Acknowledgments
We thank Claudia Heym for excellent technical
assistance. This work was supported by grants from
Deutsches Zentrum fu¨r Luft- und Raumfahrt (DLR,
Bonn, Germany) and by EU Research Training Network TIPNET (project HPRN-CT-2002-00265) obtained from Brussels, Belgium. F. Balusˇka and
J. S
ˇamaj receive partial support from the Slovak
Academy of Sciences, Grant Agency VEGA (Grant
No. 2031) and APVT (Grant No. APVT-51-00-23-02),
Bratislava, Slovakia.