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 orbit of a comet
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 and the Kuiper Belt.
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 tail 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!)
Naming a comet
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.
Predicting a comet
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.
The Stardust Mission
On February 7 1999, the joint NASA/European space probe Stardust
was launched from Cape Kennedy in Florida. The probe will make three loops
around the Sun. During the second loop, on 2 January 2004, it will catch
up with Comet Wild-2 and pass within about 150 km of the comet's nucleus.
A tennis-racket-shaped particle catcher filled with a low density foam,
or 'aerogel', will then be used to slow down and capture particles of dust
from the head of the comet. Once the encounter is over, the aerogel device
will fold into a capsule which will return the sample of comet dust to Earth.
For more information about the mission, visit the Stardust homepage.
I was sitting on the porch of the house at the trading station, looking
north. Suddenly in the north...the sky was split in two, and high above the
forest the whole northern part of the sky appeared covered with fire. I felt
a great heat, as if my shirt had caught fire... At that moment there was
a bang in the sky, and a mighty crash... I was thrown twenty feet from the
porch and lost consciousness for a moment....
The crash was followed
by a noise like stones falling from the sky, or guns firing. The earth trembled....
At the moment when the sky opened, a hot wind, as if from a cannon, blew
past the huts from the north. It damaged the onion plants. Later, we found
that many panes in the windows had been blown out and the iron hasp in the
barn door had been broken.'
- eyewitness account -
On the morning of 30 June 1908, a white light streaked across the skies
above eastern Russia. At 07.17, there was an explosion with the power of
a 20 megaton nuclear bomb above the Tunguska River in Siberia. Trees in the
forest around the river were flattened up to 30 km from the explosion, which
was heard up to 1000 km away. Since the area was so remote and relatively
unpopulated, only one person was killed and very few people saw the explosion.
That night, an eerie glow lit the sky across the world. In Western Europe,
it was bright enough that night to read a newspaper without a lamp.
now think that a small, low-density asteroid entered the Earth's atmosphere
at a glancing angle and exploded about 5–8 km above Tunguska. The mysterious
glow in the air that night would have been caused by sunlight scattering
off dust from the explosion in the upper atmosphere.
Discussion of the Tunguska impact can be found on the Tunguska home page, University of Bologna.