http://wiki.solarsails.info/index.php?title=Solar_Sailing_101&feed=atom&action=historySolar Sailing 101 - Revision history2024-03-29T12:54:46ZRevision history for this page on the wikiMediaWiki 1.21.1http://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=4190&oldid=prevBen: /* What are Solar Sails? */2010-09-10T20:26:59Z<p><span dir="auto"><span class="autocomment">What are Solar Sails?</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:WSFsailcraft_Earth.png|thumb|Square sail with steering vanes near Earth]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:WSFsailcraft_Earth.png|thumb|Square sail with steering vanes near Earth]]</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror used to propel a spacecraft using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat is propelled by deflecting the wind, a solar sail is propelled by deflecting sunlight. Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air <del class="diffchange diffchange-inline">drag </del>and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by <del class="diffchange diffchange-inline">comet </del>is pushed into brilliant tails millions of kilometers long. <del class="diffchange diffchange-inline">Spacecraft accumulate </del>small errors <del class="diffchange diffchange-inline">due </del>to <del class="diffchange diffchange-inline">sunlight </del>in <del class="diffchange diffchange-inline">their </del>orbits and <del class="diffchange diffchange-inline">their </del>spin <del class="diffchange diffchange-inline">over long time periods</del>. Even asteroids gradually change their spin over millions of years. <del class="diffchange diffchange-inline">A solar sails can accelerate quickly </del>enough to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror used to propel a spacecraft using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat is propelled by deflecting the wind, a solar sail is propelled by deflecting sunlight. Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by <ins class="diffchange diffchange-inline">comets </ins>is pushed into brilliant tails millions of kilometers long. <ins class="diffchange diffchange-inline">Sunlight causes </ins>small errors to <ins class="diffchange diffchange-inline">accumulate over time </ins>in <ins class="diffchange diffchange-inline">spacecraft </ins>orbits and spin. Even asteroids gradually change their spin over millions of years. <ins class="diffchange diffchange-inline">This gentle force is </ins>enough <ins class="diffchange diffchange-inline">for a solar sail that is sufficiently large and light weight </ins>to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*Also see: [[Solar Sail Design]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*Also see: [[Solar Sail Design]]</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3848&oldid=prevBen at 15:16, 30 June 20092009-06-30T15:16:29Z<p></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*Also see: [[Tacking Solar Sails]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*Also see: [[Tacking Solar Sails]]</div></td></tr>
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</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3739&oldid=prevBen: /* Why Don't Solar Sails Use the Solar Wind? */2009-06-08T22:50:58Z<p><span dir="auto"><span class="autocomment">Why Don't Solar Sails Use the Solar Wind?</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>At Earth's distance from the sun, sunlight has an intensity of ~1368 Watts per square meter (Joules per second per square meter). So, if a square meter object absorbs all that sunlight, its momentum will change by (1368 J) / (299,792,458 m/s) = 4.563&times;10-6 kg m/s<sup>2</sup>. If the sail also reflects the sunlight, its momentum will change again by the same amount, so the total momentum change will be 9.126&times;10-6 kg m/s<sup>2</sup>. That corresponds to a pressure of 9.126&times;10-6 N/m<sup>2</sup> - about 9 micro-Newtons per square meter as stated previously.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>At Earth's distance from the sun, sunlight has an intensity of ~1368 Watts per square meter (Joules per second per square meter). So, if a square meter object absorbs all that sunlight, its momentum will change by (1368 J) / (299,792,458 m/s) = 4.563&times;10-6 kg m/s<sup>2</sup>. If the sail also reflects the sunlight, its momentum will change again by the same amount, so the total momentum change will be 9.126&times;10-6 kg m/s<sup>2</sup>. That corresponds to a pressure of 9.126&times;10-6 N/m<sup>2</sup> - about 9 micro-Newtons per square meter as stated previously.</div></td></tr>
