twice in a lifetime, making them worth the effort to view and admire.

Picture of comet McNaught as seen from Swift's Creek, Victoria, in late January 2007.

Picture of comet McNaught as seen from Swift's Creek, Victoria, in late January 2007.

Comets in History

Observations of comets go back to virtually the beginning of recorded history, with the first by the Chinese in 1059 BC. Without the hinderance of modern urban streetlights, people all over the world were able to observe comets that visited the night sky. Their sudden appearance and often erratic behavior led to them being interpreted as an omen of nature. They were used by astrologers to predict everything from floods to pestilence, war, and the death of a king.

One legendary illustration of a comet’s effect on the outcome of history occurred during the Battle of Hastings in 1066. According to legend, a bright comet appeared near the time of Easter and shown for forty days, inspiring the invading William, the Duke of Normandy, to tell his soldiers that the comet was a sign from heaven of their coming victory. The story was immortalized in the Bayeux Tapestry, sewn by William’s wife at the time. In one panel, King Harold of England is shown fearfully gazing at the comet while his people huddle together in fear, pointing skyward at the comet. In the decisive Battle of Hastings of 1066, William’s invading army was victorious, Harold was killed, and William (later known as William the Conquer) became the king of England.

Fearful observers of the Great Comet of 1066 are shown in the Bayeux Tapestry.

Fearful observers of the Great Comet of 1066 are shown in the Bayeux Tapestry.

The Ancients’ Understanding of Comets

The ancient philosophers attempted to make sense of these strange visitors to the heavens. They didn’t behave like planets with their regular and predictable behavior; rather, they were interlopers, seemingly charting their own course across the sky. The fifth century BC Greek Pythagoreans thought that comets were planets that appeared at great intervals and only rose a little above the horizon, much like the planet Mercury.

The Greek philosopher Aristotle, writing about 350 BC, rejected the idea of the Pythagoreans and others that comets were planets. Aristotle’s dismissal of their idea comes in part six of his work Meteorology: “For all the planets appear in the circle of the zodiac, whereas many comets have been seen outside that circle.” He provides additional rationale: “Again more comets than one have often appeared simultaneously…But, as a matter of fact, no planet has been observed besides the five. And all of them are often visible about the horizon together at the same time.”

In part seven of Meteorology, Aristotle gives his explanation of comets: “The observations before us suggest the following account of the phenomena we are now considering. We know that the dry and warm exhalation is the outermost part of the terrestrial world which falls below the circular motion.” In Aristotle’s idea, a comet is a terrestrial “exhalation.” Two key words in his explanation are “terrestrial” and “circular.” If a comet is in the terrestrial realm, it cannot be part of the realm of the planets who exhibit perfect “circular” motion or in the firmament of the stars which are unchanging in his view of cosmology.

Seneca, who lived in Rome during the first century AD, was a statesman and philosopher who questioned Aristotle’s logic on comets. Seneca wrote one of the few works on the study of nature during that period called Naturales Quaestiones or “Natural Questions.” In this work he stated that comets were formed in our atmosphere from extremely dense air, a view that was in line with Aristotle’s. However, Seneca pointed out that because of their regular movement and being impervious to wind, they could not be atmospheric. He also pointed out that only the tails were transparent and cloud-like, and argued that there was no reason to confine their orbits to the zodiac. Though Seneca did not propose his own physical model of comets, his arguments did spark debate on the nature of comets in the 16th and 17th centuries.

Aristotle’s model of the universe with fixed and unchanging stars held on crystalline spheres, planets that orbited Earth, and comets that were part of the sublunar sphere (below the Moon) dominated for two thousand years. It was not until the 15th century that his model of the universe was overturned.

Aristotle’s cosmology.

Aristotle’s cosmology.

Tycho Brahe and the Comet of 1577

The 16th century Danish astronomer Tycho Brahe used the great comet of 1577 to establish critical proof that Aristotle’s model of comets was wrong. From his observatory on the island of Hven off the coast of Denmark, Tycho made critical measurements of the position of the comet and determined that the comet resided far distant from the Moon.

Tycho came to this conclusion by his repeated measurements with his accurate instruments that the comet exhibited no diurnal parallax; that is, the comet didn’t change its apparent position in the sky relative to the distant stars due to the rotation of the Earth. Tycho deduced that the comet was at least four times further than the Moon, in direct conflict to Aristotle. He established for the first time that the comet was a heavenly body, not part of the terrestrial realm.

The Comet of 1577.

The Comet of 1577.

References

  • Abell, George O., David Morrison, and Sidney C. Wolff. Realm of the Universe, 4th Edition. Philadelphia: Saunders College Publishing, 1988.
  • Aristotle and E.W. Webster (Translator). Meteorology. 1980. Available at: http://classics.mit.edu/Aristotle/meteorology.1.i.html
  • Bynum, W.F., E.J. Browne, and Roy Porter (Editors). Dictionary of the History of Science. Princeton: Princeton University Press, 1981.
  • Levy, David H. Comets: Creators and Destroyers. New York: Touchstone, 1998.
  • Pasachoff, Jay M. Astronomy: From the Earth to the Universe, 4th Edition. Philadelphia: Saunders College Publishing, 1993.
  • The New Encyclopedia Britannica, 15th edition. Chicago: Encyclopedia Britannica, Inc., 1994.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2014 Doug West

Comments

Catherine Giordano from Orlando Florida on December 02, 2014:

I just saw "Interstellar." I imagine the comet being like the ice planet they landed on.

Doug West (author) from Missouri on December 02, 2014:

Caherine:

It is amazing that we were able to land on a comet and get pictures and data. From the pictures I saw, that comet doesn't look like a place I want to visit anytime soon.

Catherine Giordano from Orlando Florida on December 02, 2014:

You must be very excited about the Philae landing. I find all things astronomical fascinating.

muhammad abdullah javed on October 15, 2014:

Very interesting hub Doug West Sir. Thanks for sharing. With all the details, write up ends with a hope that the presence of comet might be a potential threat but the human intellect will surely have command over the invisible threat. I do agree, earth is for humans to live anything that is a threat, the nature ensures one or the other way of its annihilation. We were child when the news of a Skylab hitting the earth spread like a wild fire, but it didn't. We should hope and stay positive about the human success in ensuring precautionary measures. But one question arises for how long this will happen? And from which of the potentials threats? There are innumerable threats, beneath and above the earth and still above in the heavens. The geologists say we are living on a physical hell because of the lava flowing down under whereas the scientists say the universe will attain the state of Big Crunch and ultimately we will have a cold death. Death for human and an end to the universe is sure, apart from scientific advancement for the prevention we should run towards our merciful Creator to ensure success in both the worlds.