ABC s of Solar Sails

Transcription

1 ABC s of Solar Sails I - Dream and Historical Backgroud II The Reality III The Projects IV The Adventure Union pour la Promotion de la Propulsion Photonique

2 I - DREAM AND HISTORICAL BACKGROUND 1 - FROM PULSIONS TO IMPULSIONS S OLAR S AILS ARE BUT ONE OF THE LATEST TECHNICAL AND SCIENTIFIC OUTCOMES OF A LONG, LONG EMERGENCE IN THE ASPIRATIONS OF M ANKIND TO CONQUER SPACE. The original howl, modulating like a prayer, has already detached itself from the background murmur, ascending the meanders of Evolution to reach the wolf, close relative, seated in the midst of its clearing, muzzle pointed towards the arch peopled with stars, already expressing its primal anguish, in search of a response to an imperious need for communion, already searching for a bond between the individual that he is, on the edges of his pack, and the wholeness he observes. This bond, this religion, this sovereign relationship between the individual and the whole, gives birth a little later, among a species which is a little more evolved, to a variety of gravity- defying myths (Icarus's attempt, the levitation of the monk of Cupertino, the trace of Peter Pan astride his moonbeam, or the morning dew vials of Cyrano de Bergerac) and soon the myth, the blissful incantation and the poetic impulse bloom in their turn into concrete achievement. The miracle is that we entered a fertile age with the great Leonardo leading us, he who was both an artist and an engineer, a bard and a hero. From that moment on, we took off, rising progressively higher, then suddenly coming from the beginning of time to real distinction. It is evident we are no longer in the midst of a clearing. For the past hundred or hundred and twenty years, a small number of inspired human beings descending directly from the Years of Enlightenment, devoted themselves to scholarly calculations and wham! in one stroke, one only, put a vessel into orbit. O N THE 4 TH O CTOBER 1957, SPACE A GE WAS DAWNING. Just one step, but promising. Promising in that it gives access to Space which yesterday was forbidden in that it is part of an irreversible movement, in that it guarantees us, very soon (barely a century or so), the conquest of the entire solar system... Without prejudicing future developments in the realms of space-time and energy-matter... What a program! Which goes beyond the framework of human history, a simple milestone, part of a process of a biological evolution comparable to that which came before us, and which boots us head over heels out of our familiar tracks. Reduced to this summary form, the story may look s imple. And yet, how many calculations, how many refinements, how many hazardous extrapolations, and above all, how much struggle, in the name of the right to infer and deduce without reference to the gods! In the early stages, that Spring rich in promises, there was Clerk Maxwell, a Scottish physicist who established the identity of electricity and light, and then in 1860, the reality of photons, those tiny particles which exert pressure. Then came the extremely modest and indefatigable Constantin Tsiolkovski ( ), working in his provincial isba. He used the principle of the rocket to lay the very fou ndations of the conquest of Space, publishing "Free Space" in 1883, and continuing his work for the next forty eight years. In 1912, in Saint- Petersburg, it was the turn of Frenchman, Robert Esnault- Pelterie, who was obsessed by the same ideas, to enter the history books with a talk on theory, and, some time later, by coining the word "astronautics", a new discipline designated to mobilise future generations at the end of the century. Four years later, in 1916, the American Robert Goddard, a gruff scientist completely buried in his research, took his place among the promoters of space science and in 1926, he launched the first liquid propellant rocket. Close on his heels, in 1929, the German Hermann Oberth developed a very well- researched theory of multistage rockets in the authoritative work entitled "The Road to Space Navigation", for which he was awarded 10,000 francs, contributed by Esnault- Pelterie in person, co- founder of a Prize he had created with one of his friends, a banker. Finally, Oberth's most brilliant student, Wernher Von Braun, followed in his footsteps, bringing into being the V2 bomber and the American Space Program of the 1960' s, although in the Soviet Union, through the impetus given by Tsiolkovski, the Gas Dynamics Laborato ry and later Korolev's laborato ry, were taking a similar road. After the pioneering work of Constantin Tsiolkovski, three names, therefore, dominate the beginnings of the history of astronautics: Robert Esnault-Pelterie ( ), Robert H. Goddard ( ), and Hermann Oberth ( ). All three, had, at one moment or another, envisaged photonic pressure as a mean of propulsion. What influence did these three pioneers have on one another? It is of little interest. It is probable that at first, given the unacademic nature of this type of research, and the usual distrust of the powers- that- be of innovation, all this without any publicity being given to his work, each one of them was condemned to find out the best solutions for himself. What these three inspired scientists had in common was an obsession with space, the same that haunted the aforementioned wolf and Peter Pan astride his moonbeam, or in a more structured way, the kind of intuitive certainly drawn from the anticipatory works of Jules Verne, H.G. W ells, and other e nthusiastic precursors. There is nothing surprising in the fact that three leading lights of this intermediary period should have rediscovered individually and yet almost simultaneously, the formulas established by Constantin Tsiolkovsky. Was not the escape velocity, determined in 1881 by the German, Hermann Ganswindt (11,2 kilometre/second) the vital element? No- one can escape from the law of physics. It was only left for each one of them, in his own way, to discover a mean s of succeeding in such a reckless enterprise.

3 I - DREAM AND HISTORICAL BACKGROUND As an experienced engineer, Esnault- Pelterie divided his efforts for a long time between theoretical research on space and practical work in the more familiar sphere of aeronautics. His two- directional joystick and radial engine were replicated in all industrialised countries and contributed largely to the conquest of the lower layers of the atmosphere. A little later it was the pilot in him who distinguished himself with his definition of the principle of inertial navigation and by the use of the rocket as a back- up engine. His first reference to jet propulsion went back to 1907, in fact. In 1912, in a talk given to the Society of Physics in Paris, he explained that we would soon be able to leave our planet by rocket. He purposely alternated in his argument between fantasy and strict reality, the daring nature of project and a concern for credibility. As soon as an anomaly arose in the visionary's flight of fancy, the engineer in him took over to justify and structure his discourse. He was cautious. He knew that a sedentary population is set in its ways. In 1927, he confirmed his previous work on interplanetary travel in a talk given to the Astronautical Society of France, and in 1930, published a major work on the subject of astronautics in which all eventualities were examined, including the idea, dear to supporters of photonic propulsion, of travel at constant acceleration. Meanwhile, he lost three fingers in a rocket experiment. At the same time, unlike Tsiolkovski who confined himself to theoretical research, Goddard, on the other side of the Atlantic, was an experimenter of the first order. Fleeing the glare of publicity in all parasitic glory, he registered two hundred patents during his long research into rockets. In 1909, on the verge of his discoveries, he arrived at the conclusion that liquid oxygen and hydrogen would give excellent results in rocket propulsion. In 1916, he advocated launching a rocket round the Moon, and at the same time, was one of the first to envision a system of electrostatic propulsion. In 1918, in "The Ultimate Migration" which was not published until 1972, by the way of recreation, he made himself a present of a magnificent nuclear- powered interstellar journey, and in 1918, in "Method of reaching Extreme Altitudes", he suggested lighting a dark sector of the Moon by means of a powder rocket equipped with a luminescent explosive charge. His main research, however, was concentrated on liquid- powered rockets. He started this work on 1920 and made it his speciality. In 1923, at a test- bed, a first gasoline and liquid oxygen rocket experiment proved successful. On 16th March, in Auburn, Massachusetts, Professor Goddard launched the first rocket at 56 meters. This altitude increased to 600 meters in 1930, and 2,300 meters in Pendulums and gyroscopes guided the trajectory of the rockets. The experimenter was efficient. And the way ahead clear. 2 - THREE DEDICATED RESEARCHERS Third in this brilliant areopagus was Oberth, who in 1928 made an original début into the small world of rockets produced in rudimentary workshops and the wasteland outside the city. At the age of 34, this Transylvanian, who made his career in Germany, signed a contract with Fritz Lang, the famous filmmaker who took it into his head to tell the story of a woman who travelled the moon. A real rocket was to have been launched but it exploded. Already in 1922, at the end of his studies; the young man had unsuccessfully defended his doctoral thesis on interstellar navigation before a narrow- minded jury at the University of Heidelberg. In 1923, he was made a Mathematician and a Physicist, but the thought of his thesis still rankled, and he published it under the title "By Rocketry to Space". Locked in a struggle with critics who were increasingly narrow- minded, refusing to accept the evidence of his experiments, which alas frequently happens in everyday life, he resigned himself to teaching in a provincial secondary school for almost ten years. Meanwhile, his work remained theoretical. Ignorant of the fascinating chronology of calculations already made by his predecessors in rocketry (Hermann Ganswindt, Constantin Tsiolkovski, André Bing, Robert Esnault- Pelterie, and Robert Goddard, to quote but some of his acknowledged precursors), he once again created an equation of the speed of the object of his dreams and recommended as was only right, multi- stage rockets and liquid propellants, subjects which had already been examined more than twenty years previously. But good was to come of this since in 1929 at the end of his hard labours, the young professor published a magnificent work (mentioned above), which was considered by Esnault- Pelterie to be a real "bible of scientific astronautics", and in 1931, five years after Goddard's, his "Mirak 2" rocket, designed by the league of which he became President, lifted off into the sky to a height of 1,600 metres Effets de la lumière sur les trajectoires des corps célestes observés par KEPLER à partir de l'orientation des queues des comètes à l'opposé du Soleil Travaux de MAXWELL sur l'électromagnétisme permettant d'expliquer ce phénomène FAURE et GRAFFIGNY, romanciers français de science-fiction, imaginent un vaisseau spatial utilisant un immense miroir pour recueillir la pression de la lumière solaire Pietr LEBEDEV met en évidence la pression des radiations Travaux de Yakov PEREELMAN en Russie Travaux de Fridrikh TSANDER en URSS : Pour les vols dans l'espace interplanétaire, je travaille sur l'idée d'utiliser de formidables miroirs faits de feuilles extrêmement minces et capables d'obtenir des résultats intéressants et du pionnier de l'astronautique Constantin TSIOLKOVSKI qui déclarait : La Terre est le berceau de l'humanité, mais on ne peut passer toute sa vie dans son berceau.

