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Comets, meteors and meteorites
Many bright comets appear without much warning and this unpredictability led to them being regarded as portents of disaster in historic times.
Orbital motion of comets
Edmond Halley was the first astronomer to establish how comets moved around the Sun. He realised that observations of a bright comet every 75 or 76 years were referring to the same object and predicted that it would return in 1758. By that time he was long dead but his work was vindicated when the comet came back just as he had said it would. The comet was named for him and has since returned on a further three occasions.
Most comets travel around the Sun in elliptical or open-ended parabolic and hyperbolic orbits that are highly inclined to the plane of the ecliptic. This means that they can appear anywhere in the sky.
Long-period comets take thousands of years to complete a single orbit and may only visit the inner solar system once before returning to deep space. Their orbits can be changed by the gravitational attraction of Jupiter: a long-period comet may become a short-period comet or vice versa. Comets can even be expelled from the solar system after encountering the Jovian gravitational field.
Tracking many of the orbits of long-period comets indicates that they originate at between 50,000 and 100,000 AU from the Sun in the Oort Cloud. This is a hypothetical reservoir of around 1000 billion cometary nuclei, some of which may be deflected towards the Sun when other stars pass nearby.
Short-period comets have orbital periods of less than 200 years. These originate in a second repository of comets - the Edgeworth-Kuiper belt - that lies beyond the orbit of Neptune. Bodies here orbit the Sun at distances between 30 and 50 AU and may be deflected into elliptical orbits by the gravitational influence of the outer planets.
Structure of comets
When a typical comet approaches the Sun it takes on a characteristic appearance. The icy material in the nucleus warms up and begins to evaporate. A tenuous cloud of gas and dust - the coma - forms which can extend millions of km from the nucleus. Transparent tails become visible; the gas or ion tail is composed of gases broken apart by the Sun's ultraviolet radiation into electrically charged molecules and ions. It emits light through fluorescence and is swept back by the magnetic fields associated with the solar wind, pointing directly away from the Sun.
The dust tail is created as radiation pressure from the Sun pushes dust particles away from the nucleus. It shines by reflected sunlight and curves away from the comet, pointing away from the Sun.
Cometary tails can often be as long as 100 million km. The gas tail has a blue colour whereas the dust tail often appears yellow.
The nucleus of comets
At the heart of each comet is the nucleus, an often irregular body perhaps 10 or 20 km across. Several space missions have looked at cometary nuclei at close range, including in 1986 the European Space Agency's Giotto probe. These established that nuclei consist of an interior of water, frozen carbon dioxide (dry ice), ammonia and other organic compounds. The ice is mixed with stony and metallic solids, and it is all contained within a dark black crust.
For most of the time, a nucleus is dormant and the gases are frozen solid. When the comet approaches the Sun, the gases evaporate in jets through vents in the crust. The material in these jets ultimately forms the coma and tails.
They are best observed from dark-sky sites as far as possible from sources of light pollution so that the tails and other faint features can be seen.
Unlike planets, their relatively large angular size makes comets a good target for binoculars and low-power telescopes.
Along with planets, asteroids and comets our solar system contains a large amount of dust and small pieces of rocky material, originating from various sources such as fragmented asteroids. Particles of this kind are meteoroids.
Despite this it is sometimes visible from Earth as the zodiacal light or 'false dawn' after sunset or before sunrise. A cone of light remains visible after dusk or appears before dawn, centred on and extending along the ecliptic.
In the UK this is best viewed in the spring and autumn when the ecliptic makes its steepest angle to the horizon.
Despite their prominence, meteors typically burn up at around 100 km above the surface of the Earth.
The brightest meteors can leave a visible trail of hot gas behind. This slowly fades as it cools and is dispersed by winds in the upper atmosphere.
When a large object enters the terrestrial atmosphere, charred fragments of it may survive this violent final journey and reach the ground. Objects which hit the surface of the Earth from space are meteorites.
Most meteorites are slowed down to a more modest speed - typically 500 km per hour - by the time they reach the lower atmosphere. They then hit the ground relatively gently.
On a typical clear moonless night around seven meteors an hour can be seen, randomly distributed across the sky. These are sporadic meteors and are associated with the dusty material distributed through the solar system.
During meteor showers activity is much more pronounced, with tens of meteors visible each hour. These originate from the dust particles in cometary tails, which eventually spread out along the orbital path of the parent comet.
Meteor showers take place on an annual basis on dates corresponding to the period that the Earth is in the vicinity of the dust stream.
Earth intersecting cometary material
Impact craters on Earth
One of the most famous is the Barringer Crater in Arizona in the USA. This has a diameter of 1280 m, is 180 m deep and was formed by an asteroid of 70 m striking the Earth at 20 km per second.
Although few people have been injured by meteorites in recent times, the impact of very large objects could be responsible for some of the mass extinctions of prehistory. If an asteroid-sized object a few kilometres across were to collide with the Earth, one of the consequences would be the ejection of vast quantities of dust into the atmosphere. This could darken the sky and cool the terrestrial climate for many years, collapse our civilisation and even lead to the final demise of humanity.
In 2002 geologists discovered an impact crater right next to the British Isles. The Silverpit crater lies 1 km beneath the floor of the North Sea and has a diameter of 3 km. Today it is covered by sediments that have preserved its structure from erosion. From its size and appearance, researchers believe that it was formed 60–65 million years ago when the region was covered by a shallow sea.
The incoming rock probably had a diameter of a few hundred metres, weighed around 2 million tonnes and slammed into the ground at 20–50 km/s. This would have caused an explosion equivalent to a 100 megaton nuclear weapon, creating large tidal waves (tsunami) and destruction across the region.
Fortunately collisions on this scale are very rare, but many scientists believe that the impact of a 10-km diameter asteroid in the Yucatan peninsula in Mexico was one factor in the extinction of the dinosaurs some 65 million years ago.
Questions to think about
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