The sky is an evidence of everyday, and every night, but to be cartographed, it had to be "invented", in order to draw all its possible spaces, shores, ... and "Terra Incognitas".
THE CREATION OF SPACE
For philosophers, the question was to find a third dimension to this sky : the depth. The sky, surface of a sphere including a geocentric world, had to be re-thinked as a uniform space, maybe not infinite but at least unlimited. In this resized world, the copernician revolution started at the XVI-century brought the Earth to its place, as a simple planet among others, in rotation around the Sun.
However, Képlerian laws only describe and predict a phenomenon without explaining / understanding it. For a better approach of the "nature" of space, Isaac Newton, in the XVII-century, had to take a new step. According to the hypothesis that the complex can always be reduced to the simple and the understandable (it is just a hypothesis, but the hypothesis of all the scientists), Newton builds his theory of universal gravitation on the brilliant intuition of a single law governing, in the same way, the fall of bodies - say for example of an apple falling from its tree, according with the legend - and the movement of the Moon around the Earth.
THE
PLURALITY OF WORLDS
The sky never cease revealing its extraordinary diversity. The cartography
of the "emerged lands", in the solar system archipelago, makes
think about these fractal images where a zoom on a detail creates new images
richer than the whole : There are no similar Saturn or Jupiter’s satellites,
each of them being characterized by its own color, size, atmosphere, chemical
composition, or geological activity.This is true for the planets too, or
the multitude - as yet very partially explored - of asteroids and comets.
A fast glance to the map of the solar system reveals some remarkable continents,
and stopovers of future space flights.
The Moon, born from the collision with the Earth, more than four billions years ago, of a young planet of a size probably comparable to Mars, is now a dead body without volcanic activity or atmosphere. However, it is a major strategic place, destination of the first space missions, possible relay for future interplanetary travels, inexhaustible source of ores that can be satellised at low cost, and perfect place to settle deep space observatories.
The Moon has to be discovered, or rediscovered. After the success of Apollo, the public curiosity was attracted quite naturally, but maybe too quickly, by the beautiful coloured shores of Mars. Today’s photographies of the Moon are however very incomplete and limited : we know the crater fields photographed in orbit, or the modest hills surrounding the lunar landing sites of the LEM - sites without asperities chosen for obvious questions of security, but which does not reflect the richness of the lunar landscapes. It is as if an extraterrestrial traveller only knew, about Earth, aerial photographies and Sahara dunes fields, ignoring landscapes of the Himalaya and the Alps, the Grand Canyon, the streets of Paris, or the caves of Lascaux...
A photographer should come back to the Moon - why not a simple robot, like it was already done in the past - to totally renew its image, and bring back to the Earth maybe the most important treasure we could find in space : landscapes.
In the center of the solar system, Mercure appears covered with craters, overheated the day and frozen the night, with an extremely thin atmosphere, and could be a perfect outpost for Sun observation. Further, Venus, the closest planet to the Earth, and about the same size, will remain for a long time an inhospitable place - except for scientists - hidden by a cloudy dense atmosphere of carbon dioxide and sulphuric acid, at a current temperature of 500°C.
Europa, for instance, has powerful tides due to Jupiter’s attraction, producing enough energy to heat in depth the layer of ice covering the satellite. Maybe the Earth is not the only place in the solar system to find liquid water, which makes Europa, with Mars, an interesting place for exobiologists in search for extraterrestrial life. Close to Jupiter too, Io shows evidence of volcanic activity, or, near Saturn, Titan has a dense atmosphere of methane, which could form liquid oceans or lakes.
BEYOND THE SOLAR SYSTEM
For a long time, deep space beyond the solar system will be inaccessible to man. But if we can't walk on these new worlds, we may at least observe them.
Experience confirms a not so obvious hypothesis : laws of physics seem to be the same everywhere in the universe, at least in the farest galaxies observed. The same hundred of atoms - listed in the Mendeleïev table - are found in all the stars and their associatied planets. The diversity appears only in the infinite combination of inorganic or organic chemistry. And in biology.
The light coming from the deep sky - including all the electromagnetic waves - is rich enough to provide a considerable quantity of information : spectral analysis of the light received from a star, a galaxy, or a planet, informs precisely on its chemical composition. It is thus possible to know, from the Earth, the elements constituting the surface of a star or the atmosphere of a distant planet.
The search for exo-planets - i.e. in orbit around other stars - became a major activity of astronomy. Several hundreds were already identified. The precision of observation instruments, however, must be improved to see small planets, and needs new generations of telescopes in space, or on the surface of our natural satellite : going back to Moon could be the best way to discover new worlds ...
Around the Earth, all the orbits are not equally valuable. The trajectory of a satellite only depends on its speed. Few hundred kilometers above sea-level, orbits known as "LEO" (Low Earth Orbit) concentrated all the flights of the Space Shuttle, or inhabited stations such as Skylab, Salyut, MIR, or the International Space Station (ISS). LEO is a perfect place for Earth observation : on these orbits, a revolution is completed in a very short time - nineteen minutes to a few hours - the satellite flying over the same point at regular time intervals.
The orientation of the plane of these orbits compared to the terrestrial equator is important, because it will determine the list countries flown over : enlarging international partnership for ISS supposed negotiations on the inclination of its orbit, initially too low to fly over countries located at the higher latitudes...
Two of them, "L4" and "L5", are stable : situated at the higher vertex of triangles where Earth and Moon would be the base, these points look like "virtual planets". Near "L4" and "L5", a satellite would describe an orbit around ... the void. In other words, for an object on a Lagrange point, the Earth and the Moon will seem stationary in the sky : a perfect place too, for a communication relay, or an inhabited station.