4 I - DREAM AND HISTORICAL BACKGROUND 3 - ROCKETS LEAD THE WAY All through the thirty years preceding these events, Tsiolkovski was working in his isba at Kalouga where he taught mathematics. The essential part of his manuscripts, presented in the form of a major study were published in 1903 after a wait of five years. In the eyes of the authorities, the subject was of very little interest. Nonetheless, over the years, the researcher's work was re-edited and successive publications appeared. On his desk, a body of work, an accomplishment, was taking shape. The equation he was later credited with discovering led to the need to increase the speed of the ejected matter. But this is of little importance. As early as 1903, his theoretical rocket was propelled by liquid fuel and the outline of a multi-stage rocket system appeared. Each year, he amassed a new stock of knowledge for which he had no outlet. After 1913, the outlook seemed brighter; his articles were reaching an increasingly wide audience. Attentive to his new approach to the eve of the October revolution, the great work entered into its final stages. However, 1917 onwards saw a new dawn. In the young Soviet Union, the promotion of space was enjoying a boom. In 1921, the GDL (Laboratory of Dynamic Gases) was set up in Leningrad. In 1924, following in the wake of he who was henceforth considered as a great boss, Friedrich Tsander, 37, led a section of interplanetary communications in the Academy of Sciences. In 1929, GDL had a liquid rocket section directed by Petrovitch Glouchko, 21. In 1930, Tsander, still working away, gathered together a few youthful minds attracted by this major project in the "jet-propulsion research group". Almost a thousand enthusiasts participated in this vertical-bound adventure. Amongst this prolific year we might mention the names of Tsander, the leader, Professor Rynine and Doctor Palerman, both of whom had been in the field for a long time, but also Tikhonravov, the exigent adjuster, Korolev, the newcomer, and Glouchko, of course, a specialist in liquid rockets. In the 1920's, the revolution was in an inspirational, booming phase. Tsiolkovki received appointments, honours and allowances. He was elected honorary member of the Academy of Sciences and the Air Academy. His work was widely published. Soon after, he put the finishing touches to it and made a gift of his entire work to the Soviet authorities. It would seem that he never overcame the confinement to which his deafness had condemned him since childhood. It was as though the outside world reached him through a kind of reductive filter. Lastly, to ensure he had forgotten nothing, he created, or recreated, every thing himself. Who can calculate lost opportunities? At the beginning of the 1930s, a new team of men took over. In 1932, a rocket with liquid propulsion reached a height of 400 metres. From then on, the Red Army supervised all the firings on behalf of the RNII, under the direction of Serguey Korolev. He was 25 years old. At the same time, similar developments were taking place in Germany. From 1930 onwards, a young captain of the Wehrmacht, Walter Dornberger, is commissioned to assist the young people who, in the wake of Hermann Oberth, liked to meet on a wasteland of the Capital to launch powder or liquid rockets. The following year, one of these rockets reached a height of 1,600 metres. Amongst more than eight hundred enthusiasts were Johannes Winkler, Nebel, Willy Ley, the young engineer Riedel, and the young Von Braun, aged 18. Several months later, Captain Dornberger suggested giving Wernher Von Braun, who was distinguishing himself by his initiatives, more helpers to design bigger rockets. The cause no longer needed pleading. The die was cast. In Berlin, as in Moscow, research was directed towards military applications. After Hitler came to power, the regime hardened in both camps. Both the Germans and the Soviets placed the space adventure under strict surveillance, right up to its historic conclusion. Henceforth, Von Braun and Korolev were, for the most part, at the controls. They were to meet - and oh, how much so - different destinies. But our global heritage will be restored to us Publication du premier article scientifique : «Clipper Ships of Space» de Carl WILEY dans la revue Astounding Science Fiction Hermann OBERTH développe le concept de «miroir de l'espace» dans son ouvrage «Les Hommes dans l'espace» Invention du terme «Voilier Solaire» par un ingénieur américain, Richard GARWIN, dans une étude détaillée sur le sujet (numéro de mars de «Jet Propulsion») Ted COTTER propose l'idée d'une voile tournante Publication de la nouvelle d'arthur C.CLARKE «The Wind From the Sun» qui évoque une course de voiliers solaires de la Terre à la Lune. Il est intéressant de noter que l'u3p a eu la même idée avant d'avoir eu... vent de cette nouvelle Pierre BOULLE (auteur du «Pont de la Rivière Kwaï») publie «La Planète des Singes», roman dans lequel il met en scène des voiliers solaires (absents du film) L'effet de la pression solaire est utilisé pour l'orientation de la sonde Mariner 10 durant son vol vers la planète Mercure La NASA et l'esa engagent des travaux sur les voiliers solaires afin de rejoindre la comète de Halley en Le projet est abandonné en 1977, le délai était trop court pour mettre au point cette nouvelle technologie.

