Each year between 6 and 30 November (approximately) the the Earth encounters the Leonid Meteors, one of the more spectacular of the annual meteor showers, with peak activity this year expected before dawn on Monday 18 November. Unlike most such showers, which are essentially composed of dust particles, the Leonids comprise particles of up to 8 mm across and up to 85 g in mass, leading to some spectacular fireballs, and each year the shower is thought to deposit 12-13 tonnes of material on the Earth. The Leonid Meteor Shower is so called because the meteors they appear to originate in the constellation of Leo. (Note a meteor is a 'shooting star', a piece of material visibly burning up in the atmosphere and detectable via the light it produces when doing this; a meteorite is a piece of rock that has fallen from the sky and which a geologist can physically hold; and an asteroid is a chunk of rock in orbit about the Sun, to small to be regarded as a planet.
The radiant point (apparent point of origin) of the Leonid Meteors. Greg Smye-Rumsby/Astronomy Now.
The Leonid Meteors are thought to originate from the tail of Comet 55P/Tempel-Tuttle, which orbits the Sun every 33 years, on an orbit that brings it slightly within the orbit of the Earth then out to slightly beyond the orbit of Uranus. Comets are composed largely of ice (mostly water and carbon dioxide), and when they fall into the inner Solar System the outer layers of this boil away, forming a visible tail (which always points away from the Sun, not in the direction the comet is coming from, as our Earth-bound experience would lead us to expect). Particles of rock and dust from within the comet are freed by this melting (strictly sublimation) of the comet into the tail and continue to orbit in the same path as the comet, falling behind over time.
The material in the meteor shower is densest close behind the comet, and, since Comet 55P/Tempel-Tuttle has a 33 year orbit, the Leonid Meteor Shower has a 33-year cycle, with a particularly spectacular display every thirty-third year, then a gradual decline in meteor number till the end of the cycle. The last such peak year was in 1998.
Image of Comet 55P/Tempel-Tuttle taken on 31 January 1998, 60 second exposure. Martin Mobberley.
Comet 55P/Tempel-Tuttle was discovered in December 1865 by German astronomer Wilhelm Tempel, and independently in January 1866 by the American Horace Parnell Tuttle. The designation 55/P implies that it is a Periodic Comet (comet with an orbital period of less than 200 years), and that it was the 55th such body discovered. As a Comet with a Period of less than 200 years and more than 20 years it is also regarded as a Halley-type Comet.
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