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A few words on Solar Sails ...

Like the ships which are pushed by the wind, large solar sails will some day in the future deploy in Space. With their thin reflective material, they will react to the weak thrust, a very weak thrust indeed, given by sunlight to whatever object it encounters. This mode of propulsion, using an everlasting source of energy, will enable the sailcraft to set course through Space beyond the Moon and further into the Solar System, without limits...

Half-way between the dreams of today and the realities of tomorrow,
solar sails are now ready to set off, ready to fly through Space...

Become a member of the solar sail network today !
All your questions, remarks, information are welcome ! ...

Photonic Propulsion

How to travel into Space with the only energy from sunlight ?.. the idea, which may seem strange at first, is not a joke. As early as the 17th century, Kepler has noticed that the tail of a comet did not spread along the direction of motion as expected, but in the direction opposite to the incoming sunbeams. He guessed that the fact could be explained by the effect of a pressure of the light on the particles escaping from the comet. We had to wait until the end of the 19th century and the work of Maxwell in the field of electromagnetism, to see the hypothesis confirmed by mathematical and physical theory.

Today, we know that at the distance of the Earth, the light of the Sun provides a thrust of 9 micro-newton (the weight of a coin ...) for each square meter of sail, i.e. the same force as the mere weight of a 900 gram mass for each square kilometre. This could appear negligible, but it is not when applied in Space, where there is no resistance, to thin structures with a good surface to mass ratio. In the best cases, photonic propulsion will not provide a better acceleration than a few mm/s2. However, unlike in the case of conventional propulsion with chemical propellants, the thrust will last as long as desired, with no limits in time. Quick calculations demonstrate that a solar sail in Earth orbit could reach escape velocity within a resonable period of time and then set for interplanetary travel.

Sailing in Space

The first serious considerations about photonic propulsion came around in Russia in 1915 with the work of Yakov Pelerman, and then, a few years later, in the 20's, with Fridrick Tsander, probably inspired by the writings of Space pioneer Konstantin Tsiolkovsky (see the short history of photonic propulsion). During the 20th century both engineers and writers were interested by the concept, and the expression "solar sail" eventually was coined by the American engineer Garvin in a scientific paper published in1958. In 1963, Arthur C. Clarke invented a solar sail race from the Earth to the Moon in his short essay "The Wind from the Sun". But as early of the mid-fifties the French writer Pierre Boulle had introduced solar sails in his famous book "The Planet of the Apes". Unfortunately the film ignored them...

The first operational opportunity for solar sails comes in 1973, when NASA is considering a rendezvous of a spacecraft with the Halley comet, and a research program on solar sailing is initiated. For many reasons, the project is dropped in the late 70's, but several scientists and technicians from the Jet Propulsion Laboratory decide to keep the spirit alive as a private venture, and create a non-profit association, the World Space Foundation, with a goal to prove the technical feasibility of solar sailing.

In 1981, in Europe, several Space professionals with interest in solar sails join together to create the Union pour la Promotion de la Propulsion Photonique (U3P). Originating mostly from ONERA and from CNES, the French Space Agency, their goal, as an amateur group, is similar to the goal of the World Space Foundation, with an additional new idea of racing several solar sails together in a race between the Earth and the Moon .

The idea of solar sailing is more alive than ever, moving at the speed of light on the Web. Some future day, soon, a solar sail will set off into Space !...

Technology Challenge

Photonic propulsion has already been investigated in many research projects, sometimes in co-operation with major universities and several thesis works have been conducted on the subject of solar sailing trajectories between the Earth and the Moon( see References). The range is wider than in conventional Space studies, since original and unexpected solar sail specific subjects may often pop up :

  1. ultra-light structure analysis, with physical and mechanical characteristics unknown on the surface of the Earth : some concepts envision sails with dimensions in the order of several kilometres, with a thickness of only a few micrometers.

  2. material engineering, for the manufacture of thin aluminium-coated plastic films, or light and strong booms, made of composite materials, to stiffen the structure of the sailcraft.

  3. or in totally different field, origami, the Japanese art of folding paper, in order to obtain the optimal packing and unpacking of the sail, which has to be confined in a small container until the time of deployment in orbit.

  4. and many others..

Use of Photonic Propulsion

Indirectly, solar sail studies may lead to many technical applications. We should mention for instance the folding method developed by Koryo Miura from Japan, which is commercially applied for a convenient and original way of presenting road maps.

In the field of Space, photonic propulsion has many applications :

  1. attitude control for satellites : to prevent the drifting of artificial satellites from their nominal orbits, frequent corrections must be applied, usually with small jet thrusters with a limited propellant supply. Solar sail panels can perform the same corrections without using expandables. A similar method was used in 1970 to adjust the trajectory of a probe around the planet Mercury.

  2. interplanetary travel of Space probes : solar sails offer a great method to travel through the solar system. NASA had considered solar sails for the Halley comet, today we can think of the exploration of asteroids or remote objects as Pluto.

  3. fast transportation of low mass payloads : a miniature probe with a mass of only a few hundred grams associated with an ultra-thin solar sail of a few hundred meters in diameter could reach the orbit of Jupiter in a matter of a few weeks, to reach in emergency a far away manned Space colony...