5 I - DREAM AND HISTORICAL BACKGROUND 4 - THE BLOOMING OF SOLAR SAILS Restored to us a hundredfold, since despite the uproar and abomination, the soft song of the solar sail had not been forgotten. That tiny upsurge, the gentle zephyr, the infinitely small breeze imperceptible to the naked skin, continues to blow its ten grams per hectare in one small corner of the Milky Way. It would be enough to give a carrier rocket the job of putting a load into outer space for it to be immediately entrusted to the constancy of this beneficial flow blowing towards the farthest of destinations. Alleluia! The sailors of yesterday are waking up. Make room for manoeuvres. Henry the Navigator and his lieutenants, who discovered how the wind system works in the Atlantic, are more than ever on the bridge. It is for us to use the "trade winds of space". For us to calculate the subtle adjustments which will allow us to ricochet from one planet to the next. Unimaginable speeds, inexhaustible resources... the door is open. Korolev and Von Braun placed our feet in the stirrups. Despite the staggering priority granted to the army up until the collapse of the 3 rd Reich, the solar sail has not been forgotten. And this is a determining factor. Alas! In the meantime, we are forced to observe that despite Tsander, who as early as 1924, and in the midst of popular euphoria, envisaged launching ultra- light mirrors to act as sails, this type of propulsion was still in the planning stage at the e nd of the 1980's. In 1973, however, in the United States, the JPL (Jet Propulsion Laboratory) initiated research in the matter, but despite the inspired work of the protagonists, the experiment was cut short and Jerome Wright's team, who were brewing up a skilful recipe for straddling Halley's comet, were faced with reductions in the NASA programme. Lastly, in the field of achievements, at the end of this long road, the honour of developing the first solar sail in low orbit went to the highly pragmatic Russian engineer, Vladimir Syromiatnikov, of RSC Energya. It saw the light on 4th February 1993, and was christened "Znamia", (the Flag). Will we see solar sails enter into the competition in travel between the heavenly bodies one day? It is quite probable. The Union for the Promotion of Photonic Propulsion, known as the U3P, has taken up the cause after various other attempts, and under the leadership of Guy Pignolet, had seeked an opening to launch, on the occasion of the centenary of the creation of the Aéro- Club de France, the latest version of "Znamia", equipped with a guidance system. We wish it luck, for like the square- rigger of time past (1), the solar sail is designed for the open seas and not for manoeuvres within the harbour, and at the exit to the terrestrial port, all will not be plain sailing. What is more, this is one way to see how the considerable tonnage naturally involved in the transport programs of the next centuries could be transported. (1) The square- rigger is a ship with rigging made of square sails Robert STAEHLE et des chercheurs américains du JPL** (Jet Propulsion Laboratory) décident de poursuivre l'idée de voile solaire en créant la WSF (World Space Foundation) Création en France de l'u3p (Union pour la Promotion de la Propulsion Photonique), dont le but est de construire un voilier solaire et d'organiser une course de la Terre à la Lune Création au Japon de la SSUJ (Solar Sail Union of Japan) par des ingénieurs de l'isas (Institut of Space and Astronautical Science, agence spatiale du Ministère de l'éducation Nationale) Le Conseil Régional Midi-Pyrénées relève le défi de l'u3p en décidant de financer l'organisation de la course. Après deux brillantes conférences de presse, à Paris et Stockholm, dans le cadre de l'iaf. Il n'y a malheureusement pas eu d'autre suite Klaus Heiss, un économiste américain, propose d'organiser à l'occasion du 500e anniversaire de la découverte de l'amérique, la régate de voiliers solaires Colombus 500, de la Terre à Mars. Le projet est abandonné faute de financement Le principe de la course présenté par l'u3p, la WSF et la SSUJ est approuvé par l'iaf (International Astronautical Federation), organisation mondiale regroupant la plupart des professionnels de l'espace Des ingénieurs spatiaux espagnols créent la CVS (Comición Vela Solar) et s'associent à l'u3p. Réunies à Paris ces associations décident de se constituer en Comité de Course Terre-Lune, -EMRC (Earth Moon Race Committee)- et d'organiser un lancement simultané de voiles solaires. L'IAF sera l'arbitre de la course Création de VSE (le Voilier Solaire Européen), chargé de concevoir un voilier solaire européen L'IAF crée un groupe de travail «Voile Solaire» (SSWG Solar Sail Working Group) et officialise le règlement de la course Terre-Lune auquel il donne le nom de Luna Cup Le 4 février, déploiement par les Russes du miroir solaire Znamia par le SRC (Space Regatta Consortium) qui préfigure les futures voiles solaires L'U3P ouvre son site sur Internet : L'U3P est acceptée au sein de l'iaf La voile poursuit son vol avec le lancement de la course virtuelle sur Internet et avec la création du groupe de travail DAEDALUS à l'estec (centre technique de l'esa, Agence Spatiale Européenne). ** Le JPL a été créé en Il deviendra le principal laboratoire de recherche spatiale de la NASA, créée en 1958 par le Président Eisenhower. Les sondes conçues par le JPL nous ont permis de découvrir un système solaire inattendu, à la fois varié et uniforme. Elles ont pour nom : Pionner, Voyager, Mariner,... et plus récemment Galiléo, Mars Pathfinder accompagnée de Sojourner, premier robot martien.

6 II - THE REALITY 5 - THE SOLAR SYSTEM T HE S UN PRODUCES PHOTONS, i.e. an electromagnetic wave which is equivalent to a flow of light particles; it also produces, among others elements, heavy nuclei, electrons and protons that make up the solar wind. A common mistake consists in associating solar wind and photonic propulsion. There is no connection between them. It is the photons, when reflected on the surface of the sail, which transmit a small but constant part of their energy. Solar wind heavy nuclei travel at a speed of some hundreds to some thousands kilometres per second along the lines of interplanetary magnetic fields while the solar photons move in a straight line at a speed of km per second. I N OUR ENTIRE SOLAR SYSTEM, a spacecraft is submitted to the gravitational attraction of the Sun and of the planets. If it is close to a planet, the gravitational attraction of that planet will be predominant. Whether in orbit around the Sun (as the planets are) or around a planet, a solar sail is a spacecraft which owns a unique property with respect to usual satellites : in addition to the various forces of attraction to which it is submitted and which shape up its basic traje ctory, there is a force of photonic origin that can be used to modify the initial trajectory. A SOLAR SAIL IS A MIRROR THAT REFLECTS THE INCIDENT BEAM OF LIGHT. ACTION GENERATES REACTION. The photonic pressure exerted by the sunlight applies in a direction perpendicular to the surface of the sail. So, by changing the orientation of the sail, it is possible to choose the direction in which the acceleration will be effective and therefore the way how the orbit will be modified. Two simple cases have to be considered, depending on whether the orbit is sun- centred or planet- centred (around the Earth, for instance). Comets were formed at the time of the birth of the solar system. Most of them have not evolved since their formation. They present therefore a great scientific interest. The end of the millenn ium has seen the visit of four exceptional comets : in 1986: the return of Halley, in 1994: Shoemaker- Levy SL- 9 broke into more than twenty pieces due to the gravitational field of Jupiter and smashed on the planet. in 1996, Yakutake shined across the skies of June in 1997, during 3 months, Hall Bopp offered an unforgettable show for Earthmen. Asteroids are small solid bodies, with sizes varying from a few kilometres to a few hundred kilometres. They go around the Sun, most of them between Mars and Jupiter. They are an inexhaustible source of raw materials (see p.15) Meteorites range from fine dust to asteroids. The Earth receives each year several thousand tons of dust of all sizes. Most burn through the atmosphere, forming meteors or shooting stars, some others reach the ground to the greatest delight of collectors and scientists. Meteorites are, together with the Moon rocks brought back during space missions, the only extraterrestrial bodies that can be studied in laboratories. O RBITS AROUND THE SUN have long periods (one year in the case of the Earth, for instance). Consequently, the various changes of attitude necessary to affect the orbit do not need to be implemented quickly. A ROUND A PLANET, such as the Earth, the angular speed is much greater (a satellite makes a revolution in one hour and half in low orbit or in 24 hours for the geostationary orbit at km. So, if we want to use the photonic pressure around the Earth, we need to have more precision and to act faster than in the case of the heliocentric orbit. C OMETS, ASTEROIDS, METEORITES, are small objects of the solar system that did not incorporate into a planet during the formation of the solar system by the process of accretion (accumulation of materials). They are privileged targets and several space probes were dedicated to them from the start of the space conquest. More major missions are announced for the 21st century, and the European Rosetta space probe, which is to be launched in 2003 toward the Wirtanen comet where it is due to land in 2011, is bound to make physico- chemical analyses of the soil of the comet.

