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COMETS THROUGH THE CENTURIES: FEAR AND CURIOSITY
From time to time, bright comets are visible for observers with the naked
eye, and so they are known to people all through the history. Their apparent
unpredictable behaviour, their sudden apparition somewhere in the sky, gave
often reason for fear and superstitioness. For example, people believed
that gnats became poisonous by the light of a comet and that food could
empoison when left in open air during the apparition of a comet. In many
cultures comets were regarded as messengers of death and misery.
Nevertheless, philosophers and scientists were speculating about
their physical nature.
Aristoteles (384 - 322 B.C.) used to think that comets
belong to the Earth's atmosphere, being some kind of igneous evaporation
raising up. It took many years until Tycho Brahe (1546 - 1601) who observed
a bright comet in 1577 and attempted to measure the distance to the comet.
He found that comets belong to the regions of planets and thus they are
constituents of the solar system. After the discovery of Newton's law of
gravity, Edmund Halley (1656 - 1743) determined the orbit of one bright
comet, now named after him, around the Sun showing a periodicity in the
phenomenon of comets. He detached comets from the field of myths.
In today's
view, a comet is a chunk of condensed volatiles, mainly water and carbon
monoxide, with particles of silicate dust included. The typical sizes of
these bodies, called nuclei, are some hundred meters to a few kilometers. If
one of these nuclei is approaching the Sun, the increasing solar irradiation
leads to the sublimation of the volatiles. The resulting gas steam carries
the dust particles with it. A kind of "cloud", called coma,
develops around
the nucleus, leading to the fuzzy appearence of faint comets. The prominent
tails form when the dust disperses under the influence of the solar gravity
and radiation pressure (dust tail) and when the gas ionises by
radiation and moves away from the Sun due to the solar wind (ion tail).
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ICE AND DUST FROM THE BIRTH OF THE SOLAR SYSTEM: WHY
STUDY COMETS?
Comets arrive in the inner solar system on strongly eccentric orbits.
One source region is the Kuiper-Edgeworth-belt, a ring containing
cometary nuclei and asteroids outside the orbit of Neptune. Beyond
Neptune, the density of planetesimals, building blocks of planets, was
too small to form larger bodies, and thus, the planetesimals outlast
there for approximately 4.6 billion years until today with only little
modification. Planetesimals that were formed in the region of the giant
planets could be scattered by these planets outwards where they form
another source region of comets: the Oort cloud. This spherical
reservoir of cometary nuclei extends to approximately 100 000 AU from
the Sun. Due to disturbances by passing stars, the nuclei in the Oort
cloud can be deflected to the inner solar system where they can be seen
as long period comets. When the nuclei enter the inner solar system and become visible as
a comet, they offer the opportunity to study a remnant of the birth of
the solar system.
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THE COMETARY COMA: OUR SCIENTIFIC ACTIVITY
The gases sublimating from the cometary nucleus, called parent
molecules, are exposed to solar radiation. This leads to ionisation or
destruction of the molecules. The products of the
reactions of the parent molecules are called daughter species. Radicals
like CN or C2 show strong emissions in the optical wavelengths range of
the electromagnetic spectrum. They are therefore, in contrast to the
parent molecules, easy to observe with the techniques of optical
spectroscopy. This makes it possible to observe daugther species in
faint comets or at large distances from the Sun where the parent
molecules can not be detected.
In order to determine the production
rates of the parent molecules from the observations of the daughter
radicals, the chemistry in the cometary coma has to be known. One of the
focusses of this group are the reactions leading to the formation of the
C2 and C3 radicals. It was possible to derive the production rates of
the parent molecules for C2, C2H2 and C2H6 and the proposed parent for
C3, C3H4 from optical longslit spectra of comet C/1995 O1 Hale-Bopp.
Another special research interest is the study of the long-term activity
evolution of comets. The extraordinary active comet C/1995 O1 Hale-Bopp
was target of a monitoring program that lasts from April, 1996 to
January, 2001. Optical longslit spectra and broadband filter images were
taken in the range of heliocentric distances from 4.6 AU to 2.8 AU
preperihelion and 2.9 AU to 12.8 AU postperihelion. This is the hitherto
largest interval of heliocentric distances over which a comet was
continuously observed.
The dust particles are thought to be released from the nucleus of a
comet by the stream of gases resulting from the sublimation on the
nucleus. Therefore, a correlation between the dust and the gas activity
of a comet can be expected. In this group, we are interested in the
dynamics of dust particles in the comae of comets in order to determine
the dust production rates and to come to a better understanding of the
connection between the dust and the gas activity.
MISSION TO A COMET: ROSETTA ACTIVITIES
The mission Rosetta of the European Space Agency is going to make the
most detailed analysis of a comet ever. The Rosetta spacecraft has started on March 2, 2004 on its long flight to comet
67P/Churyumov-Gerasimenko where it will arrive in 2014. The spacecraft
will orbit the comet's nucleus and sent a lander module on its surface
and follow the comet on its path towards the sun.
This working group is involved in the following Rosetta experiments:
MIRO,
COSAC,
OSIRIS
67P/CHURYUMOV-GERASIMENKO DATA BASE
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