Comets, Meteors and Meteorites

Other objects in Solar System Asteroids in orbit between Mars and Jupiter. Comets (in highly eccentric orbits). Meteors are cometary debris and are called ‘shooting-stars’. Meteorites are related to asteroids.

Comets Comets with long-period orbits come from the Oort Cloud, an icy region 1 ly from the Sun. Comets with short-period orbits come from the Kuiper Belt, a region extending from the orbit of Neptune out to 55 AU from the Sun. Pluto is a Kuiper Belt object. Comets that originate from the Oort Cloud may fall towards the Sun, a journey that takes over thousands of millions of years. As they reach the inner Solar System they may: 1)Fall into the Sun. 2)Be captured by a planet and enter a smaller orbit around the Sun. 3)Collide with a planet (Shoemaker-Levy 9 hit Jupiter in July )Pass through perihelion and continue back to the Oort cloud, reverting to a lump of ice.

Comets Comets have a small solid nucleus (few km across) made of ice. When they approach the Sun the ice starts to evaporate forming the coma, a halo around the nucleus of dust and gas. Radiation pressure from the Sun pushes some of this dust and gas outwards (away from the Sun) to form two tails - one straight and blue (gas and ions), the other curved and white (dust). The gas tail is affected by the solar wind and magnetic field lines. Not all comets have tails. Comets reflect sunlight and the gas also emits light when close to the Sun. Comet Hale-Bopp

Comets Comets have highly eccentric orbits. Some move in retrograde orbits, e.g. Halley’s comet which has an orbital period of 76 years. It was last seen (at perihelion) in 1986, the next visible point is in 2061.

Comets The orbits may have high inclinations and are perturbed by nearby planets. They are visible when moving in the inner Solar System. They move fastest at perihelion.

Comets Comet Hale-Bopp was discovered in the US on 23rd July 1995 and was visible in the sky for 18 months. It had a nucleus 40 km across and reached within 200 million km of the Earth. It won’t return for 2000 yrs.

Comets Comet Hyakutake was discovered in Japan on 30th Jan It is a long-period comet that won’t return for another yrs.

Comets Comet Encke is a periodic comet that orbits the Sun once every 3 yrs. It was first recorded in Short-period comets are seen at intervals of yrs.

Comets Halley’s Comet was studied by Halley, the Astronomer Royal in He found this comet had a similar motion to those observed in 1607 and 1531 and realised it was the same one returning approximately every 76 yrs. It was last seen in 1986 and is due again in The comet was first recorded in 240 BC and was also visible in 1066 after the Battle of Hastings. It is shown in the Bayeaux Tapestry.

Comets In 1986 the European space probe Giotto was launched to look at Halley’s comet. It took images of the icy nucleus, 15 km wide. Craters and hills could be seen and jets of dust emanating from the nucleus were visible. The nucleus consisted of 84% water ice.

Meteors Every time a comet passes perihelion it loses about tons of material. When a comet breaks up it can produce a meteor shower. Meteors are small icy rocks that flare up upon entering the Earths atmosphere - they are called ‘shooting stars’. They can reach speeds of 72 km s -1, burning up into a fine dust. Leonid shower over Great Wall of China

Meteor Showers DateNameConstellationComet of origin JanQuadrantidsBoötesUnknown MayEta AquaridsAquariusHalley 27 July - 17 AugPerseidsPerseusSwift-Tuttle OctOrionidsOrionHalley NovLeonidsLeoTempel-Tuttle DecGeminidsGeminiUnknown

Meteorites Meteorites are larger objects that reach the Earth’s surface without burning up. These are not linked to comets, they are related to asteroids. Meteorite of Murchison, 1969, Australia Gibeon meteorite, 19th Century, Namibia

Meteorites Sikhote-Alin meteorite, 1947, Siberia Canyon Diablo meteorite, yrs ago, Arizona, US

Meteorites Hoba West Meteorite, Namibia. Landed yrs ago. Mass > 60 tons. Composition: 84% iron, 16% nickel.

Meteorites Meteor Crater, Arizona, US. A meteorite impact crater created yrs ago by a nickel- iron meteorite of size 50 m moving at 13 km s -1. Diameter 1200 m, depth 170 m.

Meteorites Tunguska explosion, Russia, million trees blown flat over 2150 km 2. Thought to be due to the break up of a large meteorite or comet km above the surface. Or something more sinister?