7 II - THE REALITY 6 - TECHNOLOGY BASICS THE SOLAR SAIL CRAFT CONCEPT BASIC PRINCIPLES OF SOLAR SAILING : The pressure of radiation of the Sun is acting on the solar sail. Light, i.e. the photons, generate the pressure when they bounce off the sail. This pressure of radiation is from one hundred to one thousand times stronger than the pressure of the solar wind (wind of heavy nuclei). sail, the principle is the same but it extends over a long time. The surface of the sail is turned in order to get a reaction from the photons that is directed along with the orbital velocity, either to accelerate or to slow down the spacecraft. This leads to a trajectory shaped as an escaping spiral for an acceleration, or as a contracting spiral for a slow down. So the trajectory is continuously modified without using any fuel. Here is the potential, "here is the inherent beauty of the solar sailing. We don't need any fuel to propel ourselves; the nature provides us with it" (Louis Friedman). From the energy point of view, we see indeed the perspectives of an endless resource. HOW STRONG IS PHOTONIC THRUST? The solar sail is a mirror with a high reflecting capability. When the photons hit the mirror, they give it momentum. The bigger the sail, that is the mirror, the bigger the momentum will be. The lighter the sail, the larger the acceleration will be. The resulting thrust is perpendicular to the sail. This thrust of propulsion can be oriented by turning the sail (changing its attitude) and consequently it is possible to control the solar sail craft. PHOTONS AND GRAVITATION : On the surface of a lake the combination of the action of the aerodynamic forces and the reaction of the water make the sailboat move forward. The air acts on the sail in a controll ed way, the water reacts on the submerged part due to its inertia. Together, the two forces define the trajectory of the ship. Likewise, it is the combination of the controlled effects of the photon flow with a set direction with the effects of gravitation that allows a vehicle with a solar sail to move freely. The Sun produces a huge amount of light energy that disperses with the square of the distance. Near the Earth, the pressure exerted by the solar radiation is about 4,5 grams on a square with 100- m sides. It results into a low acceleration that is doubled and that become controllable when it exerts on a mirror. Pressure = 4, Pascal (1AU away from the Sun) The photonic pressure is acting all day long, for months, and years. Its long- term effect is far from being negligibl e. And ideall y a 800 metres side and 2,5 microns thick square sail would convey a payload of 5 tons to Mars in 500 days or 1,5 tons to Jupiter in 900 days, as shown hereafter in the diagram proposed here by Louis Friedman (d=day). But a dynamic element has to be taken into account in this analogy : without wind, the position of the sailboat remains constant, while without photonic pressure, it is the orbit of the solar sail craft that remains constant (same speeds = same orbit) In order to manoeuvre in the solar system, a space vehicle has to modify its trajectory by acting on its instantaneous speed and on the orientation of the plane of its orbit. In a first approximation, all the objects in the solar systems, small or large, are moving around the Sun, circling around elliptical orbits. Discovered by Kepler in 1609, this phenomenon that results from the laws of gravitation associates closely the speed of an object with its distance from the Sun. Mercury, the closest planet from the Sun covers its orbits faster than any other planet. The Earth covers every year 940 million of kilometres at the average speed of 30 km/s. Mars, more distant from the Sun, moves at a speed of 24 km/s only. So, to transfer an object of the Earth on an inertial trajectory (without propulsion), where the laws of astronomy set the speeds, important adjustments are needed on departure and on arrival. That is what is realised in the rockets with standard propulsion systems by creating a thrust that modifies the speed almost instantly. With a solar Thus, even if the solar sail craft is a low thrust vehicle, the continuous energy of the Sun allows ii to achieve important modifications of trajectory in a relatively short time. The operational flexibility combined with an almost infinite theoretical operating time allows speak for a bright future in the interplanetary travel systems. Orbits around the Earth can give opportunities to fascinating evolutions and races, but they are not the favoured grounds of the solar sails to come. The maximal operational efficiency will be obtained on heliocentric trajectories when solar craft will sail from one end to the other across the solar system.

8 II - THE REALITY 7 - CONSTRAINTS A solar sail will be subjected to constraints common to all the spacecraft but also to some peculiar ones. S PACE IS A VIOLENT AND HOSTILE ENVIRONMENT FOR SEVERAL REASONS: The differences in temperature are important between the parts exposed to the Sun and those in the shade. The thermal exchanges with the environment are realised due to the only radiative effect (neither conduction nor convection possible). Vacuum has also the effect of sticking the materials one to another. The lubrication of the mobile parts is a particularly sensitive problem (especially when the sails will have to be unfolded). Once in space, the only way to intervene will be the remote control. So, before declaring a satellite able to fly, it is necessary to make sure, by modelling, putting to the test in simulated surroundings, that the chances of success are reasonable. Choosing the strategies of redundancy and of systems reconfiguration constitutes an arduous problem. Les moyens de communication entre un véhicule spatial et le centre chargé de son contrôle, au sol, sont limités par la distance et l énergie disponible. Il faut donc a priori opérer une sélection drastique des informations jugées nécessaires : vitesse, attitude, éloignement, perturbations éventuelles,... Space is not a complete vacuum. It is crossed by heavy nuclei emitted by the Sun (the solar wind), micrometeorites, and even spatial debris (which are becoming a permanent threat for the satellites in activity). In some areas of the terrestrial surroundings as the radiation belts, the accumulated doses of radiation are very proving for the onboard electronics : this conditions the choice of components hardened to the radiation. During the first phases of the crossing of the atmosphere, a fairing must protect the satellite. The size of this fairing must be limited, antennas and solar panels have to be fold up. On orbit, they have to be unfolded in complete safety, the experience shows that this difficulty causes numerous problems during the space missions. S OME OF THESE CONSTRAINTS GROW IN SIZE IN THE CASE OF A SOLAR SAIL: The ratio of dimensions between the folded configuration and the operational unfold configuration is far greater than for usual satellites: the diameter has to increase from 3 metres to several dozens metres. The unfolding is surely the crucial phase of the mission of a solar sail. T all and very light, a solar sail is a relatively flexible object. The control of a solar sail must take this great flexibility of the structure into account. The choice of the material supporting the metallized layer that is to ensure the optical quality of the reflection of the flow of light is crucial. We are looking for a surface that will be smooth after its deployment. We also aimed at keeping its mechanical qualities all the way through its mission (moderate aging under particles and ultraviolet radiation in particular). The mass/surface ratio of the sail, main element of its performance, should be raised to the greater value possible, while for the usual satellite, size and mass are elements that count only for the cost and the launching conditions. They have no major importance for the functioning. The techniques evolve quickly and what is a complex problem today will be probably solved during the years to come. B UT, THE CONDITIONS ENCOUNTERED DURING THE LAUNCH TO REACH SPACE, A RE AS SEVERE AS IS THE SPACE ENVIRONMENT : During some phases of the launch, the acceleration of the carrier rocket is about 5g. This means the equipment has to be sized to stand 5 times the weight it should stand on the ground. Whatever launcher is a powerful device. The vibrations and the shocks during the different phases of the flight, as well as the acoustic pressure undergone by the satellite on board are very limiting. For financial reasons, the satellites have to be a light as possible; only 1 to 2% on the weight leaving the launch pad will be put into orbit. L'espace milieu violent et hostile collection BT ESPACE, cnes - PEMF 1994 "...L'atmosphère gazeuse qui enrobe notre planète et son champ magnétique naturel nous épargnent les rigueurs du milieu spatial dont nous ne soupçonnions pas la violence avant que la multitude d'engins spatiaux lancés depuis plus de trente ans nous en rapporte une image effrayante : un vide quasi total, des écarts thermiques redoutables, des flux de particules, des rayonnements et des débris de toutes sortes...".

