Каталог :: Астрономия

Реферат: Кометы

Comets can be spectacular objects seen in the night-time sky. They have been
associated by the superstitious with disasters and other notable historical
events. Until the 1986 opposition of Halley's comet, the true nature of a
comet's nucleus was the subject of argument amongst astronomers. The passage
of the Giotto probe close to the nucleus of Comet Halley and the many
observations that were carried out worldwide have vastly improved our
knowledge of the nature of comets.
Because comets can be seen so easily, records of the observation of comets
can be traced back over many centuries. It was from a study of the historical
observations of several comets that Halley, using Newton's new theory of
gravitation, showed that the orbits of several comets around the Sun were
almost identical. He postulated that they were all the same object and
predicted that it would be seen again at a certain time in the future. As we
know, Halley's comet did reappear around the predicted date and has been seen
since then on each of its journeys in towards the Sun.
Comets, as seen from the Earth, appear to have some sort of nucleus which is
surrounded by a bright, more or less circular region called the ‘coma’ from
which one or more tails may be seen spreading out away from the direction to
the Sun. These tails when photographed can be seen to be different colours.
There is often a filamentary structured tail which is bluish and a series of
more amorphous tails which are yellowish. The supposed nucleus of the comet is
the bright centre of the coma. The coma and the tails develop markedly as the
comet gets closer to the Sun with tail lengths sometimes growing as long as 100
million kilometres. 
     The Orbits of Comets 
The first computation of cometary orbits was made by Halley, as mentioned
above. Since then the orbits of many hundreds of comets have been determined.
They almost all fall into two types; periodic orbits, which take the form of
very eccentric ellipses, and parabolic orbits.
The orbits of many comets have periods ranging from hundreds of years to tens
of millions of years, indicating that they spend much of the time far outside
the orbits of Neptune and Pluto. The orbits of the long-period comets are not
confined to a plane, like the orbits of the planets, and these comets can
appear in any part of the sky. In order to explain the orbits of comets,
astronomers have postulated the existence of two groups of comets on the
edges of the solar system:
     The Oort Cloud: 
In 1950, Dutch Astronomer Jan Oort proposed that a large, spherical cloud of
comets surrounds the solar system. The Oort Cloud is supposed to be almost 1
light year in radius and could contain up to a trillion small, icy comets.
Small perturbations to the very slow motions of these bodies will cause one
of them to start its long, slow journey towards the inner solar system under
the gravitational pull of the Sun. The orbit of such a body will be a
parabola with the Sun as its focus. As the comet gets closer to the Sun its
velocity increases reaching a maximum at its closest point whereupon is
starts its journey back out to the outer reaches of the solar system, never
to be seen again. The Oort Cloud has never been observed, only theorised, but
its existence would explain the orbits of long period comets, which have
orbital periods greater than 200 years.
Sometimes, during its journey through the solar system, a comet may pass
close to one of the major planets. If this encounter is a close one then the
gravitational pull of the planet will dramatically change the comet's orbit
and can alter the parabolic orbit into a closed, elliptical orbit. The comet
the becomes a periodic comet with a definite period for its returns close to
the Sun. Halley's comet is the best known example of such a comet. The
existence of periodic comets, with orbital periods less than 200 years, led
to the proposal of a second source of comets:
     The Kuiper Belt: 
The Oort Cloud does not explain the existence of comets which have orbital
periods of 200 years or less. In 1951, astronomer Gerald Kuiper suggested
that another belt of comets existed beyond the orbit of Neptune, between 30
and 50 astronomical units (4.5 to 7.5 thousand million km) from the Sun. In
1988, a group of astronomers at the University of Hawaii and the University
of California at Berkeley began searching for Kuiper Belt objects using a
2.2m telescope in Hawaii. They discovered the first Kuiper Belt object in
1992. Subsequent observations from Hawaii and with the Hubble Space Telescope
have discovered dozens of icy objects, each a few hundred km in size and with
orbital periods of a few hundred years. The Kuiper Belt may be composed of
comets from the Oort Cloud, which have been deflected into smaller orbits by
Jupiter or the other outer planets.
A few comets have very short period orbits. For example, Comet Encke has a
period of 3.5 years, the shortest known, which places its orbit inside the
orbit of Jupiter. It is generally thought that these inner solar system
comets originated in the Oort Cloud or the Kuiper Belt but passed close
enough to one of the giant planets to be deflected by its gravitational pull
into a much smaller orbit.
     The Cometary Nucleus 
Until the Giotto probe showed us pictures of the nucleus of comet Halley there
was considerable discussion of the nature of a comet's nucleus. We now know
that the nucleus is small, about 10-20 kilometres across, is irregular in shape
(rather like a peanut), and is almost black. From it jets of gas and dust are
forced out by the Sun's radiation. We believe that under the black skin there
is a solid body composed of ices of various kinds, including water-ice, dry-ice
(made of carbon dioxide), ammonia, methane and many other organic carbon
compound ices all mixed together with dust. The dust contains silicates, carbon
and carbon compounds. 
     The Cometary Coma 
Surrounding the nucleus is the bright coma. This is composed of gas and dust
which has been expelled as the Sun evaporates the icy nucleus. The parent
molecules are mainly split up by energetic ultraviolet radiation from the Sun
into simple compounds. These are not necessarily like stable chemicals that we
know on the Earth but are simple combinations of atoms. For example, some of
the most numerous are CN, C2, OH, C3, H2O
+ and NH2. These are broken down pieces of larger
chemicals, such as water (H2O) and organic carbon compounds.
The expelled gas and dust form a roughly spherical ball around the nucleus.
This is many times larger than the nucleus - the coma of a bright comet can be
millions of kilometres in size, whereas the nucleus is only 10km or so across.
The coma of the Great Comet of 1811 was larger than the Sun.
The action of the Sun's radiation and the magnetic field associated with the
solar wind remove gas and dust from the coma and it is ‘blown’ away to form the
comet's tail. 
     The Tails of a Comet
The gas which is blown away from the coma is ionised by solar radiation and
becomes electrically charged. It is then affected strongly by the magnetic
fields associated with the solar wind (a stream of charged particles expelled
by the Sun). The gas tail is made visible by line-emission from the
excitation of the gas by the Sun's radiation. This gives the gas tail its
characteristic blue colour. The geometric shape of the tail is governed by
the magnetic structures in the solar wind but predominantly the gas tail
points directly away from the direction from the comet to the Sun.
The dust is blown away from the coma by radiation pressure from the sunlight
absorbed by individual dust grains. It moves in a direction which is governed
by the motion of the comet, by the size of the dust particles and by the
speed of ejection from the coma. The dust tail can be complex, multiple and
even curved but, in general, will point away from the Sun. Sometimes, due to
projection effects, part of the dust tail can be seen pointing in a sunward
direction. This is just due to the fact that the comet and the Earth are
moving and that part of the tail has been ‘left behind’ in such a place as to
appear to point towards the Sun. The dust tail is yellow because it reflects
the Sun's light to us.
The gas tail can be about 100 million km long while the dust tail is around
10 million km long. The longest observed tail on record is the Great Comet of
1843, which had a tail that was 250 million km long (greater than the
distance from the Sun to Mars!).
     The Names of Comets 
A comet takes the name of its discoverer, or discoverers. It also has a serial
number consisting of the year and a letter designation. In this way all comets
are named uniquely. Halley's comet is one of very few exceptions to the naming
rule. Halley did not discover ‘his’ comet but has the honour of having his name
attached to it because of his pioneering work in determining the orbits of
comets and showing that this comet was periodic. 
     Prediction of Comets 
Apart from the periodic comets, whose orbital periods are well known and
hence whose returns can be predicted with great accuracy, it is impossible to
predict when comets may be seen in the sky. Most of the brightest and most
spectacular comets have been ones which have appeared only once and have
never been seen again. When a comet is discovered, far from the Sun, it is
very difficult to predict how bright it will appear when it comes close to
the Earth and the Sun. Some comets seem to emit a lot of gas and dust and
produce long and spectacular tails whereas others only produce a small amount
of gas and dust and have almost no tail at all.
     