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<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>===<del class="diffchange diffchange-inline">Why Don't Solar Sails Use the </del>Solar Wind<del class="diffchange diffchange-inline">?</del>===</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>===Solar Wind===</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. In fact, the solar wind can have 1000-10,000 times less force on a solar sail than sunlight. People worry about the solar wind for other reasons. Solar storms can hurt people and electronics in space. Increases in the solar wind can cause the upper atmosphere to swell and increase drag on low Earth orbit spacecraft. The solar wind can be used to push [http://en.wikipedia.org/wiki/Magnetic_sail magnetic sail] or plasma sail spacecraft.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. In fact, the solar wind can have 1000-10,000 times less force on a solar sail than sunlight. People worry about the solar wind for other reasons. Solar storms can hurt people and electronics in space. Increases in the solar wind can cause the upper atmosphere to swell and increase drag on low Earth orbit spacecraft. The solar wind can be used to push [http://en.wikipedia.org/wiki/Magnetic_sail magnetic sail] or plasma sail spacecraft.</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3738&oldid=prevBen: /* Introduction */2009-06-08T22:50:05Z<p><span dir="auto"><span class="autocomment">Introduction</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Hopefully by reading this document, you will be able to answer these questions <del class="diffchange diffchange-inline">when the topic of </del>solar sails <del class="diffchange diffchange-inline">comes up</del>:</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Hopefully by reading this document, you will be able to answer these questions <ins class="diffchange diffchange-inline">about </ins>solar sails:</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What is a solar sail?</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What is a solar sail?</div></td></tr>
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<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <del class="diffchange diffchange-inline">Why </del>does <del class="diffchange diffchange-inline">sunlight push </del>a solar <del class="diffchange diffchange-inline">and not the solar wind</del>?</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <ins class="diffchange diffchange-inline">How </ins>does a solar <ins class="diffchange diffchange-inline">sail travel through space</ins>?</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==What are Solar Sails?==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==What are Solar Sails?==</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3737&oldid=prevBen: /* What are Solar Sails? */2009-06-08T22:47:23Z<p><span dir="auto"><span class="autocomment">What are Solar Sails?</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror used to propel a spacecraft using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat is propelled by deflecting the wind, a solar sail is propelled by deflecting sunlight. Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air drag and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by comet is pushed into brilliant tails millions of kilometers long. Spacecraft accumulate small errors due to sunlight in their orbits and their spin over long time periods. Even asteroids gradually change their spin over millions of years. A solar sails can accelerate quickly enough to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror used to propel a spacecraft using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat is propelled by deflecting the wind, a solar sail is propelled by deflecting sunlight. Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air drag and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by comet is pushed into brilliant tails millions of kilometers long. Spacecraft accumulate small errors due to sunlight in their orbits and their spin over long time periods. Even asteroids gradually change their spin over millions of years. A solar sails can accelerate quickly enough to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">*Also see: [[Solar Sail Design]]</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3736&oldid=prevBen: /* Solar Sailing */2009-06-08T22:46:06Z<p><span dir="auto"><span class="autocomment">Solar Sailing</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. In fact, the solar wind can have 1000-10,000 times less force on a solar sail than sunlight. People worry about the solar wind for other reasons. Solar storms can hurt people and electronics in space. Increases in the solar wind can cause the upper atmosphere to swell and increase drag on low Earth orbit spacecraft. The solar wind can be used to push [http://en.wikipedia.org/wiki/Magnetic_sail magnetic sail] or plasma sail spacecraft.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. In fact, the solar wind can have 1000-10,000 times less force on a solar sail than sunlight. People worry about the solar wind for other reasons. Solar storms can hurt people and electronics in space. Increases in the solar wind can cause the upper atmosphere to swell and increase drag on low Earth orbit spacecraft. The solar wind can be used to push [http://en.wikipedia.org/wiki/Magnetic_sail magnetic sail] or plasma sail spacecraft.