Non-Planetary BodiesAsteroidsDwarf PlanetsMeteors Halley's Comet


Halley's Comet Source: NASA
Halley's Comet Source: NASA



What is a Comet?

Halley's Comet Nucleus. Halley Multicolor Camera Team; Giotto Project, ESA
Halley's Comet Nucleus. Halley Multicolor Camera Team; Giotto Project, ESA
Comets are small, fragile, irregularly shaped bodies composed of ice made from nonvolatile grains and frozen gases. They are ice rich because they formed beyond the frost line, where abundant hydrogen compounds condensed into ice. Comets are basically chunks of ice mixed with rocky dust and some more complex chemicals such as water vapor, carbon dioxide, carbon monoxide, methane, and ammonia(1).

Any icy leftover planetesimal orbiting the Sun is considered a comet, regardless of its size, whether it has a tail, or where it resides or comes from(2).

Today, we know that the vast majority of comets do not have tails and never venture anywhere close to Earth. Instead, they remain in the outer reaches of our solar system, orbiting the Sun far beyond the orbit of Neptune. Like Halley’s Comet, the comets that appear in the night sky are the rare ones that have had their orbits changed by the gravitational influences of planets, other comets, or stars passing by in the distance. Their new orbits may carry them much closer to the Sun, eventually bringing them into the inner solar system, where they may grow the tails that allow us to see them in the night sky(3).

Spectra of comets confirm their distant origin because they show the presence of compounds that could have only formed in the cold outer layers of the solar nebula. Comet spectra also show evidence of many more complex molecules, including some organic molecules, leading scientists to consider that much of the organic material that made life possible on Earth was brought by comets(4).

Comets contain a nucleus, a coma, and two tails. A comet’s nucleus has been called an icy conglomerate, or “dirty snowball”(5). These dirty snowballs are a few kilometers in diameter and lose much of their ice over time since the ice sublimes when the comet nears the sun.

The coma is the part of the comet that can be seen from Earth. It is far larger than the nucleus it surrounds. It is basically an atmosphere of gas and dust surrounding the nucleus (6).

Coma of Halley's Comet http://www.virginmedia.com/digital/features/10-failed-doomsday-predictions.php?ssid=5
Coma of Halley's Comet http://www.virginmedia.com/digital/features/10-failed-doomsday-predictions.php?ssid=5

The Two Tails of Halley's Comet. http://www.weirdwarp.com/2010/04/the-ins-and-outs-of-comets/
The Two Tails of Halley's Comet. http://www.weirdwarp.com/2010/04/the-ins-and-outs-of-comets/

The comet also has two tails, a dust tail, and a plasma tail. They are pushed out from the coma and extend for approximately 1 AU. The tails also always point away from the sun due to solar wind and solar radiation(7).
The plasma tail consists of gas escaping from the coma. Ultraviolet light from the Sun ionizes the gas, and the solar wind then carries this gas straight outward from the Sun at all times. The dust tail is made of dust-size particles escaping from the coma as it enters the inner solar system. These particles are not affected by the solar wind and are instead pushed away from the Sun by the pressure of sunlight itself, radiation pressure(8). In the picture above, the blue tail is the plasma tail and the white tail is the dust tail.



Edmund Halley: The Man Behind the Comet

Edmund Halley. http://diogenesii.wordpress.com/2011/11/08/november-8-1656/
Edmund Halley. http://diogenesii.wordpress.com/2011/11/08/november-8-1656/

Edmund Halley was born October 29, 1656 in Haggerston, Shoreditch, England. He was a very wealthy and educated man. At the age of 17, he entered Queen's College Oxford, already an expert astronomer. Later in life, King George II backed Halley’s trip to St. Helena Island, the southernmost point of the British Empire, to study and catalogue the southern hemisphere stars(9).


Overtime, through his astronomical studies and observations, Halley had demonstrated that Kepler's third law implied the inverse square law of attraction, and began to question whether this implied elliptical orbits for planetary motion. Investigating the matter further, he visited his good friend, Isaac Newton in Cambridge only to learn that Newton claimed to have solved the problem four years earlier, but could not find the proof among his papers(10). Armed with Kepler's Laws of Planetary Motion, and Newton’s theories of elliptical orbit, Halley recognized that the comets recorded in 1456, 1531, 1607, and 1682 all followed similar paths. He then deduced that these were all, in fact, the same comet(11). Unfortunately, Halley did not live to see the return of the comet, which would later bear his name, on Christmas Day in 1758(12).

Other Accomplishments

Edmund Halley had many other accomplishments in his life along with the discovery of his now famous comet. During his years of scientific study he created two different diving bells to make underwater exploration possible. By studying the moon, he developed a method of determining longitude at sea, and if that were not enough, he also studied archaeology, geophysics, the history of astronomy, and the solution of polynomial equations(13).


Halley’s Comet: A Closer Look






Halley's Clockwise, Inclined Orbit http://csep10.phys.utk.edu/astr161/lect/comets/halley.html
Halley's Clockwise, Inclined Orbit http://csep10.phys.utk.edu/astr161/lect/comets/halley.html

Halley’s Comet is the only periodical comet that can become a bright naked-eye object. It has an orbital eccentricityof 0.96727 and is inclined to the ecliptic at 162.239°r. Its distance at perhelion, inside the orbit of Venus, is 0.58720 AU and at aphelion, beyond the orbit of Neptune, is 35.33 AU(14). The comet’s orbital period is approximately 76 years. It is not definite since the gravitational pull of major planets alters its travel time. It also orbits clockwise, the opposite direction of most other planetary motion(15). The nucleus of Halley’s Comet is very dark, with an albedoof .03. This makes it darker than coal as well as one of the darkest objects in the solar system. The nucleus has a very low density. It is considerably less than the density of water, suggesting that the nucleus is part ice and part empty space. Knowing this, scientists have reason to believe that Halley's nucleus is porous (16). The picture below is a diagram of Halley's orbit. It orbits in retrograde motion, meaning it travels in the opposite direction of other celestial bodies. The diagram also shows that Halley's orbit is highly eccentric, stretching from the inner solar system to beyond Neptune. Lastly, it shows that the comet's orbit is inclined, not in the same plane as most other objects in the solar system.

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Resources:
1, 2, 3, 4, 8. Bennett, Donahue, Shneider, & Voit. (2010). The Cosmic Perspective.
5. What is a Comet? http://www.solarviews.com/eng/comet/whatis.htm
6, 7. Lecture; The Cosmic Perspective
9,10,11,12,13. Edmund Halley: More Than Just a Comet. Nick Greene. __http://space.about.com/cs/astronomerbios/a/edmundhalley__.htm
14, 15, 16. Halley’s Comet http://infoman16.tripod.com/Articles/halley.htm__