Halley's Comet
Overview
Halley's Comet, officially designated 1P/Halley, is a periodic comet visible from Earth approximately every 75–76 years. It is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus has been observed and recorded since ancient times. Halley's Comet is named after the English astronomer Edmond Halley, who, in 1705, was the first to compute its orbit and predict its return.
Historical Observations
Ancient Records
Halley's Comet has been observed and recorded by various cultures throughout history. The earliest known record of the comet dates back to 240 BC in Chinese chronicles. The Babylonians and the ancient Greeks also documented its appearances. The comet's passage in 1066 was famously depicted in the Bayeux Tapestry, which commemorates the Norman conquest of England.
Edmond Halley
In the early 18th century, Edmond Halley used Newtonian physics to calculate the orbits of several comets. He noted that the comets of 1531, 1607, and 1682 had very similar orbital elements and concluded that these were the same comet returning periodically. Halley predicted its return in 1758, and although he did not live to see it, the comet's return confirmed his calculations and it was subsequently named in his honor.
Orbital Characteristics
Halley's Comet follows an elliptical orbit around the Sun, with an orbital period of approximately 75–76 years. Its perihelion (closest approach to the Sun) is about 0.586 AU, while its aphelion (farthest distance from the Sun) is about 35.1 AU, placing it beyond the orbit of Neptune. The comet's orbit is highly inclined at 162.26° to the ecliptic plane, making its path retrograde.
Physical Characteristics
Nucleus
The nucleus of Halley's Comet is an irregularly shaped body approximately 15 kilometers long, 8 kilometers wide, and 8 kilometers thick. It is composed primarily of water ice, dust, and organic compounds. The surface is covered with a dark layer of carbon-rich material, which absorbs sunlight and causes the comet to heat up and release gas and dust.
Coma and Tail
As Halley's Comet approaches the Sun, the heat causes the volatile materials in the nucleus to sublimate, forming a glowing coma around the nucleus. The solar wind and radiation pressure then push the gas and dust away from the comet, forming a tail that can extend millions of kilometers into space. The tail always points away from the Sun due to the influence of the solar wind.
Scientific Missions
Giotto Mission
The European Space Agency's Giotto mission was the first to closely observe Halley's Comet. Launched in 1985, Giotto made its closest approach to the comet on March 13, 1986, passing within 596 kilometers of the nucleus. The mission provided detailed images and data on the comet's nucleus and its activity, greatly enhancing our understanding of cometary physics.
Other Missions
Other spacecraft, including the Soviet Vega probes and Japan's Suisei and Sakigake, also studied Halley's Comet during its 1986 apparition. These missions provided additional data on the comet's composition, structure, and interaction with the solar wind.
Cultural Impact
Halley's Comet has had a significant impact on human culture and history. Its appearances have been recorded in various historical documents, and it has been associated with omens and portents. The comet's 1910 apparition caused widespread public interest and even panic, as some believed that the Earth would pass through the comet's tail and be exposed to toxic gases.
Future Observations
Halley's Comet is expected to return to the inner solar system in 2061. Advances in technology and astronomy will likely allow for even more detailed observations and studies of the comet during its next apparition. Future missions may include close flybys or even landers to study the comet's surface and composition in greater detail.