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Comets

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Comets

Date de mise en ligne : Tuesday 11 February 2014

Observatoire de Paris - PSL Centre de recherche en astronomie et

astrophysique

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Comets

What are comets made of?

Comets are small solar system bodies which turn around the Sun on elliptical orbits.

Their nuclei are dark and elongated, and their sizes are in the range of a kilometre to several tens of kilometres.

Cometary nuclei are not very dense; lacking significant cohesion, they are extremely fragile and can fragment oreven break up completely.

Cometary nuclei are made up of ices and rocks. The ices are mostly made up of water, but they can also containmore volatile materials such as carbon monoxide, methane and ammonia. The rocks contain minerals and organicmatter.



Comets

As the comets come close to the Sun, their ices sublime, which produces an atmosphere (referred to as the coma)and two main tails:

the dust tail, which is made up of dust grains swept along by the gas;and a plasma tail composed of ions accelerated by the solar wind.Comets carry important information about the origin and formation of the solar system. They are in effect theremains of the small planetesimals which ultimately agglomerated into planets.

Since their birth, comets have been confined to two "reservoirs":

• the Oort cloud, a spherical cloud at a distance of roughly 10 000 au from the Sun; and the Kuiper belt, apopulation of bodies beyond the orbit of Neptune.

Taken from the Agenda astronomique 2014, Institut de mécanique céleste et de calcul des éphémérides del'Observatoire de Paris, EDP sciences, p. 62

The Oort cloud

In 1950, the Dutch astronomer Jan Hendrik Oort realized that the distribution of the semi-major axes of cometsknown at that time with periods in excess of 200 years, peaks beyond 10 000 au: to-day this peak is referred to asthe Oort peak (1 au = the mean heliocentric distance of the Earth). This corresponds to a very small orbital energy.However, as these comets enter the planetary zone of the solar system, planets modify this energy considerably, sothat these comets will be ejected from the Oort peak.

From this, Oort deduced that these comets must have penetrated the planetary zone for the first time, and thattherefore there must be a region, more than 10 000 au from the Sun, which contains a large number of comets.

This region, now referred to as the Oort cloud, extends out to about 200 000 au from the Sun.

The comets in this cloud become observable as a consequence of the gravitational perturbations induced by starsclose to the Sun, and also by that of the entire galaxy.Numerical simulations have shown that, to obtain the rate at which comets are observed, this cloud must containalmost 1 000 billion comets! Such a large number is very hard to undersant in the context of theories for theformation of the Oort cloud and of the solar system.

Note that the hypothesis of the Oort cloud is based only on the distribution of the semi-major axes of long periodcomets; however, no candidate member of this cloud has ever actually been observed so far.


The orbits of comets



Comets

Although planets and most asteroids move on almost circular orbits, comets, comets are different in that their orbits



Comets

are much more elongated: they can be elliptical, parabolic or even hyperbolic.Comets can be classified as a function of their orbits: there are two main classes:

• short period comets (with periods between two successive close approaches to the Sun less than 200 years) ;• and long period comets (periods longer than 200 years). Yet another characteristic distinguishes comets from

the other bodies: they are made up of rocks and ices. As they approach the Sun, the ice covering of thecometary nuclei sublimes (passes directly from the solid to the gaseeous phase, with passing through the liquidphase). This process is referred to as «cometary degassing» and leads to the creation of a reaction force whichperturbs the cometary orbit. Since these bodies sometimes come close to the Sun, their orbits will be modifiedslightly by the effects of general relativity.


Comets and meteorites

It is well known that, under the influence of solar radiation, comets eject gas (mainly made up of water) as well assolid particles (metéoroids).A meteor swarm is thus created in the wake of the comet, and can extend out to many hundreds of millions ofkilometres, filling up the entire orbit.A particular feature of comets of the so-called Jupiter family is that they cross the orbit of the giant planet. Duringclose encounters with Jupiter, the gravitational force is such that the orbit of the comet can change completely,leading to the creation of a new meteor swarm, on a new orbit.



Comets

The adjoining figure shows the position of the meteor swarm ejected by the comet Churyumov-Gerasimenko beforeits close encounter with Jupiter (in 1957), as it is in 2014, during the encounter of the Rosetta probe with the comet.We note that the meteoroides are spread out over the whole solar system. The youngest meteor swarm is close tothe comet, whose position is indicated by a red point. A meteor shower is produced when a planet crosses such swarm.




comets - obspm.fr · comets are much more elongated: they can be elliptical, parabolic or even...

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