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>==<del class="diffchange diffchange-inline">Solar </del>Sailing==</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>==Sailing==</div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:Sail forces.png|thumb|Force components on a solar sail]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:Sail forces.png|thumb|Force components on a solar sail]]</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3735&oldid=prevBen: /* "Tacking" */2009-06-08T22:45:46Z<p><span dir="auto"><span class="autocomment">"Tacking"</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>When a sail is pointed directly at the sun, it experiences a force as described above - the sun pushes the sail directly away from it at a force twice that of an object that absorbs the sunlight. The reflected light generates a reaction force as it leaves the sail, much like the exhaust of a rocket. When the sail is tilted at an angle to the sunlight, two things happen: less sunlight is intercepted by the sail and the forces from absorbed and reflected sunlight are no longer in the same direction. As the sail is tilted, the total force will drop until it becomes zero when the sail is edge-on to the sun. The absorbed sunlight and a portion of the reflected sunlight will always push directly away from the sun (called the radial force), but when the sail is tilted, there will be a portion of the reflected light that pushes perpendicular or tangent to the sunlight. This force component, called the tangential force, can be used to maneuver in space.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>When a sail is pointed directly at the sun, it experiences a force as described above - the sun pushes the sail directly away from it at a force twice that of an object that absorbs the sunlight. The reflected light generates a reaction force as it leaves the sail, much like the exhaust of a rocket. When the sail is tilted at an angle to the sunlight, two things happen: less sunlight is intercepted by the sail and the forces from absorbed and reflected sunlight are no longer in the same direction. As the sail is tilted, the total force will drop until it becomes zero when the sail is edge-on to the sun. The absorbed sunlight and a portion of the reflected sunlight will always push directly away from the sun (called the radial force), but when the sail is tilted, there will be a portion of the reflected light that pushes perpendicular or tangent to the sunlight. This force component, called the tangential force, can be used to maneuver in space.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>===<del class="diffchange diffchange-inline">"Tacking"</del>===</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>===<ins class="diffchange diffchange-inline">Sailing away from or towards the sun</ins>===</div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:Sailorbtransfer.png|thumb|Solar force (blue arrow) can spiral sail</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:Sailorbtransfer.png|thumb|Solar force (blue arrow) can spiral sail</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3734&oldid=prevBen: /* Special Relativity */2009-06-08T22:44:37Z<p><span dir="auto"><span class="autocomment">Special Relativity</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Einstein's theory of Special Relativity relates the energy (E) of particles to their mass (m) and momentum (p = mass &times; speed) by the following equation where c is the speed of light (299,792,458 meters/second):</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Einstein's theory of Special Relativity relates the energy (E) of particles to their mass (m) and momentum (p = mass &times; speed) by the following equation where c is the speed of light (299,792,458 meters/second):</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><<del class="diffchange diffchange-inline">tex</del>><del class="diffchange diffchange-inline">E^</del>2 = m<del class="diffchange diffchange-inline">^</del>2 c<del class="diffchange diffchange-inline">^</del>4 + p<del class="diffchange diffchange-inline">^</del>2 c<del class="diffchange diffchange-inline">^</del>2</<del class="diffchange diffchange-inline">tex</del>></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">*''E</ins><<ins class="diffchange diffchange-inline">sup</ins>>2<ins class="diffchange diffchange-inline"></sup> </ins>= m<ins class="diffchange diffchange-inline"><sup></ins>2<ins class="diffchange diffchange-inline"></sup> </ins>c<ins class="diffchange diffchange-inline"><sup></ins>4<ins class="diffchange diffchange-inline"></sup> </ins>+ p<ins class="diffchange diffchange-inline"><sup></ins>2<ins class="diffchange diffchange-inline"></sup> </ins>c<ins class="diffchange diffchange-inline"><sup></ins>2</<ins class="diffchange diffchange-inline">sup</ins>><ins class="diffchange diffchange-inline">''</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>If the particles have no momentum, then it reduces to the familiar E = m c<sup>2</sup>. Photons (light particles) have no mass, but they do have energy that is observed when sunlight warms your skin or generates power in a solar array. The equation becomes ''E = p c'', which can be rearranged as ''p = E/c''. Therefore, if you know how much energy there is in the sunlight absorbed and reflected by a solar sail over a period of time, you can calculate the momentum transferred to it. To find the change in speed, divide the momentum by the mass of the spacecraft.