Comets

Besides planets, satellites and asteroids, the solar system contains comets (hair in Greek).
The roaming comets originate in the depths of the space in several light years.
The number of periodic comets is of the order of 2000. Comets, small spheres of ice travelling between stars, were surprised by the Sun. Their orbits have elliptic trajectories extremely eccentrics around the Sun.
The most known periodic comet is the Halley's comet which approaches the Sun every 76 years.
When a comet approaches the Sun, the outside layer vaporizes and forms a gaseous envelope (coma).
The solar radiation exercises a pressure on coma and force particles and gases to go away from the Sun by forming a brilliant tail of ionized particles. Comets shine in the memory of the humanity.

They made for a long time figure of misfortune, disasters and also herald of turnover, for a time when not satisfied with our real troubles, we accumulated it of imagination. It is not the messenger of the misfortune, it is the guard of past, the one who detains the keys of our origin.
In 19th century with the separation of the sciences and the religion, the image of the comet is not negative any more.
We can contemplate them during whole weeks, before they disappear to return, 10, 100 or 3000 years later. They are the tramps of the sky, the messengers of the space.

Image: images NASA of the comet Wild 2, taken by the probe Stardust in 2004.

We can say that a comet is a big dirty snowball mixed in some dust, having in its center a big pebble. In 1986 a probe went to photograph the very close Halley's comet (600 km) returning extraordinary images of the nucleus the size of which is 16 km on 8 km.
Comets come from a place called the cometaire sphere, the vestige of past and distant reservoir which surrounds the solar system. This sphere cometaire a date of a period when the primitive solar system had not formed planets yet. There is 5 billion years, a supernova explodes. The shock wave propagates in the space and on its road it meets a nebula.

The cloud condenses to give birth to the Sun in the center and any goshawk a gaseous disk flattened in rotation fills with body which circulate everywhere. Some of these bodies agglutinate and form planets. Numerous asteroids in the system and everything in the suburb, the cloud of Oort, reservoir of the future comets remain.
By the fate of the disturbances certain pebbles leave one day the cloud of Oort and dive in the direction of the Sun, warmed their gas evaporates, a long hair spreads, they became comets. That is why comets detain the most former archives of the solar system.

In 2003, the Rosetta probe will be launched by Ariane 5 to achieve 8 years later, the comet Virtanen.
During 17 months instruments will map the surface of the nucleus in only 10 km in height.
A module will get loose from the probe to arise on the comet and plunge the hand into a virgin matter of 4,5 billion years.

In 1994 the astronomers observe live a comet (Shoemaker-Levy 9) colliding quite hard the planet Jupiter. This fantasy of the collision becomes a tangible reality for us, in spite of the smallness of the Earth.
These celestial bodies in the whimsical orbits can appear without any warning.
In January, 1996, an amateur astronomer discovers a comet in the inner suburbs of the Earth to 15 million km only, 5 km in diameter, it been massive enough to destroy a big part of the life on our Earth. These eons of collision to reminds us the vulnerability of our existence.

Comets and asteroids brought the fundamental elements of the life on Earth and paradoxically could take back it in a single impact.
If one of these collisions can have disastrous consequences for our planet, they probably contributed to the appearance of the life on Earth, to the period when comets and bombarded asteroids the Earth in all directions. The scientists even intended to go astray delicately towards Mars, 2 or 3 of these travelers of the space, to give it an atmosphere, a hydrosphere, an ocean, brief to sow it.

The spherical hair of the comet Holmes exceeded the size of the Sun. It measures 1,4 millions kilometers in diameter.
Its thin cloud of gas and dusts diffuses the solar light, the gases and dusts result from the nucleus outstandingly asset of the comet Holmes, the diameter of which before the surge of the end of October was estimated at 3,4 kilometers.
We can see easily through the external part of the hair, stars by transparency.
The nucleus is more brilliant still.
The brilliant region of the hair, or coma, seems slightly moved away from the centre, what is coherent with the hypothesis according to which a wide fragment of the comet would have separated from the main body before splitting.

The solar system is actually much more complex than it seems, if all its objects. A considerable number of small icy objects, sizes similar to that of asteroids, are in the Kuiper Belt and even beyond in the Oort cloud. This invisible and distant region of the solar system hosts billions of light icy body, to limit the attraction of the Sun.
On this orbit very fragile, almost stationary, the most distant comets du Soleil can be disrupted by any gravitational force, stars closest to the solar system.
The solar system is subject during his trip in the galaxy, with gravitational influences that can disrupt this balance and cause falls comets at the heart of the system and therefore to the planets. The astronomers believe that stellar disruptions are very high because some stars near the sun pass through the Oort cloud.
On average, a star rises to less than 10 000 astronomical units from the Sun every 36 million years and less than 3 000 astronomical units every 400 million years.
Edgard Everhart, American astronomer, has shown that if between a comet in the solar system with an orbital inclination greater than 20 °, it has a chance on two and being ejected out of the solar attraction. The Oort cloud is a remnant of the primitive nebula that collapsed on itself there are 5 billion years.
After their training by accretion, all small objects transneptuniens would have influenced the giant gaseous planets that have pushed to the periphery of the system.

These objects Oort cloud are in a primitive state since their inception, therefore, the materials that formed the solar system at its origin.
It is possible that life came comets at the time of incessant meteorite bombardment, as they are made of oxygen, carbon, nitrogen, magnesium, silicon, iron... which constitute a little water, organic soup leading to the first living organisms.
Comets could not form in the Oort cloud, where they reside today, because these distances, material is too sparse to condense. The only possible place of creation is the planetary system.
Jan Oort, comets were created in the asteroid belt (between the orbits of Mars and Jupiter) and ejected by the giant planets during the formation of the Solar System.
However, comets are frozen bodies, as large dirty snowballs, and the belt of asteroids was too hot for ice to condense. In 1951, one year after the publication of the article Oort, Gerard Kuiper suggested that comets condense farther from the Sun, among the giant planets in the belt that bears his name, the Kuiper Belt, located in the plan the ecliptic beyond the giant planets.