Getting to the sky leaves footprints on the ground. Through its clean space initiative, this is the creation of an "ECO-friendly" approach to space activities. On the ground, this means adopting more environmentally friendly materials, processes and technologies. In space, it means preserving the orbital environment of the earth as a safe, debris-free zone.
Source: See: http://www.esa.int/Our_Activities/Space_Engineering_Technology/Clean_Space/The_Challenge