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>If the particles have no momentum, then it reduces to the familiar <ins class="diffchange diffchange-inline">''</ins>E = m c<sup>2</sup><ins class="diffchange diffchange-inline">''</ins>. Photons (light particles) have no mass, but they do have energy that is observed when sunlight warms your skin or generates power in a solar array. The equation becomes ''E = p c'', which can be rearranged as ''p = E / c''. Therefore, if you know how much energy there is in the sunlight absorbed and reflected by a solar sail over a period of time, you can calculate the momentum transferred to it. To find the change in speed, divide the momentum by the mass of the spacecraft.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>At Earth's distance from the sun, sunlight has an intensity of ~1368 Watts per square meter (Joules per second per square meter). So, if a square meter object absorbs all that sunlight, its momentum will change by (1368 J) / (299,792,458 m/s) = 4.563&times;10-6 kg m/s<sup>2</sup>. If the sail also reflects the sunlight, its momentum will change again by the same amount, so the total momentum change will be 9.126&times;10-6 kg m/s<sup>2</sup>. That corresponds to a pressure of 9.126&times;10-6 N/m<sup>2</sup> - about 9 micro-Newtons per square meter as stated previously.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>At Earth's distance from the sun, sunlight has an intensity of ~1368 Watts per square meter (Joules per second per square meter). So, if a square meter object absorbs all that sunlight, its momentum will change by (1368 J) / (299,792,458 m/s) = 4.563&times;10-6 kg m/s<sup>2</sup>. If the sail also reflects the sunlight, its momentum will change again by the same amount, so the total momentum change will be 9.126&times;10-6 kg m/s<sup>2</sup>. That corresponds to a pressure of 9.126&times;10-6 N/m<sup>2</sup> - about 9 micro-Newtons per square meter as stated previously.</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3733&oldid=prevBen: /* How Does Light Push a Solar Sail? */ modified wording2009-06-08T22:41:39Z<p><span dir="auto"><span class="autocomment">How Does Light Push a Solar Sail?: </span> modified wording</span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">Light pressure was predicted in the late 1800s by James Clerk Maxwell using electromagnetism and Adolfo Bartoli using thermodynamics. It was first measured in 1900 by Peter Lebedev and 1901 by Ernest Nichols and Gordon Hull. Before then, the technology didn't exist to measure such a gentle force. Today, quantum mechanics and relativity can also be used to predict light pressure, and mission controllers deal every day with the subtle influence sunlight has on spacecraft.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>===Electromagnetism===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>===Electromagnetism===</div></td></tr>
<tr><td colspan="2" class="diff-lineno">Line 19:</td>
<td colspan="2" class="diff-lineno">Line 21:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:EmreflectBW.png|thumb|Electric and magnetic field components of sunlight exerting force on a sail]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:EmreflectBW.png|thumb|Electric and magnetic field components of sunlight exerting force on a sail]]</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Before anyone ever took a beam of light and measured how much it could push, there were predictions that light could exert a very gentle push on objects it hits. James Clerk Maxwell developed </del>the <del class="diffchange diffchange-inline">laws describing electromagnetism and concluded that light is </del>an electromagnetic wave. <del class="diffchange diffchange-inline">Maxwell predicted that when light hits an object </del>and is absorbed <del class="diffchange diffchange-inline">or reflected</del>, the <del class="diffchange diffchange-inline">light wave pushes on </del>electric <del class="diffchange diffchange-inline">charges </del>in the surface of the <del class="diffchange diffchange-inline">object, which in turn push on </del>the <del class="diffchange diffchange-inline">rest </del>of the <del class="diffchange diffchange-inline">object</del>. If the light is reflected<del class="diffchange diffchange-inline">, </del>the object <del class="diffchange diffchange-inline">gets pushed twice as hard</del>, <del class="diffchange diffchange-inline">just as if you would </del>be <del class="diffchange diffchange-inline">pushed twice </del>as <del class="diffchange