9 II - THE REALITY 8 - LES MOYENS La voile solaire, reste un sentier difficile qui n est pas encore intégré dans la vie économique. L obtention de budget n est pas toujours facile. C est pourquoi tous les équipements non spécifiques doivent être «choisis sur étagère» parmi ce qui est disponible sur le marché. CONCEPTION ET CONSTRUCTION DE DIVERS TYPES DE VOILE 2 La voile disque dont la forme est maintenue par un mouvement de rotation : elle permet de résoudre de nombreux aléas de déploiement mais son mécanisme de pilotage (création d'un couple par déplacement relatif des centres de gravité et de poussée cf. 11) est complexe. La seule voile qui n'ait jamais été déployée à ce jour (Znamia 2, conçue par l ingénieur russe Vladimir Syromiatnikov) est rotative mais pas encore pilotée. LE MATÉRIAU : La voile doit être une feuille légère, lisse, fine, réfléchissante. Ce sera donc pour commencer un film plastique servant de support à une couche de métal réfléchissant, en général de l aluminium. Dans ses études sur la voile solaire promise au survol de la comète de Halley, l'équipe du JPL avait testé tous les types de polymères. Le mylar et le kapton s'avéraient les meilleurs. Sous une épaisseur de 8 microns, ils pourraient assurer le support d'une voile qui, bien ouvragée pour éviter les charges électrostatiques, les boursouflures locales, les plissements parasites, les déchirures et autres points délicats, n'excéderait pas une densité de 10 g/m 2. À un stade ultérieur et pour accroître la charge utile, l'épaisseur pourrait être réduite à 2 microns grâce à la stratification nanométrique. Ce procédé de pulvérisation sous vide par couches successives de l'ordre du millionième de millimètre permet de contrôler le dépôt du métal jusqu à des finesses de 0,1 micron (en dessous de 0,02 microns, les photons traverseraient le métal au lieu de rebondir ; il n y aurait plus de poussée photonique). 3 L'héliogyro la solution audacieuse qui fut imaginée à Pasadena par Richard Mc Neal et John Hedgepath, pour un vol de conserve avec la comète de Halley, aurait eu 12 pales de 8 mètres de large et 7500 mètres de long chacune maintenues en rotation lente par la force centrifuge. L'inclinaison des pales aurait permis les changements d'attitude. On envisage même (projet Aurora cf. 15) qu après le déploiement, le support plastique puisse être éliminé par évaporation pour obtenir une voile ultra légère, entièrement métallique. LES DIFFERENTS TYPES DE VOILES : Pour faire face à tous les écueils que sont : ondulations, vibrations, phénomènes de résonance et autres désordres engendrés par les manœuvres de changements d'attitude, on a imaginé divers types de voiles et de gréements. 1 La voile carrée (dont la forme pourrait aussi bien être un triangle, un rectangle ou un hexagone) : elle est stable et tenue à poste par le moyen d'un gréement classique (tangons et haubans) mais en raison de son rapport masse/surface élevé, ses performances sont modestes. POUR L'INSTANT, les technologies se limitent à ces 3 possibilités. Que privilégier? La rigidification de l'ensemble, la stabilisation de l'attitude, la précision du pilotage? À l'heure des choix il faudra bien également penser à la charge utile, scientifique ou marchande. Mais avant de dresser l'inventaire des soutes, ne serait-il pas plus urgent, dans la chronologie, de découvrir le moyen d'empaqueter sous un volume réduit, de ficeler et plus tard de redéployer ces étranges voilures de plusieurs milliers de mètres carrés? Les rouler en cylindre? Les compacter par couches superposées? Ce nouveau casse-tête, c'est «l'origami», l'art du pliage ; domaine inédit pour les maîtres-voiliers. À moins bien sûr de fabriquer directement en orbite la pellicule réfléchissante idéale!

10 III - THE PROJECTS 9 - GOOD TRIES T HE FIRST USE OF THE PHOTONIC PROPULSION took place in the seventies with the probe Mariner 10, launched to Mercury on Nov 4th An over consumption of nitrogen was detected, due to a bad condition of the attitude control system. To avoid the exhaust of nitrogen, and so the end of the mission, technicians have modified the solar panel orientation in order to modify the orbit by the effect of the photonic pressure in such a way that Mariner 10 and Mercury could meet together every 176 days. The spacecraft was not a real sail craft, but it is in that way that the solar sailing principle has been applied for the first time. At the Jet Propulsion Laboratory (JPL), in 1976, NASA engineers started studies on the subject with the mission goal to accompany the Halley comet during its sun approach in Two concepts were born, the heliogyro, a huge sail constituted by rotating strips, and the square sail, sustained by diagonal booms or sprits. For budgetary reasons, NASA did not pursue any of the concepts. At the same time, similar studies have been carried out by ESA for a one kilometre side square sail. In the first scenario envisaged in 1992 for the Luna Cup, the sail craft were to be launched by ARIANE in a double launch. They would have been put on an starting orbit beyond the geostationary orbit. After being separated, the sails were to be deployed and oriented relative to the sun to start the race. The unfolding is still the most risky step in a sail craft life. A few tests have been performed by U3P with models in the CNES Caravelle Zero- G (an aircraft used for low gravity experiments ) In 1993, a Russian team, led by Vladimir Syromiatnikov, deployed a 20 meter diameter sail named Znamia, the flag, from a Progress spacecraft after its separation from MIR. In 1996, the unfolding with inflatable booms of the experiment Spartan 207 (14 meter diameter circular reflector) from the US space shuttle was another technological demonstration useful for the development of future solar sail craft. Today, concerning spin- stabilised sails or square boomed sails, the technologies are available. Nothing can prevent to build one. Vladimir Syromiatnikov During the second half of the fifties, Vladimir Syromiatnikov was recruited by Serguey Korolev, C onstructor Principal of the Soviet space launchers, and he worked with him to build Sputnik - 1. Then he conceived and realised the androgynous docking system used for the first time for the junction of a US spacecraft Apollo and a Soviet s pacecraft Soyuz on the 17th of July He received the Lenin Awards. In 1995, he developed a new system (APAS) to dock the US Space Shuttle with the Russian Space Station MIR. He was nominated engineer of the year by the Americans. In between, the 4th of February 1993, his work as a solar sail designer has reached a top achievement with the launch of Znamia 2. Jerome Wright With an aerospace engineer diploma and a 4 year period in US Air Force, he joins the Batelle Memorial Institute of Columbus (Ohio). On Donald Edgecombe's request, he studies a trajectory showing the possibility to encounter the Halley comet with a sail craft. Called by Louis Friedman and Chauncey Uphof, he joins the JPL, where he leads the solar sail missions designer team. In spite of encouraging results and the support from Bruce Murray, JPL director, the studies are stopped for budgetary reasons. Then he creates a company, General Astronautics, with the goal to develop low cost launchers. He wrote down his experience in the book Space Sailing published by Gordon & Breach in Mariner Space probes launched from 1962 to 1974 by the US: Mariner 2, first probe flying by Venus while still operating the 13/12/62, Mariner 4, first probe flying by Mars while still operating the 17/05/65; Mariner 10, first use of gravitational swing by, Venus as it is, to reach Mercury. Dans le 1er scénario envisagé en 1992 pour la Luna Cup, les voiliers devaient être lancés par une fusée Ariane à l'occasion d'un lancement double. Ils auraient été injectés sur une orbite de départ au-delà de l'orbite géostationnaire. Après séparation, les voiles devaient être déployées et orientées par rapport au Soleil pour commencer la course. Le déploiement reste la partie la plus délicate du fonctionnement d'un voilier. Des essais ont été réalisés par l'u3p avec des maquettes dans la Caravelle Zéro-G du cnes. Progress Unmanned automatic spacecraft used for supplying the Russian space station MIR with equipment, water, food, fuel, The craft can be used for scientific experiment, as it was the case for Znamia 2. At the end of the mission, the craft, containing the waste of the station and the obsolete pieces of equipment, is pushed down to the Earth. It disintegrates and burns in the dense layers of the atmosphere.

11 III - THE PROJECTS 10 - NAVIGATING WITH A SOLAR CRAFT A PRESSURE EXERTED ALONG THE VELOCITY VECTOR MAKES THE SAIL CRAFT SPIRAL OUTWARDS. A PRESSURE EXERTED AGAINST THE VELOCITY VECTOR MAKES THE SAIL CRAFT SPIRAL INWARDS. A PRESSURE PERPENDICULAR TO THE DIRECTION OF THE VELOCI TY VECTOR CHANGES THE INCLINATION OF THE ORBITAL PLANE OF THE SAIL CRAFT. Thus, TO CHANGE THEIR ORBITS, standard satellites are fitted with small thrusters that exhaust at a great speed the gas produced by a chemical reaction. Considering the orbital period of the satellite, these manoeuvres can be seen as instantaneous. Their effects is to transfer the satellite from an initial orbit to a new orbit defined by the point where the manoeuvre takes place and by the velocity vector obtained during this manoeuvre. In the case of a solar sail, THE MODIFICATION OF THE TRAJECTORY IS A PERMANENT PROCESS, the effect on the orbit can be assessed only in the long term, and it involves more complex mathematical analyses. The key factor to control the orbit of a solar sail is the angle made by the normal of the sail and its velocity vector. It is worth noticing that in a heliocentric orbit the attracting gravitational acceleration and the repelling photonic pressure both act with an intensity in a square inverse ratio to the distance to the Sun. Consequently, any size of sail leads to an identical performance wherever the sail is in the solar system. Some simple cases : An orientation of the normal of the sail along the velocity vector and directed in the direction of motion leads to a spiral towards the outside. On the contrary, if the sail is directed so that its normal is lined up with the velocity vector, but in the opposite direction to the motion, the energ y of the sail is continuously lowered down. Thus in this braking configuration, the sail spirals in closer and closer towards the attracting body. The so- called "levitation" of the orbit is an especially interesting case: if the normal of the sail, on a heliocentric orbit, is continuously directed toward the Sun, the photonic pressure is deducted to the gravitational pull of the Sun. It is as if the Sun was less massive as seen from the sail, and the orbits of a given period will be achieved at a distance from the Sun lower than would be usual for this period.

