Acknowledgement
One part of the graphical material used in the head of this web page belongs to NASA ( http://www.nasa.gov) . It has been used in agreement with the general NASA copyright guidelines (http://www.nasa.gov/multimedia/guidelines/index.html). Another part of the graphical material belongs to Tomas Bodnar (http://cvut.academia.edu/TomasBodnar) and has been used with his permission.