diffchange-inline">hard by </del>a <del class="diffchange diffchange-inline">rubber ball hitting you as a ball </del>of <del class="diffchange diffchange-inline">clay. In 1901</del>-<del class="diffchange diffchange-inline">1903</del>, the <del class="diffchange diffchange-inline">Americans Nichols and Hull and Russian Lebedev were able to measure light pressure </del>as <del class="diffchange diffchange-inline">predicted by Maxwell</del>. <del class="diffchange diffchange-inline">Find a physics text that discussed electromagnetism</del>, <del class="diffchange diffchange-inline">like Physics, by Halliday, Resnick, and Krane, to see how </del>the force <del class="diffchange diffchange-inline">is derived from Maxwell's equations</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">The electromagnetic explanation begins with </ins>the <ins class="diffchange diffchange-inline">description of sunlight as </ins>an electromagnetic wave. <ins class="diffchange diffchange-inline">An oscillating electric field causes a perpendicular magnetic field to form, </ins>and <ins class="diffchange diffchange-inline">vice versa. Together, this self-maintaining electromagnetic wave can travel through space, only losing energy or changing the frequency of oscillation by interaction with solid matter. When the wave </ins>is absorbed <ins class="diffchange diffchange-inline">by a solid object</ins>, <ins class="diffchange diffchange-inline">electric field theory predicts that </ins>the electric <ins class="diffchange diffchange-inline">field will cause an electric current to flow </ins>in the surface of the <ins class="diffchange diffchange-inline">material. Magnetic field theory predicts that </ins>the <ins class="diffchange diffchange-inline">magnetic component </ins>of the <ins class="diffchange diffchange-inline">light wave will exert a pressure into any material with a current flowing through it - just as a magnet pulls on a wire with a current</ins>. If the light <ins class="diffchange diffchange-inline">wave </ins>is reflected <ins class="diffchange diffchange-inline">from </ins>the object, <ins class="diffchange diffchange-inline">the electric and magnetic fields will </ins>be <ins class="diffchange diffchange-inline">in opposite directions </ins>as <ins class="diffchange diffchange-inline">the originals, resulting in </ins>a <ins class="diffchange diffchange-inline">force opposite the direction </ins>of <ins class="diffchange diffchange-inline">the wave </ins>- <ins class="diffchange diffchange-inline">that is</ins>, the <ins class="diffchange diffchange-inline">same direction </ins>as <ins class="diffchange diffchange-inline">the force from the original light wave</ins>. <ins class="diffchange diffchange-inline">Thus</ins>, <ins class="diffchange diffchange-inline">a mirrored object will experience twice </ins>the force <ins class="diffchange diffchange-inline">of one that absorbs all the light</ins>.</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div> </div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">===Einstein===</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">When Einstein developed his theories of relativity, and gave us the equation <tex>E </del>= <del class="diffchange diffchange-inline">m c^2</tex>, it allowed us to calculate light pressure a lot easier. <tex>E </del>= <del class="diffchange diffchange-inline">m c^2</tex> compares energy, which can be easily measured in light, to mass and movement, which can easily be used to find forces.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>==<ins class="diffchange diffchange-inline">=Special Relativity===</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">* E is an amount </del>of energy</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Einstein's theory </ins>of <ins class="diffchange diffchange-inline">Special Relativity relates the </ins>energy <ins class="diffchange diffchange-inline">(E) </ins>of <ins class="diffchange diffchange-inline">particles to their </ins>mass <ins class="diffchange diffchange-inline">(m) and momentum (p = mass &times; speed) by the following equation where </ins>c is the speed of light <ins class="diffchange diffchange-inline">(299,792</ins>,<ins class="diffchange diffchange-inline">458 </ins>meters<ins class="diffchange diffchange-inline">/</ins>second<ins class="diffchange diffchange-inline">):</ins></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">* m is an amount </del>of mass</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">* </del>c is the speed of light, <del class="diffchange diffchange-inline">about 300 million </del>meters <del class="diffchange diffchange-inline">per </del>second</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">If you fiddle around with </del><tex>E = m c^2</tex><del class="diffchange diffchange-inline">, you find that the force light exerts is the power of the sunlight divided by the speed of light. Like I stated above, you get twice as much force from an object that reflects all the light as you do from an object that absorbs all the light. In order to get this simple formula, force equals power divided by speed of light, the steps taken by Maxwell and others had to be taken first.