SPACE PHYSICS Written project : Sailing in space

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1 Labrousse November, 2009 Pierre SPACE PHYSICS Written project : Sailing in space I) Introduction Human beings have always been looking for adventure and that's why they have never stopped to explore the world around them. In order to do so, they have imagined many ways to travel faster and further (from taming horses to flying a spacecraft). In this report, i would like to speak about one of the oldest way to travel that has been used, so as to know : sailing. Contrarily to cars, trains and airplanes that have been developed since the 19 th century, we know that human beings have traveled on water for almost 5000 years... So we can assume that we have a quite good knowledge of sailing! Nowadays people don't talk anymore about discovering new lands on Earth but rather new planets, new solar systems in the wide universe above us. We are looking much more forward and thus we have to invent new ways to travel beyond the unknown. Over the past few years, scientists (and SF writers as well) have begun to consider the following facts : we know that there is no proper vacuum in space but a very small particles density, and we also know that there is a solar wind blowing from the sun. Considering these facts and what we know about sailing, it's time to consider the idea of «sailing in space». In the first hand I will present the main concepts concerning physics of sailing and, on the second hand, I will talk more longer about what's going on in space.

2 II) Physics of sailing In order to understand physics of sailing, we have to introduce a few basic concepts. To make it simple, we can say that sailing is mainly due to the fact that the wind will apply pressure on the sail (we will also quickly talk about the role of water). First let us do a short recall: Pressure is a fundamental notion in physics. If we consider an elementary surface ds (with a normal vector n) subjected to a force F, we can define the pressure P as: F.n = PdS In the case where the force is perpendicular to the surface, this relation can be rewritten: P = F /S We see that the pressure is always perpendicular to the surface it is applying on. Now what about sailing? When a ship is moving forward in the same direction than the incident wind, we are in the situation where the induced force is perpendicular to the sail. The pressure is thus given by F /S. But it is known in sailing that this configuration is not the best option so we should rather consider the case where there is a definite angle between the motion of the ship and the direction of the wind. Another thing that we have to take into account when we are talking about fluid mechanics (here we are dealing with air) is about velocities.

3 Let us consider the following configuration: We can see that the air have two possible paths when it encounters the sail. The air passing on the right-hand side of the sail may have a higher speed relative to the boat than does the air on the lefthand side. This gives a lower pressure on the right-hand side, pulling the sail in that direction (this is a simple explanation of Bernoulli's equation) We can summarize the situation as it follows: And using Newton s laws we can easily understand how the ship is able to move forward thanks to the action of the wind on the sail (and using a rudder of course but here we focus on the wind and won t go into details concerning what happens with the water). Finally we get the following force pattern: One more thing I would like to talk about is called the apparent wind. It is the wind as it appears to the sailor on a moving ship. It differs in speed and direction from the true wind that is experienced by a stationary observer. Therefore the apparent wind can also be defined as a vector sum: the velocity of the wind plus the headwind velocity (note that the headwind velocity is just the opposite of the ship velocity). So we have the following configuration (from above) :

4 One question we could ask ourselves is : why doesn't the boat drift sideway? Well, it does! But when it happens, the keel (a large nearly flat area under the boat) has to push a lot of water sideways. The water resists and exerts an opposite force F k on the keel that cancels the sideway component of F w. III) Sailing in space Now let us talk about space physics! We know that space is not filled with ether as it was believed in the past and that it cannot be described as proper vacuum. Actually there is a small (but not negligible) density of particles blowing in a so called solar wind, and also photons that are very interesting particles (even if they don't have a rest mass, but that is another story!). So the idea that many scientists have worked on for some time now is the following : what about sailing the particles traveling into space in the same way? Notice that even if we could think that the particles forming the solar wind (such as electrons or protons for example) can also contribute to the thrust, their contribution would be very small and can be neglected. Nevertheless, we could think about using the magnetic field created by these charged particles if we were dealing with a magnetic sail. But here we will more longer talk about the solar sail. If we consider Einstein's relation : E = p c, we can see that photons do have a momentum. Thus when light is reflected from a surface, it apply a certain amount of what is called radiation pressure. In 1924, a Russian space engineer named F. Zander proposed that, since light can provides a small amount of thrust, it could be used as a form of space propulsion that requires no fuel! If the size of the reflecting surface is large enough, this thrust could provide a significant acceleration and, over time, lead to considerable speed. Likewise a ship using the strength of both water and wind, a sailing space-vehicle could use the gravitational force and the photonic thrust to travel into space.

