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).
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.
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