But why prioritize Mars among all the possible objectives of space exploration? After all this is only a small planet (half the diameter of the Earth and only 1/10th of its mass); it has a very tenuous atmosphere (6 millibars at the “datum”, its “zero altitude”); its surface is arid because, at this pressure, water (even brackish) can only be exceptionally liquid; on its surface perchlorates salts, very aggressive for organic matters, are ubiquitous; solar and galactic radiation are relatively unfiltered by the atmosphere and the absence of magnetosphere, so that their intensity is comparable to that measured in the Earth's environment at the altitude of the ISS; finally it evolves between 56 and 400 million km (between 3 and 23 light-minutes) from us and it takes between 6 and 9 months every 26 months to reach it, under optimal energy conditions, after having traveled an elliptical arc more than 500 million km long. Well !

However these weaknesses or these defects, are largely offset by relative and absolute advantages compared to the other celestial objects that we can observe.
First, Mars is “far away” but it is “not so far”. It is still, although at the limit, within the “habitable zone” of our stellar system (notion of temperature depending on the irradiance of the Sun). Jupiter and its moon Europa, which probably contains an ocean of salty water under its ice crust, evolve between 591 and 965 million km and it takes 4.23 years to electromagnetic radiations to reach us from the Proxima-b planet, located in the “habitable zone” of Proxima Centauri, the closest neighbor of our Sun (this star is also a red-dwarf, which presents unacceptable conditions for real habitability). Then, Mars atmosphere, equivalent to that at 35 km altitude above sea level on Earth (Space is officially at 100 km), has at least “the merit of existence”. There is practically no gas envelope wrapped around the Moon or Mercury and that of Venus is so thick that it maintains a surface pressure and temperature of a “pressure cooker” (450°C and 90 bar). Similarly the amount of water on Mars is far from nil. It is present as ice at the poles and in the immediate underground everywhere. Above all, it might have been liquid in the deep underground for some 4 billions of years and it could run on the surface in exceptional conditions (at the bottom of the Hellas basin, in fair weather and within a span of a few degrees). During the early ages (Phyllosian and Theiikian eras) well before 4 billion years ago and up to 3.5 billion years ago, then episodically, it marked the soil of its print and hydrated the rocks within a relatively warm environment. This long work of active water on materials very similar to ours, accompanied by a powerful volcanism, resulted in a deep transformation of the primitive environment. Indeed Mars was a living planet in the meaning that we may use this term for the Earth, down to the time when the first traces of microbial life appeared in our terrestrial “home". As erosion has been very weak since then, areas of ​​unchanged surfaces that can bear witness of this early period, amount to millions of square km. This is most likely where someday we are to find traces of life (fossils or even active life) elsewhere than on Earth and even more so than on Earth, traces of some advanced evolution of organic molecules towards life. Nothing like our neighbors, the Moon whose active geological life has probably lasted only a few million years or Venus, on the surface of which greenhouse effect has led to conditions under which magmatic spilling have erased all geological traces really old and which is very difficult to explore.

And forget about lunar water! There is certainly some water ice at the bottom of a few craters located at the moon's poles never lit by the sun, but the quantities are infinitesimal compared to the Martian water masses and it is most likely very difficult to access. With respect to the project of allowing Man to settle “out of his cradle”, Mars offers undoubted advantages: some atmospheric protection against radiations, a circadian rhythm comparable to that of Earth (24h39) which, associated to the presence of atmosphere, somewhat smooths out the peaks of temperature (think of the 14 days of light in a row on the Moon followed by as many “days” of night!), a softened temperature (at the Martian equator the temperature can exceed 15°C during the day and, though it might go down to -80°C during the night, this is only for a dozen hours); the possibility of using interesting local resources, starting with water but also the atmosphere of carbon dioxide that could easily be converted into oxygen and methane; the possibility of using the atmosphere to slow down the descent of vessels to the surface; of giving some lift to ultralight planetary vehicles; the advantage of offering a difference in surface gravity less disabling than that of the Moon (roughly 1/3 of that of the Earth but twice that of the Moon).

Finally, Mars was an “Almost Earth” and it could become a “New Earth”. In the event of a global Earth disaster, always possible (hypothesis are not lacking), Mars would be the only planet on which a handful of the best of us could survive and thus perpetuate our species and our civilization. This is not a negligible motivation for settling Mars as soon as possible, because acquiring a minimum autonomy will be difficult and long. After, if you are still not convinced of the interest of the inhabited missions, you may consider Mars, this little distant dot of light, as the archetype of places inaccessible yesterday but which are becoming accessible today, albeit very difficultly and, it must be acknowledged, rather “perilously”. Do imagine the planet as some of the highest alpine peaks, or the great deserts of Africa or Asia, or Antarctic, or a kind of Moon but more worth the effort and the risk; a challenge that is posed to us and that we want to answer simply because it is there, a little red dot stuck on the vault of the sky but within range of our rockets. Our curiosity, our desire and our taste for adventure are typically human. Why deny them and renounce satisfying them; why also refuse the pleasure of designing and realizing the complex systems and machines which would allow them? These Martian characteristics and this potential of robotic and human missions, must not lead us to forget the interest of other celestial objects but they justify that we allocate a maximum of our scarce resources to Mars because, in the near future, the “satisfaction return” and surely in the more distant future, the “financial return” on investment will be multiple and huge.