Comets are balls of rock and ice that form tails as they approach the sun. These tails makes them visible. Bright comets appear the inthe night sky about once every 10 years.
Short period comets have an orbital period of less than 200 years. Generally originate from the Kuiper Belt. (approx 30 - 50AU from the sun)
Most famous is Halley's Comet with a period of 76 years. Last seen in the inner solar system in 1986. Observations showed Halley's nucleus to be peanut shaped and much darker than expected. After further observation it was concluded that the nucleus was formed 4.5 billion years ago by (mainly water) ices condensing into dust particles.
Long period comets originate from the Oort Cloud, a spherical distribution of cometary nuclei at the outer reaches of the solar system (approx 50,000AU from the sun).
This was proposed by the Dutch astronomer Jan Oort in 1950. Although there is no evidence for the Oort Cloud there is strong support for its existence.
Observations indicate that their paths are either clockwise or anticlockwise around the sun, tend to be very eliptical, tend to originate a long way from the sun (+50,000AU)
Most famous Long-period comet observed in recent years was Hale-Bopp in 1997.
As a comet approaches the Sun its temperature increases causing a spherical COMA around the nucleus up to 100,000km across.
Some comets can even form 2 tails:
Blue coloured straight ION TAIL consisting of atoms and molecules of gas (mainly carbon monoxide) that have been ionised by the solar wind. When they de-excite, atoms emit a light by flourescence.
Lighter-coloured shorter, broader, and slightly curved DUST TAIL. Created by radiation pressure that pushes the particles out of the nucleus. This tail of dust and grit shines by reflecting sunlight and its curvature is because individual particles follow their own independent solar orbit.
Comets tails can be several million km long.
Small rocky irregular lumps of debris ( with perhaps some iron-nickel content) ranging form micromilimetres to several metres in size.
It was recently proposed that the size boundary between meteoroid and small asteroid be 10m by the Royal Astronomical Society.
Some are formed by broken fragments of colliding asteroids while others are formed through impacts with the surface of either Moon or Mars. They orbit the sun at up to 40km/s
Some meteoroids come from dust tails of comets forming a group called METEOROID STREAMS which create meteor showers.
Many meteors intersect the Earth's orbit. When they enter the atmosphere, friction causes the meteoroid and surrounding air to heat up producing a short streak of light - meteor or shooting star.
Meteors with a magnitude of -3 or brighter are called FIREBALLS
If part of the meteoroid surives its passage through the atmosphere it lands on earth and is called a METEORITE.
These can then be classified into three groups depending on their ratio of rock to metal
1 - Stones
2 - Irons
3 - Stony-Irons
When the earth passes through an area of increased dust particles it causes a METEOR SHOWER as the dust particles enter and burn up in the atmosphere.
A meteor shower is names after the constellating in which the radiant is found
Pereids from Perseus constellation (August)
Quantrantids (January); Leonids (November); Geminids (December)
Potentially Hazardous Objects - PHO's
Asteroids and comets whose orbit is close to the Earth's are called NEAR EARTH OBJECTS (NEO's). These have a trajectory which might bring them closer to Earth then 0.3AU (45 million km). In October 2008 approx 5600 NEO's were known.
Some have orbits which bring them even closer to the Earth (less than 0.05AU or 7.5million km). In 2008, approx 1000 have been identified.
Examples of collisions of astronomical bodies in the solar system:
The Moon, several planets, moons and asteroids are covered in impact craters
Venus (Backwards) and Uranus (Sideways) rotate on their axis in unusual ways. Both are thought to have been a result of a gigantic collision early in the solar system's history.
Geological evidence brought back by Apollo shows that the Moon was once part of the Earth and formed through a giant impact
In 1994 several fragments of Comet Shoemaker-Levy 9 collided with Jupiter (Observed)
Evidence of previous impacts on Earth - Barringer Crater, Arizona; Chicxulub Crater, Coast of Mexico, Tunguska Event (1908) in Siberia.
Created in 1999 to categorize the impact hazards and to communicate risks
0 to 1 = likely misses or collisions with objects so small that no damage would be seen
8, 9 and 10 = certain collision capable of causing localised, regional or world-wide devastation eg land destruction, tsunamis or global climatic change that could threaten civilisation.