2023-06-26 03:02:14
Posted on June 26, 2023
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Earthlings are ungrateful. Many consider the planet Mars to be a hostile and distant land that should therefore be ignored. They are wrong. Instead, they should see that it is exactly the springboard humanity needs to one day take off into deep space. They should therefore give thanks to nature which placed it there, such as it is, in the place where it is.
The fundamental advantages that Mars offers for us human beings, compared to the other planets of the solar system and compared to the planets of other systems which would not present the same configuration as our couple Earth/Mars, are related to the mass and to the the distance. More precisely, they are on the one hand, the mass ratio between the two planets and the mass of Mars in absolute terms; on the other hand, the distance from Mars to the Earth and the distance from Mars to the Sun.
I will develop this week the advantages given by the mass.
For the record, Mars has a relatively small mass, a tenth of that of the Earth, for a diameter equal to half that of the Earth (therefore approximately the same density) but an area equal to the totality of the emerged lands of our planet. This low mass is not “normal”. Given the likely homogeneity of the protoplanetary disk, Mars could have had the same mass as Earth or Venus. It should even have had this mass if, as the astrophysicist Alessandro Morbidelli brilliantly demonstrated to us (theory of the “great tack” that is to say the “great cusp”), Jupiter, formed beyond the line of ice, had not come to make an incursion into this region of the inner solar system (below the Asteroid Belt) while it was still in formation (with a certain delay compared to the gas giants).
Luckily, Saturn having entered into resonance with it, stopped its descent towards the Sun and led it to leave together in the external system (beyond the Asteroid Belt). The damage from the incursion, however, was already enormous, most of the material in the Asteroid Belt turned upside down, scattered, absorbed, absorbed, most of the material that could have created a planet Mars of the size of the Earth. However, Jupiter had not descended long enough in this region to absorb all the matter, and luckily it had remained there long enough for what remained of matter to be accreted by gravity into a planet much less massive than the Earth. Earth, but significantly more massive than the Moon, which was exactly what we would need later.
Indeed, this mass of Mars implies a gravity on the ground of about a third of that which we have on Earth (0.38 g) and an escape velocity of 5.03 km/s (compared to 11.2 km /s for Earth).
The consequences in astronautics are that:
The weight of the possible Starship which will land on Mars almost empty of propellants will only be 100 tons for a maximum mass of some 300 tons (i.e. 100 tons of structures, 50 tons of residual propellants, 150 tons of load useful) The weight on departure from Mars, once filled with propellants, will be 370 tonnes for a mass of 1400 tonnes (100 tonnes of structure, 1200 tonnes of propellant, 100 tonnes of payload).
This weight is to be compared to that of the Starship with its launcher, SuperHeavy, at the start of the Earth, 4000 tons (i.e. for the structure, 200 tons for the SuperHeavy and 100 tons for the Starship, and for the propellants, 3400 tons for the SuperHeavy and 150 tons for the Starship – waiting to refuel in orbit – plus 150 tons of payloads). It should also be compared to the weight on departure from Earth of the 538 tonnes of the heaviest version of a loaded Falcon-9 or the 780 tonnes of an Ariane-5.
It is clear that the difficulties in landing and then leaving a “hypothetical-Mars” planet of the mass of the Earth would be incomparable to the difficulties to be overcome in order to take off from the Earth. By analogy, wanting to land on the unprepared ground of such a planet and especially wanting to leave would pose almost insurmountable problems. Considering only the take-off, it would be necessary to have on site a launcher equivalent to the SuperHeavy which it would have been necessary to have been able to bring to Mars as well as the propellants necessary to supply it (or the laboratory capable of producing them from the local resources in sufficient quantity and with sufficient speed).
It is therefore a machine much more powerful than the Starship-integrated (with its SuperHeavy) that we would need from Earth. However, the take-off test for orbital flight of the integrated Starship has amply demonstrated that we had reached the maximum of what it was possible to attempt with our current means of propulsion. We can therefore only hope today to carry out manned missions on our “real-Mars” planet because it has a mass much smaller than that of the Earth. To continue the reasoning, any mission on the surface of Venus (or planet of the same mass), in addition to the fact that one could not descend there because of the atmospheric pressure on the surface (90 bars) and the temperature (450°) would be completely excluded because we could no longer leave it simply because of gravity. Any mission on a “super-earth” (by definition of mass greater than the Earth) would a fortiori also be totally excluded.
