Press Release

Once a myth, now an object of study – How the perception of comets has changed over the centuries

By SpaceRef Editor
February 24, 2004
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Once a myth, now an object of study – How the perception of comets has changed over the centuries
comet

In February 2004, Rosetta will be setting off on its long journey through
our solar system to meet up with Comet Churyumov-Gerasimenko. It will take
the European Space Agency (ESA) space probe ten years to reach its
destination. The comet, which moves in an elliptical orbit around the Sun,
will at rendezvous be some 675 million kilometres from the Sun, near the
point in its orbit farthest from the Sun. The meeting point was not chosen
at random: at this point the comet is still barely active, it is still in
fact a frozen lump of ice and interplanetary dust, in all probability the
matter from which our solar system emerged four and a half billion years
ago. Rosetta’s job is to find out more about these strange bodies that
travel through our solar system.


As it moves on, the comet will begin to change. As it approaches the Sun,
it will – like all comets – become active: in the warmth of the Sun’s
rays, the ices evaporate, tearing small dust particles from the surface.
This produces the comet head (the coma) and tail.


Only these two phenomena are visible from Earth. The comet nucleus itself
is far too tiny – Churyumov-Gerasimenko measures about 4 kilometres
across – to be viewed from Earth. As Dr Uwe Keller of the Max Planck
Institute for Aeronomy in Kaltenburg-Lindau, the scientist responsible for
the Osiris camera carried by Rosetta, explains, "Formation of the coma and
tail during solar flyby skims several metres of matter off the comet’s
surface. In the case of a small comet like Churyumov-Gerasimenko, the
shrinkage is a good 1% each time round." As it flies past the Sun every
6.6 years it can look forward to a short future, especially on a cosmic
timescale.


Comets – a mystical view


Visible cometary phenomena have fascinated human beings from time
immemorial – and frightened them too. Even today mystical explanations
prevail among some of the Earth’s peoples. The Andaman islanders, a
primitive people living in the Gulf of Bengal, see comets as burning
torches hurled into the air by forest spirits – the more easily to detect
humans foolish enough to stay out at night. For some Australian
aborigines, comets are flaming sticks ridden by mighty shamans.


Efforts to provide a scientific explanation of the "cometa aster" ("hairy
star") phenomenon stretch back to ancient times. A widely held view was
that comets were in some way connected with processes at work in the
atmosphere. In Meteorologica, Aristotle (384-322 BC) described how
inflammable gases escape from clefts in rocks, collect in the upper layers
of the sub-lunar world ("world under the Moon") and ignite. Rapid release
of such gases produced a shooting star; when let out slowly, they gave
rise to a comet. That was Aristotle’s best shot – and he was well aware of
his limited insight into the question. As he himself acknowledged: "As we
have no demonstrable basis for assertions about comets, I have to settle
for an interpretation that does not conflict with established truths."
Admittedly such truths were thin on the ground at the time.


Comets – something of a disaster


As the centuries unfolded, what could be called the opposite view – that
the comets were responsible for intense heat spells – also gained a
considerable audience, though there was just as little truth in it. The
natural philosophers went one further. They said comets lead to heat, heat
to storms and storms to natural disasters. Pliny the Elder for example
(born circa 23 AD) listed twelve cometary phenomena according to their
external appearances. And he assigned one natural disaster to each class.


The Christian Middle Ages no longer saw cometary phenomena as the blind
raging of an even blinder nature, preferring to interpret them as signs
from God. Theologians such as Saint Hildegard of Bingen (1098-1179) and
Albert Magnus (1200-1280) cited holy scripture. The Book of Jeremiah for
example (1:11,12), in which God caused a fearsome "rod of an almond tree"
to appear in the sky, a symbol of the prophet’s empowerment. Or again Luke
21:11: "And great earthquakes shall be in divers places, and famines, and
pestilences; and fearful sights and great signs shall there be from
heaven."


In 1066, Halley’s Comet appeared to many as a harbinger of the Norman
conquest of Britain, so vividly portrayed in the Bayeux tapestry, with its
scenes from the Battle of Hastings.


The decisive step towards overturning the view that comets are atmospheric
phenomena was taken in 1577 by Danish astronomer, Tycho Brahe. For two and
a half months he observed from his observatory in Uranienburg the progress
of a comet across the heavens. Relying on the phenomenon of the daily
parallax – an apparent "shuddering" motion of heavenly bodies in fact
attributable to the observer’s position on the revolving Earth – he was
able to establish that the comet had to be located beyond the lunar orbit.


Halley discovers an elliptical orbit


The scientific description of comets took another major step forward in
1705 thanks to the work of the British astronomer and physicist, Edmond
Halley, a friend and patron of Isaac Newton. Investigating recorded comet
measurements, he observed that the orbits of a number of bright comets
were very similar: his own calculation of the orbit of a comet observed in
1682 coincided with the data recorded by Johannes Kepler in 1607 and by
Apianus in 1531. He concluded that various comet observations were
attributable to one and the same comet. Halley was proved right when in
December 1758, the comet whose return he had predicted, thenceforth named
after him, did indeed make a repeat appearance. This confirmed his theory
that apparently parabolic comet orbits were in fact "simply" sections of
one enormous elliptical orbit. Since then observations recorded in China
in 240 BC have been identified as relating to a sighting of Halley’s
comet, the oldest known document dealing with this phenomenon.


What was described in the Bible as a sign from God was seen by Fred Hoyle,
the British astrophysicist, as a possible explanation for the great hiatal
breaks in history. He took the view that such extraordinary developments
as the extinction of the mammoth were attributable to strikes by comet
fragments. His views incorporated the theory advanced by British
astronomers Victor Clube and Bill Napier in 1982 that a giant comet was
trapped by our solar system 15000 years ago. With the return of that comet
every 1600 years, the accompanying debris – so the argument goes –
prompted some of the world’s great turning points. This might also be an
explanation for such legends as the Flood.


A lump of icy sludge


So what does the actual nucleus of a comet look like? One answer was
supplied by the Giotto space probe in a mission masterminded by ESA. The
probe was named after the major Italian painter Giotto di Bondone, who, in
the early 14th century, portrayed a comet in his fresco in the Scrovegni
Chapel in Padua. On 14 March 1986, the probe succeeded in taking
100-metre-resolution pictures of the nucleus of Halley’s Comet from only
600 kilometres away. In the words of Uwe Keller: "The mission forced us to
revisit our long-standing image of a comet nucleus as a ‘dirty snowball’.
The pictures showed that it was more like a lump of icy sludge. The solid
part of the nucleus is much larger than the icy part."


Bur hardly had Giotto trained its electronic eye on the heavenly body than
the photo opportunity was already over; a dust particle measuring about a
millimetre hit the probe. As the velocity differential between probe and
comet was at that point 68.4 km per second, the force of the involuntary
encounter was enough to put paid to any further snapshots. All the same,
despite the damage to the camera, it proved possible to go on with the
mission. Following two periods of "hibernation", Giotto achieved a
successful flyby of the Grigg-Skjellerup comet on 10 July 1992.


Rosetta should now bring us entirely new knowledge about comet nuclei. It
will orbit the comet and deposit a small lander probe on its surface. So
for the first time in history a comet travelling sunwards will be
investigated from close quarters.


For further information on Rosetta and ESA projects, please consult our
portal at : http://www.esa.int/science or http:// www.esa.int/rosetta

SpaceRef staff editor.