A solar veil or photovoile is a device of propulsion using the Pression of radiation emitted by the stars to move in space with the manner of a sailing ship.

Taking into account the weak generated propulsion, the process does not make it possible to leave the surface of a planet (even stripped of atmosphere, and thus of friction). It is on the other hand usable on an apparatus having already reached the Speed of orbiting, even the Escape velocity.

General principle

The solar sail is propelled by the pressure produced by the Photon S which come to strike it. More the sail is large and reflective, larger is the force of propulsion. One then can by inclining the sail or while acting on his aerofoil, to modify the surface offered to the light and thus to proportion the balance of the forces for thus to control the sail. With the manner of a sailing boat, using the force of water and the wind, an space engine with solar veil, can use the gravitational force and forces it of photonic push to sail in space.

The principal interest lies in the absence of fuel for a vehicle provided with such a device. That makes it possible to consider a very great autonomy of displacement in the Solar system.

Action of the pressure of radiation on the solar sail

Impulse of an “average” photon

An animated massive body a speed compared to a reference mark has in this one a Quantité of movement which is the product $m\; \backslash \; mathbf\; \{v\}$. A particle without mass (as a Photon) has a characteristic being expressed in the same unit and which names the impulse : the report/ratio of sound energy on its celerity.

For reasons of conservation, the particle absorption by a target generates a retreat of this one according to the formula:

$p\; =\; \backslash \; frac\; \{H\; \backslash \; naked\}\; \{C\}\; =\; \backslash \; frac\; \{H\}\; \{\backslash \; lambda\}$

with

$h$: Constant of Planck 6,6e-34 J.s

$\backslash \; nu$: Frequency (in hertz)

$c$: the Speed of light 3e8 m.s^-1

$\backslash \; lambda$: the Wavelength (in m)

To simplify calculations, let us suppose that the Sun does not emit a mixing wavelengths, but just one of 0.5 µm.

• the impulse of each photon will be thus close to 10^-27 kg.m.s^-1
• and its E_p energy = 3.96x10^-19 Joule, is 2.5 eV.

Macroscopic use

The Sun emits a total power of E_s = 3,9x10²⁶ W. That represents E_s/E_p = 10⁴⁵ photons a second.

On terrestrial orbit, these photons are distributed on a sphere of 150 million kilometers of ray, that is to say a surface of 3.10²³ m ².

That represents a density of 10⁴⁵/3.10²³ is 3.10²¹photons/m ².

One multiplies by the impulse of each one to obtain the maximum impulse which can collect 1 m ² of solar veil on terrestrial orbit, that is to say 5x10^-6 kg.m.s^-1 per m ².

To accelerate 1 kg of 1 m.s^-2, one thus needs at least a surface of 120 113.5 m ², that is to say a square of 346 × 346 m, the mass of this sail being to deduce to obtain the payload . A sail of this size using less than 1 kg would break immediately on Earth under any draft, but this problem does not exist obviously with this width in space.

Importance of the solar wind

One can also think of the Solar wind (flow of electrically charged particles, dispatched out of the Sun) like contribution to the phenomenon. Here a small calculation to fix the ideas.

The characteristic of the solar wind on the level of the Earth are (see on this subject):

Speed of the particles approximately 500 km/s = 5.10⁵ m/s

Density approximately 10/cm³, is per cubic meter: 10⁷ particles.

In one second and out of 1 m ², it thus arrives NR = 5.10⁵ X 10⁷ = 5.10¹² particles.

The composition is variable, but contains primarily electrons and atoms of ionized hydrogen (i.e. Proton S). The mass of each particle is thus with the ladle: m = 2.10^-27 kg.

If they rebound completely on the sail, each one will transmit to the veil an impulse equalizes with the double of his, that is to say p = 2 m.v = 2.10^-21 kg.m/s.

The acceleration undergone by 1 m ² is thus: = NR X has p/1 S = 10^-8 kg.m/s ²

All this is a high estimate. If one compares with the radiative pressure found above (that is to say 5.10^-6), the solar wind contributes as well as possible only to 0,2% of the total. And in fact, if one holds of the account of the proportion of electrons (at sight of nose, half), plus the round-offs, the weak reflection of the ions, which rather will be encrusted in the sail, one could at least divide by 4 this order of magnitude, that is to say with final contribution of about 0,05%, to say as much negligible.

On the other hand, one can plan to use the magnetic field created by particles electrically charged to be propelled, using a magnetic veil.

Design of solar veils

It is included/understood since the surface offered by the sail is a paramount characteristic in the performances of this means of propulsion (a sail of 220.000 m ² is necessary to obtain a push of 1 kg.m.s^-1). The difficulty will be then to transport this sail at the time of the placing in orbits of these machines, to deploy it and direct it in the space vacuum.

The various shapes of veils are found:

• square sails solid and easy to direct but complex to deploy and less powerful because offering less usable surface to the solar rays.
• the round veils deployed by rotation movement are easier to transport but very complex to direct.
• the veils héliogyres made up of blades fixed around a central axis easier to deploy and direct but less rigid and thus more fragile.

The quality of the aerofoil is also important. It must be solid and light and to have a maximum capacity of reflection of the light. When designing means of propulsion for the future satellite of study of the Halley's Comet in 1973, the solar sail was designed in Polymère Mylar and of Krypton of only some micrometers thickness.

History

• 1616 : Effects of the light on the trajectories of the celestial bodies observed by Johannes Kepler starting from the orientation of the tails of comets contrary to the Sun.
• 1873 : Work of James Clerk Maxwell on electromagnetism allowing to explain this phenomenon.
• 1889: Faure and Graffigny, novelists French of science fiction, imagine a spaceship using an immense mirror to collect the pressure of solar light.
• 1901 : Pyotr Lebedev highlights the pressure of radiations
• 1915: Work of Yakov Pereelmann in Russia.
• 1924 : Work of Fridrikh Tsander, in the USSR, states the idea to use mirrors for the flights in interplanetary space “For the flights in interplanetary space, I work on the idea to use formidable made mirrors of sheets extremely thin and able to obtain interesting results”
• 1955: Hermann Oberth develops the concept of mirror of space being used to collect the solar radiations
• 1958: Invention of the Solar term “ Sailing ” by Richard Garwin, in the March issue of of Jet Propulsion
• 1970: The effect of the solar pressure is used for the orientation of the probe Mariner 10 during its flight towards Mercure.
• 1973 : NASA and ESA engage of work on the solar sailing ships in order to join the Halley's Comet in 1986 (the project is abandoned in 1977 because the development of this technology is too slow)
• 2005: June 21st, launching of Cosmos 1, the first space engine with solar veil: it is a failure at the time of the placing in orbit.
• 2007 Japan considers a probe propelled by solar veil, and studies with the ISAS (Institute off Space and Astronautical Science, which launched a call for offer) the scientific material which could be carried by this probe, which should be launched by a Japanese rocket H-IIA with the satellite PLANET-C which must explore Venus (launching envisaged about 2010).

See too

• magnetic Veil
• Solar energy

The solar sail is also a topic of Science-fiction:

• the Sailing ships of the sun (1961), novel of Gerard Klein
• the Planet of the monkeys (1963), novel of Pierre Swell
• Dust in the eye of God (1974), novel of Larry Niven and Jerry Pournelle
• the Butterfly of the stars (2005), novel of Bernard Werber

External bonds

• Animation of a solar sail, on the site of NASA (QuickTime format)

• the solar sail: Cosmos-1
• U3P (Union for the Promotion of the Photonic Propulsion: race the Ground-Moon, veil Dragonfly-Dragonfly, topicality,…)