12 III - THE PROJECTS 11 - PILOTING TECHNOLOGIES T O CONTROL THE DIRECTION OF THE THRUST AND ITS EFFECT on the orbit, a solar sail must have a definite orientation with respect to the Sun and to its own trajectory (velocity vector). It must have sensors that indicates the direction of the Sun and its own orientation (attitude), for instance with respect to the fixed stars. The information has to be processed either directly by its onboard computer or on the ground after transmission, to know how this attitude is modified. When it is necessary, actuators (systems that produce an action) have to be operated to change the orientation of the sail. The actuators can be of various kinds, such as reaction systems, (gas jets), mobile flaps that use themselves the photonic pressure (rotation, dihedral). They depend closely on the conception of the solar sail, where the two most crucial aspects are the deployment and the orientation control system. Thus, TO AVOID BEING AN AUTUMN LEAF blown by the wind, a solar sail will have its own equipment that will ensure at least the following functions: communication with the stations on Earth: transmitter- receiver, antennas; attitude control with solar, stellar, terrestrial, and magnetic sensors ; management of the onboard information: computer, memory, clock; electrical power supply: solar cells, batteries. Other functions can also appears necessary during some part of the flight, such as the use of a thrust system to raise up the initial orbit, or control systems for the deployment of the sail. T WO TECHNIQUES ARE FORESEEN TO MAINTAIN A SOLAR SAIL DEPLOYED : 1 the use of a large rigid structure 2 the centrifugal force created by a rotation. 1 RIGID STRUCTURES : The photonic force is small and a very thin and light structure in the shape of masts or circular rings would be enough to maintain the sail tight under tension; but some other effects take place, such as the inertia during the manoeuvres of orientation, the thermal distortion, depending on the angle of the solar aspect, and above all the stress during the deployment of the structure itself. These are the effects that will dimens i on the structure. Two main kinds of rigid structures are possible. the articulated mechanical masts the inflatable structures that can be hardened.. 2 T HE CENTRIFUGAL FORCE The tension exerted on the material by the centrifugal force in a rotating sail maintains it in a tight shape, the tighter the faster the rotation. The sail can have the shape of a disc (Znamia), or can be formed with long blades (Heliogyro). To sail, you have to know in which state (position + velocity) is the sail and what later state is expected, in order to manage the modifications of trajectory. The control techniques of solar sails show analogies with the navigation of terrestrial vehicles: the motion of the flaps at the end of the masts or at the edge of the sails correspond to the navigation of the light dinghy fitted with only a main sail and a jib. the moving of a mass toward the centre of pressure is similar to the manoeuvre of the windsurfer that moves on its board. the torsion of the Heli ogyro's blades is similar to the cyclic variations of pitch that exists on the rotors of helicopters.

13 III - THE PROJECTS 12 - FROM THE EARTH TO THE MOON T HE NEW MILLENN IUM will see the launching of the Luna Cup, first Earth- Moon regatta. Sail craft, without crew on board, will be launched from the E arth. They'll be piloted from the E arth and will sail to the M oon only pushed by the strength of light. They'll symbolize again, the adventure of new worlds' pioneers gone on the oceans and will foreshadow... the pacific conquest of solar system, the main work of the new millenn ium. U3P has a mission to organize the race, to supervise the launching of the sail craft on board of an Ariane (E uropean) or a Proton (R ussian) rocket, to navigate the sail craft and to promote it by mass medias and cultural actions, where the mobile control center and the space exhibition will be spearheads. T HE RACE WILL TAKE A LONG TIME : At the least 10 months, related to the project in question, that is to say one hundred orbits more and more elliptical around the E arth. For each orbit, sail craft crews will attempt to gain some thousand kilometers until the trajectory goes beyond the M oon. THE WINNER will be the first sail craft that has transmitted a photography of the center of the dark side of the Moon. " The interest in sail craft is for distant flights in such situation like spacecraft that need to bring fuel for an outward and back journey. The sail craft, which does need no fuel, is in a way to be freed of this constraint. On another side, the weak but constant acceleration allows to get velocities out of reach of actual space probes and make possible hovering flights with other celestial bodies such as comets. " Louis Friedman In a script thought of in 1992, the E uropean sail craft, with 45m side, more than 2000m2 of sail, will be driven from the E arth from a center of mobile shipping. The sail craft crew will receive, analyse, design strategies and will keep operations under control. This sail craft crew will move along with a space exhibition and a sponsor's village through fifty towns of France and Europe, and when they'll stop over, an exceptional event will happen there. Sail craft crew will be linked to the control center of Toulouse that will assume all links to the sail craft. About the Luna Cup "... The solar sail is even more functional in deep space and for interplanetary travel than in the neighbourhood of the Ea rth where it has to make multiple manoeuvres. The race will be much demanding for a "first sail" : the navigation will be sophisticated, attitude control will have to be perfect and the dynamic stability without any fault... manoeuvring capability and accuracy will be a must.. " Louis Friedman W AITING THE L UNA C UP THE VIRTUAL RACE W HEN OPENED IN 1995, the Web server allowed numerous and beneficial exchanges. That's how is born the idea to adapt on the web simul a tor s softwares of the E arth- M oon race. T HE VIRTUAL RACE ON I NTERNET, offering a standard configuration of interface, is going to permit at any sail craft s simulator to move around in space with any navigation software. That virtual race, available for the general public, will make it easier for schools and universities to do research and it will allow to start new challenges.

14 IV - THE ADVENTURE 13 - MIRRORS OF THE FUTURE T HE AIM OF A SOLAR SAIL IS TO FLY ON INTERPLANETARY - OR INTERSTELLAR - TRAJECTORIES. They should logically find their place in the exploration of the solar system, started in the sixties, and to be pursued in the 21st century with any kind of scientific or economical objective. The sails will carry scientific probes studying the solar system, and bringing back to the E arth equipment and samples. Among the objects to be explored, the asteroids have a great scientific interest, as they are, like comets, witnesses of the birth of the solar system. They have an economical interest too, as they can be seen as a huge reserve of raw materials, if they run short on the Earth in the next decades. Cf 15 Thomas, 7 years old school boy at St-Joseph-de-Rivière (Isère), 1993 (free translation of his poem) A dog in a bike? I can't believe it! A cat in a boat? I can't believe it! A mouse in a whale? I can't believe it! But, believe me, A Thomas In a sailing boat Racing to the Moon The 4th of February 1993, Znamia 2, the "flag", the first solar sail like, was unfolded in space. The objective was to create a space mirror in order to reflect the sun light on the Earth. Znamia 2 is the first sail of the Novy Sviet project, the New Light - New World. Vladimir Syromiatnikov, its project manager, thinks that, in a long- term future, a hundred of solar mirrors, in a swarm at altitudes between and to km, could create an artificial "sun" : for a town in the polar night, for rescues during the night in case of catastrophe, for a spectacle,... We must precise that this project has opponents : astronomers afraid by the increasing light pollution, and ecologists, afraid by the possible changes in life cycles for plants and animals. But it could also contribute to find a smart and economical solution to the problem of street lighting in urban areas. I will see it soon! Excerpt from "The Wind from the Sun" by Arthur C. Clarke, May " The full force of the Sun... He smiled wryly, remembering all his attempts to explain solar sailing to those lecture audiences back on Earth. /... / " Hold your hands out to the Sun " he'd said. " What do you feel? Heat, of course. But there's pressure as well - though you've never noticed it, because it's so tiny. Over the area of your hands, it come only to one millionth of an ounce. " But out in space, even a pressure as small as that can be important, for it's acting all the time, hour after hour, day after day. Unlike rocket fuel, it's free and unlimited. If you want to, you can use it. We can build sails to catch the radiation blowing from the Sun. " S AILING IS A ROMANTIC IDEA, in harmony with nature. Sailing in space with mirrors is a similar idea. Peter Pan travell ing on a Moon beam, Icarus flying near the Sun,... T HE PHOTONIC LITERATURE is, for now, the domain of science fiction authors, poets, and visionary engineers. But after the first solar sails running in space, we can expect that intrepid skippers, like the French sailor Florence Arthaud who is already volunteer, will challenge for a race to the Moon, and later for other planets. One of these days, will the books be based on a true life experience? Excerpt from "The Planet of the Apes" by Pierre Boulle, 1963 Chapter 1 : " In those times, interplanetary travel was commonplace, though interstellar ventures were still an exception. Rocket ships would take the tourists to fabulous locations on Sirius, or the finance people to the stock markets of Arcturus and Aldebaran. But Jinn and Phyllis, a wealthy and free couple, were known through the Cosmos to be young originals, with a bit of craziness, and they would cruse through the Universe just for the fun of it - with their sail craft. " Their ship was a kind of sphere with a shell - the sail - made of amazingly thin material, and it would move through space, just pushed by the pressure of light beams. "