Lagrange points could be found in any double system, for example the imposing couple Jupiter-Sun maintaining, on two symmetrical points located on the orbit of the giant planet, stable concentrations of asteroids called the "Trojan planets".
The question becomes quite complex for a system of more than two bodies - of hundreds, thousands, or hundred of thousands - like the rocks forming the Saturn's rings, in a local disorder creating, surprisingly, at a large scale, a coherent, simple, and permanent form.
Between stars and planets, the gravitation law draws in the sky favorable streams, zones of turbulence and equilibrium points. The map of space is ready for space travelers ...
What the Earth, by its new position, lost in prestige, the sky gained in intelligibility: it became possible to go beyond a purely descriptive astronomy, and to conceive "models" of the world. This new point of view was necessary to formulate, for instance, on the basis of pure observations, the great "laws of Klépler" decribing the satellites courses around the Sun: they first state that planets trajectories are ellipses, a figure obtained while cutting a cone by a plan. Astonishing return of the pythagoricians ideas, in search of a mathematical and geometrical order hidden in nature...
Previously, astronomy had always been an essential discipline, but treating, actually, more about time than about space : in the sky were distinguished fixed and eternal stars from the wandering one - etymologically, the planets - whose unpredictable trajectories through the constellations of the zodiac had paradoxically to deal with the prediction of the future.
From a mathematical point of view, we just need the multiplication and division to formulate this law, written in two lines : "the attraction of two bodies is proportional to their masses, and inversely proportional to the square of their distances". This formula is based on elementary geometrical considerations, similar to the fact that a walker - in the same way - is splashed by a water-jet in proportion with its flow, and inversely to the square of its distance...
The important point, in the law of universal gravitation, is that it is sufficent to explain all the Keplerian laws, to simulate the evolution of complex planetary systems, to study ballistics, and to calculate the trajectory of Jules Verne's Columbiad, or Apollo capsules. Even if the model of the world built thanks to Newton's laws will be reinterpreted - it is the destiny of any model - in a more powerful theory developped some time later by Einstein, its efficiency is sufficient, on our scale and on the scale of the solar system, to conceive and plan space flights.
From the concepts of the XVII-century, space exploration was not a theoretical problem anymore, but appeared first as a technological question. The sky was not only the place for theologists, philosophers and scientists, it became a place for engineers too, as it will be for architects tomorrow. This new territory could be cartographed, like the continents when the concept of a spherical world was accepted and were developed navigation and time measuring instruments.
However, a map is just a tool. Its purpose is not only to contemplate a world, but to travel inside. And move "from the scientia contemplativa to the scientia activa which transformed the man as a spectator of nature into its owner and master".
Close to the Earth, the Moon appears, for more than one reason, as a providential neighbor. It is the primarily cause of terrestrial cycles, such as tides, essential for the development of life. But it is also a guarantee of stability, its heavy mass acting as a counterweight able to fix, by the laws of mechanics, the orientation of Earth’s axis of rotation. Beyond the climatic variations observed through the ages, and the slow continental drift, the same territories thus remained longer close to the same equatorial, tropical, temperate or polar zones, with all the benefifs of this continuity for the survival and the evolution of species. Mars, for instance, is not in that case, whose two little satellites Phobos and Deimos are to small to play the same part, the instability of Mars’ axis of rotation producing, on short geological times, violent and permanent climatic changes.
The dust of micro-meteorites accumulated on lunar surface during the last billions years concentrates elements, such as helium 3, which could be useful one day for energy production. Oxygen, necessary to the consumption of inhabited stations and source of propergol, exists in a great quantity into the ground as it composes 40 % of its rocks. And perhaps, in the dark valleys of polar craters, constantly protected from the solar radiation, have to be discovered important quantities of ice, i.e. of water, resulting from the collision with comets.
Just a little more than one second is necessary to communicate with the Moon by radio, which makes our satellite the only "natural ground" preserving a quasi-direct connection with the Earth. It will be different with the distant planets : messages will need at least fifteen minutes - or several hours - to reach destination, deeply modifying - in a technological as well a psychological, and even philosophical way - any relation with the mother planet.
Mars is the next step after the terrestrial orbit. This future Far West has the same ochre colors, deserts and canyons. On Mars, water is scarse too, but in a remote past it ran in abundance, forming oceans. There is enough water today to feed the needs of visitors, captive in the polar icecape, or in the ground at middle or deep depth. In the solar system archipelago, Mars is the first island which could be tomorrow, more than an exploration place, the first human colony in space.
CARTOGRAPHY OF THE VOID
But a cartography of the sky only describing planets, comets, stars or asteroids, would be incomplete. Space is full of immaterial curiosities important for space travellers, such as equilibrium points or particular trajectories.
Other orbits will be perpendicular to the equator plan, flying above the poles. Correctly positioned, they will have a very particular interest : to always fly over the same territory at the same hour of the day.
The duration of a revolution, in an orbit, increases with its diameter. At a particular altitude, 36.000 kilometers, this duration is precisely 24 hours : a satellite positioned on this orbit called "GEO" (Geostationary Earth Orbit), will appear stationary in the sky. Since 1954, scientists stressed on the importance of these "anchoring point" where could be installed, like in the top of mountains, communication relays.
There are other trajectories and singular points to draw on this "map of the void". Among them, the Lagrange points, strategic for a space traveler. Discovered by the mathematician Joseph Louis Lagrange in the XVIII-century, these points result of complex equations but could be explained by a simple and intuitive idea : in a double system, such as the Earth-Moon couple, where each planet exerts a gravitational attraction, there are necessarily equilibrium points. Are they stable, or not ? Mathematics are useful to help intuition : there is, under these conditions, five equilibirum points, known as "Lagrange points".
