Comets

Comets are considered leftovers of the solar system formation. Comets may be of the periodic type, that is that they regularly come into the inner solar system. Most of the periodic comet however never were officially seen as such as most weren't seen more than twice at their perihelion. From the thousands of comets seen, 190 only are classified periodic. Short-period comets are orbiting the Sun in less than 200 years and they grossly do in the ecliptic plane. Most such comets are thought to come from the Kuiper Belt, this region of comets and diverse leftovers which lies beyond Neptune's orbit. Long-period comets, as far as they are concerned, may have orbits of a duration of up to

Comet Borrelly's nucleus, jets, and coma. picture NASA/JPL

30 million years, as they have more varied inclinations to the ecliptic. Such comets are originating in a more distant zone, the Oort Cloud, a huge cloud of leftovers dating back to the early solar system. Some comet, at last, may have hyperbolic or parabolic orbits, meaning they are passing just once in the inner solar system. Most comets are passing between 0.05 and 2.5 AU from the Sun where they come at their perihelion, that is at their nearest Sun. The famous Halley comet, which passes each 76 years in the inner solar system, is now heading to its farthest from the Sun. It will not be back until 2061

The Kuiper Belt is thought to extend beyond Neptune, between 2.8 billion and 28.6 billion miles (4.5 billion and 46 billion kilometers), that is, for its largest, up to 300 Astronomical Units (AU) -300 times the distance Sun-Earth. Important comets, those with a nucleus of about 25-mile (40-kilometer) wide, are thought to be mostly within 50 AU from the Sun. Much of the short-period comets have diameters of about 3,300 ft to 6 miles (about 1 to 10 kilometers). The Kuiper Belt's exact content -and even extent- is still in doubt

Comets are icy and dusty bodies. As they come nearer to the Sun, their nucleus is vaporized by heat. It yields then a tail which may extend up to millions of miles (millions of km).

Comet Borrelly's 5-mile (8 km) long nucleus as seen from 2,000 miles (3,200 km). picture NASA/JPL

Such a tail is generally directed away from Sun, no matter whether comet is approaching the Sun or receding from it. Another smaller tail, only made of dust, may exist too, with no preferential direction except that the heaviest dust stay behind the nucleus, along the comet's path. The "coma" is the nebulous halo which is surrounding the comet's nucleus. It's a cloud of dust and gas. It may be considered the comet's atmosphere. The "coma" is produced by the same processes which are yielding the comet's tails. Nuclei are small bodies, with a diameter of about 6 miles (9.5 km), as comae may exceed Jupiter in size! Comets are now believed to be carriers of well-preserved materials dating back to the formation of the solar system, some 4.6 billion years ago. Comets now, too, are seen like bodies which, along with asteroids, took part to the Heavy Bombardment Period. Such a period occurred around 3.8 billion years ago when the planets, once formed, just swept, all the debris and various objects which were remaining from the solar system's formation. Such objects heavily struck the planets. Comets and asteroids are thought to have thus brought water, and the fundamental blocks of life! Conspicuously naked-eye comets are representing less than 10 percent of all the comets ever recorded. The term "comet" is originating from a Latin world, "comata", wich means "hairy", refering to the comets' tails. Water, carbon monoxide, ammonia and methanol are four of the most abundant compounds and gases found at a comet

What is Composing a Comet?
From the data collected by the Spitzer Telescope's infrared spectrometer during the impact of mission Deep Impact at comet Tempel 1, this particular comet was found made of these following elements: clays, ice, and dry ice. Iron-containing compounds, carbonates, the minerals in seashells, crystallized silicates, such as the green olivine minerals, and polycyclic aromatic hydrocarbons, these carbon-containing compounds found in car exhaust or on burnt toast. Hints of the mineral found in the reddish-brown gem spinel were also observed. Additional molecules include water vapor and carbon dioxide gas. Is such composition typical of any comet?

Much Variety Among the Comets Materials
As comets, until now, were thought to be composed from interstellar dust and ice, no less than 10 percent of comet Wild2, that comet samples of which were taken back Earth by the Stardust mission, originated from the inner solar system. Another finding is that they were likely different stages along which various materials formed and/or aggregated during the solar system formation, which are found back in comets
It might that some comets, on the other hand, formed in the inner, hot, regions of the fledgling solar system, and then were transported to beyond the orbit of Neptune!

The protoplanetary disk from which the planets formed around the Sun might not have extended beyond current Neptune's orbit. Neptune and other gas giants would have formed nearer to the Sun, along with the Kuiper Belt objects. Mostly Uranus and Neptune would have moved outward then, Neptune pushing away some material and the Kuiper Belt objects. Neptune eventually stopped where the protoplanetary disk was ending