Comet Halley

Comet Halley has been observed since ancient times by Chinese, Mesopotamian and medieval European astronomers.

Diameter: 1.1 x 104m (mean)
Mass: 2.2 x 1014kg
Composition: Ice and dust
Orbital Period: 75.3 Earth years

Halley’s Comet is the best known and most recognised comet that is visible from the Earth with the naked eye. It appears in the Earth’s skies once every 75 to 76 years. The comet was named after the British astronomer Edmond Halley, who first calculated its periodicity.

Halley’s Comet has also been observed in great detail by the spacecraft of the European Space Agency’s Giotto and former Soviet Union’s Vega missions. This data provided the first evidence about the structure of a comet’s nucleus and the formation of its coma and tail. From this information, it was possible to deduce that comets are made up of water, ammonia and carbon-dioxide ices and dust (rock particles). In Halley’s case, only a small proportion of its surface is ice. Astronomers likened it to a “dirty snowball” as it resembled pieces of rubble held loosely together. The comet’s topography includes hills, depressions and at least one crater.

Edmond Halley’s was the first direct calculation that comets were periodic space bodies. Ancient astronomers believed that they were atmospheric disturbances, although a possibility exists that first-century Jewish astronomers had recognised the comet as periodic. The Talmud, a central Judaic text, describes a star appearing every 70 years in the sky.

A 1989 analysis of the records of 46 Comet Halley observations from Earth by Russian physicist Boris Chirikov indicated that its lifetime could be 10 million years. Later studies suggested that it could split in two, evaporate or be ejected from the Solar System in the next hundred thousand years. Over the last 2,000 to 3,000 orbits, the comet’s nucleus may have been reduced by up to 80%.

Halley’s orbit is classified as short-period as it takes less than 100 years. Long-period comets are those with orbits that take thousands of years. It is next expected to pass by the Earth in September 2060 at a distance of 0.09 astronomical units (AU), or 13 million kilometres. Its perihelion, the closest approach to the Sun, is expected in July 2061 at 0.5 AU, or 75 million kilometres. It is also expected to pass within 0.05 AU, or 8.1 million kilometres, of Venus and 0.98 AU, or 147 million kilometres, of Jupiter.

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