# LaGrange 'effect' question

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#### ihwip

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Say we put up a gigantic shield to block out the sun in an attempt to cool a planet. To make it constantly in tandem with the sun/planet system it would need to be at the LaGrange point between them.<br /><br />The problem is that the sunlight would have a 'solar sail' effect and the solar wind would also produce a force. Would this make the sunshield's orbit unstable? I am thinking moving it closer to the sun just slightly would counterbalance any kind of constant force with minor adjustments needed occassionally to adjust for solar flares and other bursts.<br /><br />If I am thinking correctly the slightly smaller orbit would normally need to be faster but because the sun is giving it a slight boost it could be slow enough to follow the Earth. Would this work?

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#### brellis

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The side facing the sun could harness solar power to feed some small thrusters. <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

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#### Saiph

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eburacum45:<br /><br />Your "anchor" solution won't work. The first is that the orbital speed of an object is determined by where it's center of mass is, not where various parts of it are distributed. As such you would still have to park the center of mass on the LaGrange point in order to have a synchronous orbit. The solar sail effect would still operate on the mirror, and still push the satellite off the inherently unstable lagrange point.<br /><br /><br />Now, having an "anchor" would work to stabilize the satellites orientation since any rotation of the satellite would be countered by a restoring torque on the anchor. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### brellis

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What if the sail were in a 'halo orbit' around the L point that favored sunblocking angles? <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

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#### dragon04

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I would think that the effects of the Solar Wind would be directly proportional to the relative cross-section of our shield that was exposed to it.<br /><br />When we think about "sails" we picture a structure that is "concave" relative to the wind it catches. If we don't care about relative direction of travel or velocity, but only having a nice, shady spot, a sail that was 'convex" relative to the wind would serve the purpose just as well.<br /><br />It appears to me that the principle of a solar sail is identical to the sail of an ocean vessel. We catch wind and it moves us.<br /><br />Conversely, ocean vessels have a streamlined construction with a pointed bow to mitigate resistance to forward motion; we do our best to maximize the mechanical advantage we gain by filling our sails.<br /><br />This would apply in the opposite to a "shield" used for the purpose of cooling, I'd think.<br /><br />To keep it in a "fixed" L-position, we want to mitigate the "push" of the Solar wind. We need to make that "push" as minimal as practical, and then deal with gravitational influences through the use of thrusters.<br /><br />Unless of course, our Shield is made of Unobtainium which has sufficient mass to lock itself into a stable Lagrangian position. <img src="/images/icons/wink.gif" /><br /><br /><br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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#### tony873004

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The pressure from the Sun's light would not pose a problem.<br /><br />Think about what makes the L1 point special. Neglecting the solar wind, an object at the L1 point feels the pull of gravity from the Sun and a much weaker pull of gravity from the Earth in the opposite direction. When you add these force vectors together you get a force vector pointing to the Sun that is a little weaker then it would be without Earth's contribution. The effect is the same as if the object orbited a star slightly less massive than the Sun without being in an L1 point. So the effect is that it has an orbital velocity that is slightly slower than it would have around a star of 1 solar mass. This is what makes the L1 point special. Normally, an object orbiting the Sun interior to Earth has a shorter orbital period. But at the L1 point, the modified force vector causes its period to exactly match that of Earth's, which makes it stable on 2 of 3 axes: perpendicular to its orbital plane, and in the direction of its orbital motion. Correctional burns are needed to keep it stable along the Earth / Sun line.<br /><br />To account for the effect of solar light pressure, instead of adding 2 forces together, you would add 3 forces together: The Sun's gravity pulling one way, and the Earth's gravity and light pressure in the opposite direction. The result is a force vector pointing towards the Sun that is slightly weaker than it would be if the Earth were not there and the Sun were not shining. Find the point where an object in the influence of this weaker force would have the same angular velocity as the Earth. This would be a new L1 point custom for your sail. It would be slightly closer to the Sun than the traditional L1 point. But since light pressure is not very strong, it wouldn't be very far from the traditional L1 point.<br /><br />To find this point requires some math. There is no formula for computing the distance to the L1 point. The Hill Sphere formula gives a very accurat

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#### CalliArcale

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<blockquote><font class="small">In reply to:</font><hr /><p>The problem is that the sunlight would have a 'solar sail' effect and the solar wind would also produce a force. Would this make the sunshield's orbit unstable? I am thinking moving it closer to the sun just slightly would counterbalance any kind of constant force with minor adjustments needed occassionally to adjust for solar flares and other bursts.<p><hr /></p></p></blockquote><br /><br />Actually, the orbit will be unstable anyway -- L1 is not a stable point. Spacecraft situated in halo orbits around it need to perform regular maneuvers to correct their position, as they will tend to "roll" off of it.<br /><br />Whether this is more significant than the solar sail effect I do not know. However, I understand that only L4 and L5 are stable; the others require regular course corrections. (One exception: Genesis did not need to make regular course corrections during its time at L1. However, this was by design so that it would return automatically at the end of the collection phase of its mission.) <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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