Because we can. In the past 60 years we have witnessed a most remarkable adventure: the in-situ exploration of our solar system. Space missions like the Voyagers1, Magellan2, Giotto3, Cassini–Huygens4, or, more recently, Rosetta5, New Horizon6, Hayabusa (2)7, Parker Solar Probe8, OSIRIS-Rex9, and now Solar Orbiter10 allow(ed) us to visit the Sun, all planets and many of their moons as well as comets, asteroids, and even Kuiper belt objects. There is no reason to stop this story of success, to which also the Hubble Space Telescope added immensely.
However, did we learn for our human society? Yes, and the key term here is comparative planetology, or, more appropriately, comparative Earth science. Often, comparisons of more and different objects are necessary to reach a comprehensive understanding. This holds for processes in and on Earth as well. Living on Earth reached a nonlinear stage. We are no more just using terrestrial resources. We are changing the resource bearing environment in a dramatic way, reaching tipping points and crossing planetary boundaries11. An important source of information on this negative feedback is global observations using satellites and space science. Earth observation with missions like ENVISAT deliver necessary contingent information and process understanding on global change, either natural or human made.
But, comparative Earth science is mandatory for deeper insight. The diversity of geological processes in the solar system is fascinating. Earth currently operates an ecosystem which supports biological activity. Since the age of space exploration, we know that Earth is the only planetary body in our solar system with human life forms. The slowly rotating planet Mercury is too hot on its dayside to support life. Venus has a very thick atmosphere with climate conditions hostile to biogenic activity. Saturn’s satellite Titan is covered by a thick nitrogen atmosphere with temperature about −180 °C, methane clouds and rain, a very hostile environment as well. Did other planetary bodies reach their natural planetary boundaries? Studying the strange worlds in our solar system puts terrestrial geological processes into a proper perspective. The various planetary objects studied in the past, currently under investigation, and object of future space explorer missions, are not only entangled by the gravitational field of the Sun, but also by the need for comparative planetology (Fig. 1). The term habitable zone was coined to describe regions around stars where carbon-based life is possible, based on our new, space-based knowledge. Doing space exploration is an important means for human and scientific self-assertion.