15 IV - THE ADVENTURE 14 - THE BILLION PARTNER ADVENTURE spectators Hilaire Belloc , an english writer No wonder if the long sailing ships have unceasingly caught the man s eyes and looked so magnificent. Arthur C.Clarke, Solar wind The unique feeling inspired by ancient sailing ships will be revived, even more vivid and intense, should one day new ships drone to and for across space ACCORDING TO THE ENTHUSIASM it will generate, the Luna Cup departure will be a planetary event. The audio- visual means of communication and information will allow the Earthlings to share such an adventure: launching of the rocket, spreading of the sails, the departure of the race, the major operations and the arrival But the race will undoubtedly last much more than expected: a hundred orbits or so, sophisticated manoeuvres reserved for those in the know, During about ten months, the climax of such a proof will have to be preserved. RIGHT FROM THE BEGINNING, the Luna Cup inventors aimed at taking advantage of the race being so popular to heighten Earthmen s awareness of need for Solar system exploration. Contacts with the media have been taken and the project rose general enthusiasm, many a chronicle have been dedicated to it. Then several outstanding ideas followed: in 1990, the mobile navigation centre. In 1994, the common documentation packaging for all the children thr ough out the world. In 1995, everything about photonic power on Internet. APPROACHING THE LUNA CUP, media- aids will be provided to retain interest and attend the Luna Cup: press articles, radio and T.V. broadcastings, new games, slogans, associated objects and flight simulators,... THE OUTSTANDING IDEA AT THE END OF 1990: MAKE THE CONTROL- CENTRE OF THE EUROPEAN SAILING SPACE SHIP CIRCULATE FROM ONE TO the other of EVERY on of the FIFTY FRENCH AND EUROPEAN regional capitals. It aims at regionalizing the event by activating the media and the local key figures and by allowing as many students as possible to take part in the Adventure, subsequently ensuring the race its popularity. Imagine a trailer full of monitors and computers from which you could attend the space ship s route and send instructions to the command computer. It is the cockpit. Next to the trailer, there is a relay- truck with a satellite- antenna to communicate with the control centre in Toulouse from where the sailing space ship will be directed through out the CNES satellite control network. Near the trucks, under a canopy, an exhibition presents the main space objects of the 21st century amidst the future applications derived from the solar sails. The caravan is to stay for a week in each scheduled town. Two big tours are planned: the first- one will be organised during the building of the ship, allowing the visitors to follow its construction. The navigation rehearsals will be coached by local skipper teams. The second one will allow the visitors to attend the live race. In 1994, during an Educational Forum on pedagogical applications of the Luna Cup, the decision has been made to create a common documentation dedicated to the children in the whole world, allowing them to attend the Luna Cup. Imagine schools from all the continents teaming up together in order to follow and share the great feelings that the Luna Cup will undoubtedly bring to them. The U3P association has played one of the main parts on the development of the Solar sails projects. It has originated the official rules of the Luna Cup. It has played a prevalent part among the Solar Sail Working Group in the I.A.F. It regularly takes part in congresses and exhibitions addressing indistinctly to dreamers or technicians. WITH THE EXISTENCE OF THE WEB IN 1995, new doors open widely on photonic power. The contacts have subsequently been fast, numerous and efficient. That is the way the virtual race was born. VIRTUAL RACE ON INTERNET The point is to define a standard interface between virtual sailing ships and navigation strategy. A virtual sailing- ship is a mathematical model of the structure and the performance of a solar sailing- ship which could be real. The space ship will be defined by constant physical parameters. The out- put- data sent to the standard interface will consist of dynamic parameters calculated in response to in- put piloting- orders received from the standard interface. A navigation strategy is a trajectory- application program which simulates the performance of a solar- sail, featured by physical parameters which gives out- put piloting- orders through the standard interface and which calculates a trajectory in response to in- put dynamic parameters received through the standard interface. Such a system will allow us to make any sailingship run on any strategy and subsequently to simulate races. The rules and standard interface are supervised by the I.A.F. expert committee. Let s imagine what will follow, schools, universities and also individuals, all fond of computing and space, will be challenging one another on a virtual Luna Cup.

16 IV - THE ADVENTURE 15 - TECHNOLOGICAL RESEARCH LA VOILE SOLAIRE APPORTE UN ELEMENT NOUVEAU dans le domaine des missions lointaines au sein du Système Solaire et plus loin encore. La capacité d'emmagasiner de l'énergie cinétique par des passages à proximité du Soleil permet d'atteindre rapidement n'importe quelle destination jusqu à l'héliopause et même au-delà. AU SEIN DU SYSTEME SOLAIRE, la voile permet des missions qui seraient irréalisable par le moyen de la propulsion chimique. Plus de combustible. La notion même de durée d'autonomie disparaît ou, plus précisément, se réduit à la durée de vie des équipements. DANS UN TEL CONTEXTE, LA MINIATURISATION DES EQUIPEMENTS DEVIENT UN ELEMENT PRIMORDIAL. De même, pour ce qui est de la structure des voiles, la recherche devra figurer au chapitre de l'hyper-résistant et de l'ultra-léger sans jamais perdre de vue le milieu très particulier dans lequel les matériaux seront utilisés : densité infinitésimale, micro-gravité, agression du milieu spatial. Les systèmes conduisant à des modifications de l'orientation de la voile n'ont en pratique jamais été réalisés. Une modélisation s'impose. La mise en œuvre de tels moyens suppose une recherche théorique préalable. Les sondes traditionnelles, avec des durées de missions très longues, limitent la recherche scientifique. La réalisation de certains projets de recherche passe par des vaisseaux à poussée faible mais continue. Les projets fleurissent : projets Aurora, Vigiwind, l'exploration des astéroïdes,... Tous ces projets rendent quasiment indispensable le développement de la propulsion photonique. LE PROJET AURORA, CONÇU PAR L'ITALIEN GIOVANNI VULPETTI, est un projet de mission pour étudier les structures de l'héliopause, limite supposée de l'héliosphère, où le vent solaire rencontre le milieu interstellaire. Ce projet est autofinancé par l'italie, le Royaume Uni et la Suisse. Il a pour ambition de créer une voile solaire métallique de 0,1 km2 soit 316 mètres de côté, comportant bôme, mât, vergue métallique, écoutes, haubans et drisses en plastique, fibre de carbone et fil d'acier. Cette voile serait construite en orbite et lancée au-delà des planètes du Système Solaire pour atteindre une vitesse de 12 UA par an, soit 57 km/s. L'étude a commencée en 1992, elle devrait aboutir au début des années LE PROJET VIGIWIND, CONÇU PAR LE FRANÇAIS J-Y PRADO, consiste à envoyer une voile solaire vers un point situé sur la ligne Terre-Soleil, à une distance de 3 millions de km, en amont du point de Lagrange L1, pour y placer un satellite d'alerte avancée pour les tempêtes solaires. La pression photonique est le seul moyen de maintenir un vaisseau spatial à ce poste pendant une longue durée. Ce satellite d'alerte offrira un laps de temps précieux pour protéger les spationautes, les satellites et les nombreuses installations terrestres des jaillissements énergétiques issus du Soleil lors de ses états de crise. La réalisation du projet Vigiwind permettrait à l'europe de se doter d'un ensemble de compétences nouvelles, pour envisager des missions futures encore plus ambitieuses. EXPLORATION A L'AIDE DE VOILES SOLAIRES projet présenté par Philippe Martimort au concours : "L'Espace à l'horizon 2025" Toulouse Il y a plus d'un million d'objets de diamètre supérieur à 1 km dans la ceinture d'astéroïdes. Les voiliers solaires semblent être adaptés à des missions de prospection de ces astéroïdes. A partir d'une orbite basse, une navette spécialisée amènera la sonde jusqu'à un voilier déjà en orbite haute d'attente et les injectera ensuite sur une trajectoire de libération. Le voilier décrira une trajectoire en forme de spirale qui l'amènera au voisinage de l'astéroïde. À proximité, la sonde se libérera du voilier et celui-ci reviendra se placer sur une orbite terrestre d'attente. La sonde freinée par un propulseur chimique se posera en douceur sur l'astéroïde. Lorsque la mission de collecte sera terminée la sonde décollera à l'aide Dun propulseur chimique et s'arrimera à un autre voilier qui la ramènera vers la Terre avec sa précieuse cargaison. Fig.5 H-Reversal Demonstration Flight Trajectory Profile : L = const. In HOF Trajectoire étudiée par Telespazio (Italie) pour la poursuite de l'héliopause (à 15 milliards de km, 100 UA) Héliopause : Limite de l'héliosphère, où le vent solaire crée une onde de choc à la rencontre du milieu interstellaire. Héliosphère : Région de l'espace où la densité d'énergie du vent solaire est supérieure à celle du milieu interstellaire. Unité Astronomique -UA- : Unité de distance égale à la distance moyenne entre le Soleil et la Terre environ 150 millions de km.

