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Halley's Comet:
Halley's Comet:
Most famous comet, returns every 76 years.
Most famous comet, returns every 76 years.
Orbital Period and Characteristics: Halley's Comet has an average orbital period of 76 Earth years. However, this period can vary due to the gravitational influence of planets, particularly Jupiter and Saturn. In fact, the gravitational pull of the major planets alters the orbital period from revolution to revolution. Measured from one perihelion passage to the next, Halley's period has been as short as 74.42 years (1835-1910) and as long as 79.25 years (451-530).
Orbit Path: The comet's orbit is highly elliptical, with an eccentricity of 0.967. This means its path around the Sun takes it from a point relatively close to the Sun (perihelion) to a point far beyond Neptune's orbit (aphelion).
Orbit is Retrograde: Halley's Comet orbits the Sun in a retrograde direction, opposite to the direction of most planets. Its orbit is also inclined by 18 degrees to the ecliptic, the plane of Earth's orbit.
History of Observations: Halley's Comet has been observed and recorded by astronomers for millennia. Before Halley's prediction of its return in 1705, these appearances were not recognized as being the same object. Ancient civilizations, particularly the Chinese, meticulously documented cometary appearances, providing valuable historical records. It is a remarkable fact that observations made with the naked eye 2,000 years ago are still of value today. In the centuries leading up to Halley's prediction, observations from China, Babylon, and Europe helped to track the comet's appearances. These observations, even those made with the naked eye, have proven valuable for modern astronomers studying the comet's long-term behavior.
The comet's appearance in 1066, shortly before the Norman Conquest of England, is perhaps its most famous historical sighting. It was depicted in the Bayeux Tapestry, a significant historical artifact. It is said that William the Conqueror believed the comet heralded his success.
The 1910 apparition of Halley's Comet was also notable, as it was the first time the comet was captured on camera.
The 1986 apparition marked a turning point in our understanding of comets. For the first time, spacecraft were sent to encounter a comet, providing close-up images and data about its nucleus and composition. This included:
Giotto (European Space Agency), which provided the first close-up images of the nucleus.
Vega 1 and 2 (Soviet Union), which provided data and images.
Suisei and Sakigake (Japan). These probes studied the comet's coma and tail.
International Cometary Explorer (NASA).
Key dates:
Halley's Comet reached perihelion on February 9, 1986.
Halley reached its closest approach to Earth on April 11, 1986 at a distance of 0.42 AU or 62.4 million km.
Halley\'s Comet reached aphelion most recently on December 9, 2023.
It will reach perihelion next on July 28th, 2061 and then aphelion again on November 21, 2097.
Halley's Meteor Showers: Each time Halley returns to the inner solar system, its nucleus sprays ice and rock into space. This debris stream results in two weak meteor showers each year: the Eta Aquarids in May and the Orionids in October.
Physical Characteristics:
Halley's Comet is a relatively small object, with a nucleus measuring approximately 15 kilometers long, 8 kilometers wide, and 8 kilometers thick. Its shape has been likened to a peanut or a potato.
Its low density indicates that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile.
The comet's nucleus is a dark, dusty conglomerate of ice and rock, often described as a "dirty snowball."
It reflects only 4% of the light that falls on it. This makes it one of the darkest objects in the solar system. This dark coloration is likely due to a layer of dust and complex organic molecules covering much of its surface.
Tail or Coma: As Halley approaches the Sun, its volatile compounds, such as water, carbon monoxide, and carbon dioxide, begin to sublimate, forming a coma, or atmosphere, around the nucleus. This coma can extend up to 100,000 kilometers across. The sublimation process also releases dust particles, which contribute to the comet's tail.
Next Predicted Perihelion Passage: Halley's Comet is expected to reach its next perihelion, or closest approach to the Sun, on July 28, 2061. During this apparition, Earth will be in a more favorable position for viewing the comet compared to its 1986 appearance. It is predicted to be as bright as some of the brightest objects in the sky (apparent magnitude -0.3). Even more exciting, in 2134, Halley's Comet will pass very close to the earth (0.09 AU, or about 13 million km) and should be much brighter than in 2061 (apparent magnitude -2).
Paloma's Orrery Plots:
Plot of Halley's orbit at aphelion on December 9, 2023. (Very Large File: 93.2 MB)
Animation of Halley's comet at aphelion on December 9, 2023. (Very Large File: 90.1 MB)
Animation of Halley's comet at perihelion, 60 days from January 9, 1986. (Very Large File: 88.8 MB)
Animation of Halley's comet at closest approach to Earth, 60 days from March 11, 1986. (Very Large File: 88.8 MB)
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