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><tex>E<ins class="diffchange diffchange-inline">^2 </ins>= m<ins class="diffchange diffchange-inline">^2 c^4 + p^2 </ins>c^2</tex></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>===<del class="diffchange diffchange-inline">Very</del>, <del class="diffchange diffchange-inline">Very</del>, <del class="diffchange diffchange-inline">Gentle Force===</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">If the particles have no momentum, then it reduces to the familiar E </ins>= <ins class="diffchange diffchange-inline">m c<sup>2</sup>. Photons (light particles) have no mass, but they do have energy that is observed when sunlight warms your skin or generates power in a solar array. The equation becomes ''E </ins>= <ins class="diffchange diffchange-inline">p c'', which can be rearranged as ''p </ins>= <ins class="diffchange diffchange-inline">E/c''. Therefore</ins>, <ins class="diffchange diffchange-inline">if you know how much energy there is in the sunlight absorbed and reflected by a solar sail over a period of time</ins>, <ins class="diffchange diffchange-inline">you can calculate the momentum transferred to it. To find the change in speed, divide the momentum by the mass of the spacecraft.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Sunlight exerts a very gentle force. The power of sunlight in space at </del>Earth's distance from the sun <del class="diffchange diffchange-inline">is between 1.3-1.4 kilowatts </del>per square meter. <del class="diffchange diffchange-inline">When you divide 1.4 kilowatts </del>by <del class="diffchange diffchange-inline">the speed of light</del>, <del class="diffchange diffchange-inline">about 300 million meters per second</del>, <del class="diffchange diffchange-inline">the result is very small</del>. <del class="diffchange diffchange-inline">A square mirror 1 kilometer on a side would only feel about 9 Newtons or </del>2 <del class="diffchange diffchange-inline">pounds of force</del>. <del class="diffchange diffchange-inline">Fortunately</del>, <del class="diffchange diffchange-inline">space is very empty and clean compared to Earth</del>, so <del class="diffchange diffchange-inline">there is plenty of room for a 1 kilometer wide sail to maneuver, and there is no noticeable friction to interfere with your </del>9 <del class="diffchange diffchange-inline">Newtons of thrust</del>. <del class="diffchange diffchange-inline">A sailboat on Earth wouldn't be going anywhere with that little force because </del>of <del class="diffchange diffchange-inline">drag from the water and air</del>. <del class="diffchange diffchange-inline">Some rockets can push millions of </del>times <del class="diffchange diffchange-inline">harder, but the sail keeps pulling so long </del>as <del class="diffchange diffchange-inline">light shines on it. Months or years after the rocket runs out of fuel, the sail is still pulling</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">At </ins>Earth's distance from the sun<ins class="diffchange diffchange-inline">, sunlight has an intensity of ~1368 Watts per square meter (Joules per second </ins>per square meter<ins class="diffchange diffchange-inline">)</ins>. <ins class="diffchange diffchange-inline">So, if a square meter object absorbs all that sunlight, its momentum will change </ins>by <ins class="diffchange diffchange-inline">(1368 J) / (299</ins>,<ins class="diffchange diffchange-inline">792</ins>,<ins class="diffchange diffchange-inline">458 m/s) = 4</ins>.<ins class="diffchange diffchange-inline">563&times;10-6 kg m/s<sup></ins>2<ins class="diffchange diffchange-inline"></sup></ins>. <ins class="diffchange diffchange-inline">If the sail also reflects the sunlight</ins>, <ins class="diffchange diffchange-inline">its momentum will change again by the same amount</ins>, so <ins class="diffchange diffchange-inline">the total momentum change will be </ins>9.<ins class="diffchange diffchange-inline">126&times;10-6 kg m/s<sup>2</sup>. That corresponds to a pressure </ins>of <ins class="diffchange diffchange-inline">9</ins>.<ins class="diffchange diffchange-inline">126&</ins>times<ins class="diffchange diffchange-inline">;10-6 N/m<sup>2</sup> - about 9 micro-Newtons per square meter </ins>as <ins class="diffchange diffchange-inline">stated previously</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>===Why Don't Solar Sails Use the Solar Wind?===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>===Why Don't Solar Sails Use the Solar Wind?===</div></td></tr>