After lift-off from the actual Mars, it will then be much easier to reach orbit before interplanetary injection to Earth, because the astronauts will not have to overcome the test of Max-Q, which represents the peak of danger after leaving the surface of the Earth (or any other planet with a dense atmosphere). Let us recall that Max-Q is the maximum aerodynamic tension through which we must pass when the atmospheric pressure is still high enough so that, depending on the speed already acquired, the density of the atmosphere imposes the hardest constraints on the structure of the rocket.
This voltage then decreases rapidly as a function of the reduction in atmospheric pressure, which itself depends on the altitude. In the Martian atmosphere, the Max-Q is much lower (not to say negligible) because the initial atmospheric pressure is already very low (615 pascals at the “Datum” ie the equivalent of sea level here) , corresponding to that which we have at an altitude of some 30 km above the Earth.
It was at this altitude that the Starship-orbital crossed its Max-Q, which undoubtedly contributed to its destabilization which became evident a few km higher. When the rocket leaving Mars reaches an altitude of 21 km above the Datum, i.e. the altitude of the summit of Olympus Mons (the highest volcano on the planet) and probably its Max-Q, the atmospheric pressure will not be more than 30 pascals (three tenths of a millibar), clearly almost nothing (and in any case, he will not have needed to carry out the delicate maneuver of dropping his launcher since this first stage will not be necessary due to lower gravity).
Once on Mars, astronauts will need to wear spacesuits for all outdoor activities and possibly a vest plus a radiation helmet in partially shielded surface habitats (like the transparent domes seen in many habitat projects) , unless of course they decide to live under a thick protection of regolith or rock. This is good, because the mass corresponding to this life support (equipped spacesuit) and this anti-radiation protection (vest and helmet), will be perfectly adapted to the muscular and bone capacity of the astronauts, and will even be beneficial for them to maintain bone tissue. and muscles in good condition, whereas it would be totally unbearable on a planet of mass, and therefore of gravity, equal (or greater!) to that of the Earth.
In terms of research, Mars also has two major advantages resulting from its mass, and therefore its gravity. This mass has indeed allowed a much more advanced geological activity than on the surface of the Moon but has not allowed the development of a significant tectonic activity as on Earth.
The geological activity on primitive Mars allowed the geological transformation by diagenesis and metamorphism associating liquid water whereas this evolution linked to water was almost nil on the Moon since our natural satellite, of too small a mass, has very soon became a dead star. Mars began a geological history similar to that of Earth with numerous early reactivations, following changes in the inclination of its axis of rotation on its ecliptic plane or due to powerful volcanic episodes. Luckily, for scientists, this activity slowed down, almost stopped, a few hundred million years after having started (around -3.5 billion years) at the same time as the atmosphere was thinning extreme and the surface water was disappearing.
In the mantle of the planet, a smaller quantity of water, also linked to the lower mass and therefore to the lower gravity of Mars compared to Earth, therefore to a less strong attractiveness of Mars for comets carrying ice , did not allow the development of such powerful convection movements linked to such a thin crust as on Earth. As a result, the convection movements could only outline a very weak plate tectonics (Valles Marineris, or Isis Planitia?) which allowed the almost complete preservation of a very old planetary surface extended over tens of million km2, whereas on Earth these same surfaces witnessing the first beginnings of life, today only occupy a few tens of km2 in Australia and Greenland.
Only negative note, the low mass of Mars did not allow the creation in the center of the planet, of a ferrous metallic core as pure and with a periphery as clearly delimited as in the center of the Earth, which did not allowed differential rotation efficient enough to generate protective global magnetic fields (if not at the very beginning of geologic history). Since the atmosphere has been depleted (around -4 billion years except for increasingly rare interludes), there has been no longer any protection on the ground against solar and galactic radiation. These conditions were obviously unfavorable to life.
Despite this last negative note, Mars therefore constitutes, by its mass, an optimal laboratory to deduce what could have been the oldest terrestrial surface, and a place where the gravitational conditions should allow man to live. under acceptable conditions.
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