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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 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:
Fall into the Sun.
Be captured by a planet and enter a smaller orbit around the Sun.
Collide with a planet (Shoemaker-Levy 9 hit Jupiter in July 1994.
Pass through perihelion and continue back to the Oort cloud, reverting to a lump of ice.
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
Comets reflect sunlight and the gas also emits light
when close to the Sun.
Comet Hale-Bopp
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.
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.
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.
Comet Hyakutake was discovered in Japan on 30th Jan 1996. It is a long-period comet that
won’t return for another 15 000 yrs.
Comet Encke is a periodic comet that orbits the Sun once every 3 yrs. It was first recorded in
1786. Short-period comets are seen at intervals of 3 - 150 yrs.
Halley’s Comet was studied by Halley, the Astronomer Royal in 1682. 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 2061.
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.
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.
Every time a comet passes perihelion it loses about 300 000 000 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
Comet of origin
1 - 6 Jan
2 - 7 May
Eta Aquarids
27 July - 17 Aug
16 - 26 Oct
15 - 19 Nov
7 - 16 Dec
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
Sikhote-Alin meteorite, 1947, Siberia
Canyon Diablo meteorite, 50 000 yrs ago,
Arizona, US
Hoba West Meteorite, Namibia. Landed 80 000 yrs ago. Mass > 60 tons. Composition:
84% iron, 16% nickel.
Meteor Crater, Arizona, US. A meteorite impact crater created 50 000 yrs ago by a nickeliron meteorite of size 50 m moving at 13 km s-1. Diameter 1200 m, depth 170 m.
Tunguska explosion, Russia, 1908. 80 million trees blown flat over 2150 km2. Thought to
be due to the break up of a large meteorite or comet 5 - 10 km above the surface. Or
something more sinister?