Rosetta A Cometary Mission Full of Answers

After the remarkable success of the cometary GIOTTO mission in 1986, the European Space Agency (ESA) directed main parts of its activities on cometary research and aimed - for the very first time - to create a mission capable of landing on a cometary nucleus. Such landing - so the assumption - would open possibilities for picturing in previously unknown detail, as well as doing chemical and physical analysis of cometary matter in hitherto unknown precision. Consequently, the cometary mission ROSETTA was designed and constructed and became ESA's cornerstone mission with an important budget of about 1 billion Euros.

Originally, 600 m radius comet 46P/Wirtanen (Jorda and Rickman 1995) was selected as a target comet of mission ROSETTA. Due to substantial problems with the Ariane 5 launcher at the beginning of 2003, resulting in a one-year launch delay, another comet had to be selected. Finally, the ROSETTA mission was successfully launched in March 2004 targeting the considerably larger comet 67P/

Churyumov-Gerasimenko. The radius of this comet's nucleus was estimated with the help of data from Hubble Space Telescope to be between 2 000 m (Lamy et al. 2003) and 4 740 m (Lamy et al. 2007).

The ROSETTA spacecraft carries its small subsatellite, the ROSETTA Lander Philae (Ulamec et al. 1997; Bibring et al. 2007), to be detached from the orbiter and set down on the surface of the comet's nucleus at about 3 astronomical units (AU) from the sun with near-zero gravity and negligible gaseous atmosphere (Fig. 9.3). Philae was constructed under scientific guidance of Helmut Rosenbauer from the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

One of the scientific interests of the ROSETTA Mission is to find out whether chiral organic compounds, such as amino acids in cometary matter brought to the Earth by cometary impacts might have had, due to corresponding enantiomeric excesses, a seed function in determining the handedness which is characteristic of homochiral compounds employed by life on Earth. For this reason, we have developed an experiment for the ROSETTA mission, named Cometary Sampling and Composition Experiment (COSAC). The COSAC experiment was installed on the ROSETTA Lander Philae and its Chirality-Module is dedicated to the enantiose-lective gas chromatographic and mass spectrometric (GC-MS) analysis of chiral organic constituents of the cometary nucleus (Goesmann et al. 2007b).

Fig. 9.3 This artist's impression shows the ROSETTA Lander Philae anchored to the comet's surface. Philae will work for a minimum mission target of 65 hours, but its operations may continue for many months. The Lander's structure consists of a baseplate, an instrument platform, and a polygonal sandwich construction, all made of carbon fibre. Some of the instruments and subsystems are beneath a hood, which is covered with solar cells. An antenna transmits data from the surface to Earth via the orbiter. The lander carries nine experiments, with a total mass of 21 kilograms. It also carries a drilling system to take samples of subsurface material. Credits: ESA/AOES Medialab

Fig. 9.3 This artist's impression shows the ROSETTA Lander Philae anchored to the comet's surface. Philae will work for a minimum mission target of 65 hours, but its operations may continue for many months. The Lander's structure consists of a baseplate, an instrument platform, and a polygonal sandwich construction, all made of carbon fibre. Some of the instruments and subsystems are beneath a hood, which is covered with solar cells. An antenna transmits data from the surface to Earth via the orbiter. The lander carries nine experiments, with a total mass of 21 kilograms. It also carries a drilling system to take samples of subsurface material. Credits: ESA/AOES Medialab

9.1.3 The Chirality-Module of Mission ROSETTA

The simulation experiments of interstellar environments presented in Chap. 7 strongly suggest that chiral amino acid structures are present in both the surface of interstellar dust particles and in comets, because comets are - in a first approximation -aggregates of interstellar dust particles. In order to resolve and quantify chiral organic molecules in cometary matter in situ, dedicated enantiomer-separating chro-matography will be used.

The cometary sample will be filled by the Lander's Sample Drill and Distribution Subsystem (Ercoli Finzi et al. 2007) in small ovens, mounted on a "carrousel", and moved by rotation, from a position where the sample will be filled in, to the so-called "tapping station", where the ovens are going to be closed and heated for obtaining material in gaseous phase. The ovens will be heated stepwise to levels programmable by ground commands. The pressure developed during each heating step will be measured and recorded, too, because it is indicative of the total amount of gas released (Rosenbauer et al. 1999, Goesmann et al. 2007b). To volatilize the organic compounds sampled on the cometary nucleus, where macromolecules and complex organic polymers of low volatility are expected to make up a major part of organic matter, the combination of two pre-processing techniques will be applied:

1. Evaporation and pyrolysis by stepwise heating of the cometary samples in the ovens of the pyrolysis section, and

2. Chemical derivatization, in which a reagent transforms the parent compound by esterification into a volatile form to be analyzed by gas chromatography.

The analytes in the gaseous phase will be injected into capillary columns coated with chiral as well as non-chiral stationary phases. The chromatographically separated analytes are going to be detected, identified, and quantified on miniature thermal-conductivity detectors, which are coupled with a multi-reflectron TOF mass-spectrometer as schematically illustrated in Fig. 9.4.

Further technical descriptions of the instrument including a test gas chromatogram of an argon, butane, methanol, isopropanol mixture in helium, and mass spectra of a H2O/D2O mixture and perfluorotributylamine can be found in Goesmann et al. (2007b). Here, the interested reader will also find a description of the international COSAC project team including individual contributions under scientific guidance of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

ROSETTA's capillary columns coated with chirally active liquid films received considerable attention, because they were designed for the resolution of non-complex enantiomers to allow the determination of enantiomeric ratios of cometary chiral organic compounds (Fig. 9.5). Consequently, they might provide crucial information about the origin of molecular parity violation in biomolecules (Meierhenrich et al. 1999, 2001b, 2001d, 2003a; Thiemann and Meierhenrich 2001; Szopa et al. 2002; Meierhenrich 2002; Goesmann et al. 2007b).

I would like to point out, that ROSETTA's Chirality-Module is not designed to resolve one or two specific families of chiral organic molecules. It is rather planned to

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    How many cometary missions?
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