Name

Orbital Period

Perihelion Date

Perihelion Distance

Halley76.1 yrs.1986-02-090.587 AU
Encke3.30 yrs.2003-12-280.340 AU
d'Arrest6.51 yrs.2008-08-011.346 AU
Tempel 15.51 yrs.2005-07-071.500 AU
Borrelly6.86 yrs.2001-09-141.358 AU
Giacobini-Zinner6.52 yrs.1998-11-210.996 AU
Grigg-Skjellerup5.09 yrs.1992-07-220.989 AU
Crommelin27.89 yrs.1984-09-010.743 AU
Honda-Mrkos-Pajdusakova5.29 yrs.1995-12-250.528 AU
Wirtanen5.46 yrs.2013-10-211.063 AU
Tempel-Tuttle32.92 yrs.1998-02-280.982 AU
Schwassmann-Wachmann 35.36 yrs.2006-06-020.937 AU
Kohoutek6.24 yrs.1973-12-281.571 AU
West-Kohoutek-Ikemura6.46 yrs.2000-06-011.596 AU
Wild 26.39 yrs.2003-09-251.583 AU
Chiron50.7 yrs.1996-02-148.460 AU
Wilson-Harrington4.29 yrs.2001-03-261.000 AU
Hale-Bopp4000 yrs.1997-03-310.914 AU
Hyakutake~40000 yrs.1996-05-010.230 AU