I have already asked the question whether space represents the setting, the opportunity to explore or to conquer. I have already shown how this distinction, as pertinent as it is, was very often caught up with by the reality of our human activity and our propensity to leave tracks behind us, even when we have decided to explore (but not to conquer) the unknown world that surrounds us. In fact, as soon as we leave the surface of our Earth, we pollute outer space physically, chemically or biologically. In return, we cannot exclude or ignore the possibility that we may pollute Earth with elements brought back from other celestial bodies. The question was already asked during the Apollo programme and will become topical again in a few years or dozens of years" time when samples gathered from the surface of Mars or the surface of asteroids are brought back to Earth. In short, any future exploration must account for possible invaders, terrestrial or extraterrestrial.