17 IV - THE ADVENTURE 16 - A NEW WORLD A LONG- TIME ON STAND BY, THE START OF FIRST SOLAR SAIL WILL BE ONE OF THE MOST IMPORTANT MILESTONES OF THE CONQUEST OF SPACE, LIKE SPUTNIK, YOURI GAGARINE, NEIL ARMSTRONG, VOYAGER... Controlled from the Earth, it will navigate only by light- power. It will allow to find again the innovative spirit which a long time ago had driven the great explorers on their way to adventure. The stepped- up activity in astronautics with the renewed exploration of the Moon and Mars, with the elaboration of large structures in orbit or with the development of new means for interplanetary propulsion requires unprecedented technological jumps. The study, the fabrication and the launch of a solar sail by Europe may represent an important step in this direction. Work on solar sails will provide to research centres and Europeans industries a concrete objective, both motivating and open to the development of many future technologies: Low thrust navigation, dynamics of the large structures, miniaturisation of equipment, programs for autonomous control... In new horizons for significant actions and for opening of 3rd millennium, ESA, in part sustained by CNES, plays a driving role to structure this approach with the manufacturers, the scientists and the other space agencies. T ODAY, THE PROJECTS THAT ANTICIPATE THE USE OF A SOLAR SAIL ARE MORE AND MORE CONCRETE; from now on the solar sail is included in the range of the propulsion systems envisioned for space projects. For some of them there is no alternative in the middle term. The WIGIWIND project first, AURORA and asteroids exploration, later, are coming out of the realm of science- fiction to become tangible reality. In these difficult economic times, one of the great advantages of a solar sail project is to be favoured by the public on which it will have a strong impact. The scientific interest and the usefulness of the missions maybe understood by all and the imaginary fiction of Arthur C. Clarke can make this projects fascinating for the European citizens. As ARIANE was in the beginning of the 80's, solar sails maybe a major asset for a renewal of the interest in solar system exploration. I N A DIFFICULT ECONOMIC ENVIRONMENT WHERE PARTNERS ARE OFTEN LOOKING FOR EACH OTHER, SOLAR SAIL CRAFT HAS A INCOMPARABLE PUBLIC IMAGE AND MAYBE A EXCELLENT FEDERATING PROJECT. A MBITIOUS SCIENTIFIC PROJECTS USING SOLAR SAILS HAVE GOOD CHANCES TO BE SOON VALIDATED, and the purpose of U3P will be reached. We can bet that a day will come where as for all new means of transport, stakes will become evident. Competitors will take over the solar sail and will take up big challenges to the M oon and beyond. They will find with strong and enthusiast supporters. YAKA A-T'IL RAISON? YAKA, c'est la manifestation spontanée de l'enthousiasme face à la virtualité de la propulsion photonique. Mais la prise de conscience de la complexité des systèmes à mettre en œuvre agit comme un frein. YAKA FAUKON ne résout rien, seules persévérance et rigueur peuvent conduire à un résultat. ADHESION U3P, CASSETTE ZNAMIA L'U3P -Union pour la Promotion de la Propulsion Photonique- est une association loi 1901 créée en L'U3P a joué un rôle essentiel dans l'évolution des projets de voiliers solaires. L'U3P est à l'origine du règlement officiel de la «Luna Cup» reconnu par l'iaf -Fédération Astronautique Internationale- en 1992 et elle joue un rôle moteur au sein du SSWG -Solar Sail Working Group de l'iaf-. Une AG annuelle permet de faire le bilan des activités de l'u3p et des avancées «photoniques» mondiales. Le bureau se réunit régulièrement pour faire le point sur les actions en cours -Congrès, Exposition, Science en Fête,...-. Le poste de Président est traditionnellement occupé par un ingénieur aérospatial mais les membres du bureau sont des passionnés qui viennent de tous les horizons. Toute personne intéressée peut adhérer soit pour travailler au sein d'une commission soit pour être tenue informée des activités et des avancées par l'intermédiaire d'un bulletin de liaison. Des documents vidéo sur Znamia 2 et des images de synthèses sur les voiles solaires et la Luna Cup sont disponibles à l'u3p. Les activités étant très variées (réalisation de documentation, recherche d'informations, participation aux congrès, expositions, conférences,...) les «rêveurs» y trouvent leur place autant que les «techniciens». U3P Secrétariat : 53, rue Amélie Vincendeau NAZELLES-NEGRON

18 Bibliography STARSAILING (Voilier de l'espace) Louis FRIEDMAN - Édition L'Étincelle SPACE SAILING (Naviguer à la voile dans l'espace) Jerome WRIGHT - Gordon et Breach Le vent venu du Soleil Arthur C.CLARKE - Presse Pocket La planète des singes Pierre Boulle - Presse Pocket L ESPACE À QUOI ÇA SERT? COMMENT ÇA MARCHE? cnes - sep - Palais de la Découverte Le système solaire Palais de la Découverte L'ESPACE milieu violent et hostile BT espace - cnes- pemf AÉROFRANCE SPECIAL ESPACE - N 76 Étapes et techniques de l'astronautique J-P OEHMICHEN - Les compacts - Bordas Les sciences du Ciel Flammarion Dictionnaire de l'espace Larousse La lumière Bernard MAITTE - Points Sciences Le grand atlas de l'espace Universalis AUTHORS Georges BALLINI Éric DUTET Alain PERRET Guy PIGNOLET Jean-Yves PRADO avec la collaboration de : Cyril BAZIN, Myriam BELMIHOUB, Alexandre BESTOUGEFF, Olivier BOISARD, Bernard CHARLES, Marie-Louise DUTET, Chantal MONTEILLET, Paul NICOLAS, Roland PAISANT, Carolyne WESLEY en remerciant particulièrement Georges BALLINI pour la recherche historique de la première partie ainsi que Vladimir SYROMIATNIKOV et Kirill SIMON pour les documents ZNAMIA et Louis FRIEDMAN dont le livre Starsailing reste la référence en matière de voile solaire ENGLISH TRANSLATORS Olivier BOISARD, François ESTEVE, René DUMONTET, Guy PIGNOLET ILLUSTRATIONS Jean-Michel MEYER ( Li An ) avec la collaboration de : Pierre-Louis MANGEARD du Cri du Margouillat (La Réunion) SCHÉMAS Olivier BOISARD Thomas DUTET Starsailing Louis FRIEDMAN Phillipe WILLERS RÉALISATION U3P Le Parc aux Etoiles Château de l Hautil TRIEL SUR SEINE Juillet 1997 mise à jour Décembre 2002 Cette plaquette accompagne l exposition : «La voile solaire : Comment ça marche? À quoi ça sert?»