After talking about the impact that the use of space has on the ecology of the circumterrestrial environment, as well as on that of the celestial bodies that have been explored so far, we broaden our vision to include interactions between the terrestrial environment and the other bodies of the solar system, touching on a topic that is destined to assume an increasingly important role in the years to come: planetary defense. It is a term that evokes almost catastrophic scenarios where we must defend our planet from the impact of killer asteroids. While this is undoubtedly a correct interpretation, it is not the only one. Planetary defense also has a biological dimension that sees us doubly protagonists, since we must avoid contaminating celestial bodies we visit with terrestrial organisms and, conversely, we must ensure that samples of extraterrestrial material, laboriously collected and brought back to earth, do not contaminate our world.