</table>Benhttp://wiki.solarsails.info/index.php?title=Solar_Sailing_101&diff=3732&oldid=prevBen: /* What is a Solar Sail? */ modified wording2009-06-08T22:28:43Z<p><span dir="auto"><span class="autocomment">What is a Solar Sail?: </span> modified wording</span></p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 22:28, 8 June 2009</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Why does sunlight push a solar and not the solar wind?</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Why does sunlight push a solar and not the solar wind?</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>==What <del class="diffchange diffchange-inline">is a </del>Solar <del class="diffchange diffchange-inline">Sail</del>?==</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>==What <ins class="diffchange diffchange-inline">are </ins>Solar <ins class="diffchange diffchange-inline">Sails</ins>?==</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:WSFsailcraft_Earth.png|thumb|Square sail with steering vanes near Earth]]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>[[Image:WSFsailcraft_Earth.png|thumb|Square sail with steering vanes near Earth]]</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror <del class="diffchange diffchange-inline">attached </del>to a spacecraft <del class="diffchange diffchange-inline">that </del>is <del class="diffchange diffchange-inline">pushed through space </del>by the pressure of <del class="diffchange diffchange-inline">light reflecting off </del>of <del class="diffchange diffchange-inline">it. Look here for [[Solar Sail Design]]. The light to push </del>a <del class="diffchange diffchange-inline">sail can come </del>from the sun <del class="diffchange diffchange-inline">or large lasers</del>. <del class="diffchange diffchange-inline">Satellites in orbit around the Earth can survive for many years without any maintenance while using only a </del>little <del class="diffchange diffchange-inline">bit </del>of <del class="diffchange diffchange-inline">rocket propellant </del>to <del class="diffchange diffchange-inline">hold their positions</del>. <del class="diffchange diffchange-inline">Solar sails </del>can <del class="diffchange diffchange-inline">be made </del>to <del class="diffchange diffchange-inline">survive </del>in <del class="diffchange diffchange-inline">space for many </del>years <del class="diffchange diffchange-inline">as well</del>. <del class="diffchange diffchange-inline">Solar </del>sails <del class="diffchange diffchange-inline">use sunlight that never runs out like rocket propellant. During those years in space, a sail </del>can <del class="diffchange diffchange-inline">move around as much as you want it </del>to<del class="diffchange diffchange-inline">, such as from Earth to Mars and back, possibly several times if </del>the <del class="diffchange diffchange-inline">sail remains in good condition. A similarly equipped rocket would either be ridiculously huge because it has to carry the fuel for all the trips, </del>or <del class="diffchange diffchange-inline">would need to be refueled regularly</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A solar sail is a large, lightweight mirror <ins class="diffchange diffchange-inline">used </ins>to <ins class="diffchange diffchange-inline">propel </ins>a spacecraft <ins class="diffchange diffchange-inline">using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat </ins>is <ins class="diffchange diffchange-inline">propelled </ins>by <ins class="diffchange diffchange-inline">deflecting </ins>the <ins class="diffchange diffchange-inline">wind, a solar sail is propelled by deflecting sunlight. Solar </ins>pressure <ins class="diffchange diffchange-inline">is very weak - about 9 millionths </ins>of <ins class="diffchange diffchange-inline">a Newton (micro-Newtons) or 2 millionths </ins>of a <ins class="diffchange diffchange-inline">pound (micro-pounds) of force on a square meter at Earth's distance </ins>from the sun. <ins class="diffchange diffchange-inline">This is far too </ins>little <ins class="diffchange diffchange-inline">pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air drag and objects fall freely under the influence </ins>of <ins class="diffchange diffchange-inline">gravity without the ground </ins>to <ins class="diffchange diffchange-inline">constrain them</ins>. <ins class="diffchange diffchange-inline">Sunlight </ins>can <ins class="diffchange diffchange-inline">have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by comet is pushed into brilliant tails millions of kilometers long. Spacecraft accumulate small errors due </ins>to <ins class="diffchange diffchange-inline">sunlight </ins>in <ins class="diffchange diffchange-inline">their orbits and their spin over long time periods. Even asteroids gradually change their spin over millions of </ins>years. <ins class="diffchange diffchange-inline">A solar </ins>sails can <ins class="diffchange diffchange-inline">accelerate quickly enough </ins>to <ins class="diffchange diffchange-inline">travel between planets or change </ins>the <ins class="diffchange diffchange-inline">behavior of its orbit around a planet </ins>or <ins class="diffchange diffchange-inline">the sun - without consuming any propellant</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==How Does Light Push a Solar Sail?==</div></td></tr>
</table>Ben