Comet Swift-Tuttle

Swift-Tuttle is a periodic comet first seen in 1862. It was observed independently by the American astronomers Lewis Swift and Horace Parnell Tuttle.

Diameter: 2.6 x10 4 m
Mass: 7.5 x 1015 kg
Composition: Ice and rock
Orbital Period: 133.2 Earth years

Swift-Tuttle is the largest comet to orbit periodically around the Earth. After its discovery in 1862, Giovanni Schiaparelli, an Italian astronomer, realised that its orbit was very similar to that of the dust showers that produce the Perseid meteor shower every August. Hundreds, or even thousands, of meteors can be seen at this time from Earth. In 1973, Brian Marsden, an astronomer at the Harvard-Smithsonian Center for Astrophysics, suggested that Swift-Tuttle may be the same comet that was reported in 1737 by Ignatius Kegler, a Jesuit missionary in China.

The comet’s next near approach to Earth was expected in 1982. As this did not occur, astronomers believed that part of its mass may have evaporated during its close orbit around the Sun. It was observed again in 1992 by Japanese astronomer Tsuruhiko Kiuchi using six-inch binoculars as it passed 177 million kilometres from Earth. The observations also conformed that Swift Tuttle was the same comet as observed by Kegler. Swift-Tuttle’s closest approach to Earth was 17 days later than predicted by astronomers, which means there is a possibility that it could collide with the Earth or the Moon on a later orbit.

Marsden continued to refine his observations and traced Swift-Tuttle’s orbits back 2,000 years. He matched it to comets that were observed in 188 AD and even 69 BC. According to his calculations, Swift-Tuttle has a very stable orbit and it is not greatly affected by any jets of gas (water vapour) emitted from its tail. He also predicted that there was no danger of a collision with Earth for at least another 2,000 years. One unusual aspect of this comet is that it completes one orbit around the Sun for every 11 orbits of Jupiter around the Sun.

Swift-Tuttle will next pass close to the Earth in August 2126, at approximately 24 million kilometres. It should be visible to the naked eye at this time. Its nearest approach to Earth is expected in 3044, when the comet will come within 1.6 million kilometres of the Earth’s surface.

At its closest approach, the comet will travel at a velocity of approximately 60 kilometres per second. The chances of a collision with the Earth are regarded as very slight. But if this were to happen, the energy produced would be about 27 times that of the Cretaceous-Palaeogene meteor impact that caused the extinction of the dinosaurs.

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