5 III.1)Testing solar sails Over the past, a few experiments have been lead by several agencies. NASA has successfully tested small solar sails in vacuum chambers but it has never be used in space as primary propulsion system. Some other space missions (Mariner 10, Messenger for example) demonstrated the use of solar pressure as a method of attitude control in order to conserve attitudecontrol propellant. But until now, no space mission have succeded in using a solar sail as a real propulsion device. The following table summarizes what have been done in the past : Time February, 2003 August, 2004 June, 2005 Mission leading agency(ies) RFSA (Russian Federal Space Agency) ISAS (Institute of Space and Astronautical Science, Japan) Planetary society, Cosmos studios and Russian Academy of Science (private project) Description of the mission - Znamya 2 : 20-meter wide aluminized-mylar reflector, tested from the Russian Mir space station. - Znamya 2.5 Two different sails deployed from a sounding rocket (at different altitudes). Both used 7,5 μm thick film. Cosmos 1 : rocket launched from a submarine (Barents sea) See picture below. February, 2006 Japanese space mission A 15 meters diameter solar sail was launched with Astro F on a MV rocket August 2008 NASA (Space flight center and Research center) Nanosail-D : made of aluminum and plastic, about 100 square feet (9,3m 2 ) of light catching surface. Results - Successful deployment test but the experiment didn t demonstrate propulsion. - Failed to deploy properly. Pure test of the deployment mechanism, not of propulsion. The spacecraft didn't reach its orbit (a solar sail would have gradually raise the spacecraft to a higher earth orbit). It deployed from the stage but opened incompletely Lost in launch failure. This mission was supposed to test deployment technologies. Similar missions are supposed to take place in the year(s) to come. For example : on the 75th anniversary of astronomer Carl Sagan's birth, the Planetary Society announced their plans to sail a spacecraft on sunlight alone by the end of Called LightSail, the project will launch three separate spacecraft over the course of several years, beginning with LightSail-1, which will demonstrate that sunlight alone can propel a spacecraft in Earth orbit. LightSails 2 and 3, will travel farther into space.

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7 III.2) Materials The most common material in current designs is aluminized 2 μm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side. The sails of Cosmos 1 were made of aluminized PET film. In 2000, Energy Science Laboratories developed a new carbon fiber material which might be useful for solar sails. The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same mass. The rigidity and durability of this material could make solar sails that are significantly stronger than plastic films. The material could self-deploy and should withstand higher temperatures. There has been some theoretical speculation about using molecular manufacturing techniques to create advanced, strong, hyper-light sail material (based on nanotube mesh weaves). Such materials could mass less than 0.1 g/m², making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick Mylar sail material mass 7 g/m², aluminized Kapton films have a mass as much as 12 g/m², and Energy Science Laboratories' new carbon fiber material masses 3 g/m². III.3) Applications Beam propelled : Most theoretical studies of interstellar missions with a solar sail plan to push the sail with a very large laser beam-powered propulsion direct impulse beam. The thrust vector (spatial vector) would therefore be away from the Sun and toward the target. In theory a lightsail driven by a laser or other beam from Earth can be used to decelerate a spacecraft approaching a distant star or planet, by detaching part of the sail and using it to focus the beam on the forward-facing surface of the rest of the sail. Escaping planetary orbit : Almost all missions would use the sail to change orbit, rather than thrusting directly away from a planet or the sun. The sail is rotated slowly as the sail orbits around a planet so the thrust is in the direction of the orbital movement to move to a higher orbit or against it to move to a lower orbit. When an orbit is far enough away from a planet, the sail then begins similar maneuvers in orbit around the sun.

8 IV) Conclusion Solar sails still don't work very well in low Earth orbit below about 800 km altitude due to erosion or air drag. Above that altitude they give very small accelerations that take months to build up to useful speeds. They also have to be physically large, and payload size is often small. Moreover deploying them is still highly challenging to accomplish. Another problem is that solar sails have to face the sun to decelerate. Therefore, on trips away from the sun, they must arrange to loop behind the outer planet, and decelerate into the sunlight. There is also a common misunderstanding that solar sails cannot go towards their light source. This is false. In particular, sails can go toward the sun by thrusting against their orbital motion. This reduces the energy of their orbit, spiraling the sail toward the sun. Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. However, when in Earth orbit, the majority of mass on most interplanetary missions is taken up by fuel. A robotic solar sail could therefore multiply an interplanetary payload by several times by reducing this significant fuel mass, and create a reusable, multimission spacecraft. Most near-term planetary missions involve robotic exploration craft, in which the directness of the course is unimportant compared to the fuel mass savings and fast transit times of a solar sail. We could also believe that solar sails capture energy primarily from the solar wind high speed charged particles emitted from the sun. These particles would impart a small amount of momentum upon striking the sail, but this effect would be small compared to the force due to radiation pressure from light reflected from the sail. The force due to light pressure is about 5,000 times as strong as that due to solar wind. A much larger type of sail called a Magsail would employ the solar wind. Despite the losses of Cosmos 1 and NanoSail-D (which were due to failure of their launchers), scientists and engineers around the world remain encouraged and continue to work on solar sails. While most direct applications created so far intend to use the sails as inexpensive modes of cargo transport, some scientists are investigating the possibility of using solar sails as a means of transporting humans. This goal is strongly related to the management of very large (i.e. well above 1 km²) surfaces in space and the sail making advancements. Thus, in the near/medium term, solar sail propulsion is aimed chiefly at accomplishing a very high number of non-crewed missions in any part of the solar system and beyond.