Date: Thu, 11 Oct 1990 01:29-EDT From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #72 Contents: Marc Ringuette Lagrange point L2 and earth warming Phil Fraering NASA's Commercial Experiment Carrier (query) ------------------------------------------------------------ Date: Wed, 3 Oct 1990 00:34-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Lagrange point L2 and earth warming A neat idea from sci.space. And yes, space-tech is still alive and well, if dormant. Feel free to start something up! --Marc From: pfennige@uni2a.unige.ch Subject: Lagrange point L2 and earth warming Date: 28 Sep 90 16:27:55 GMT Just an idea. Some people propose to circumvent the greenhouse effect by installing a lot of mirrors in deserts. This technical fix has many drawbacks, as each mirror would work less than half of time, due to earth rotation, and the atmosphere would keep a part of the reflected light. The needed surface would be of the order of a few percents of the earth surface. Wouldn't it be simpler to hide a few percents of the sun disk (as seen from earth) by one or several screens orbiting around the Earth-Sun Lagrangian point L2? One could imagine that cleverly shaped screens could be stabilized by radiation pressure, or that some rocketry would do the job. If L2 would be stable, it would even be more cost effective to put dust there. Dust reradiates incident light in far-infrared in all directions! As L2 is unstable, dust would diffuse away with a typical time-scale. Possibly it might be cheaper to send occasionally a few containers of dust there (from the moon?). Daniel Pfenniger, Geneva Observatory ------------------------------ From: pfennige@uni2a.unige.ch Subject: Re: Lagrange point L2 and earth warming Date: 1 Oct 90 09:28:50 GMT In article <15625@yunexus.YorkU.CA>, tony@nexus.yorku.ca (Tony Wallis) writes: > > Does the sun-earth L2 point really exist ? Wouldn't perturbations from > the moon or Venus muck it up ? > L2 exists in the same sense as geostationnary orbits exist. Both are good approximations to reality coming from an idealized model. For the Lagrangian points the model is called the "restricted three body problem", where two massive bodies are assumed to be on circular orbit around their common center of mass, and the orbit of the supposed massless third body is determined by Newton law. In this simplified problem, there are five stationnary points (the Lagrangian points) in the frame of reference rotating with the massive bodies. In reality one has to check if the model is good enough with respect to the actual perturbations as the Moon to preserve the Lagrangian points. A recent poster by Henri Spencer mentions that "halo orbits" around L2 exist, and that satellites are already there needing periodic ajustements for compensating perturbations. The same is required for geostationnary satellites. Daniel Pfenniger, Geneva Observatory ------------------------------ Date: Mon, 1 Oct 1990 00:45-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: pfennige@uni2a.unige.ch Subject: Re: Lagrange point L2 and earth warming I think there's a fatal problem with a sunblock at the L2 point. The sun is 1/2 degree in diameter. At the distance of the L2 point, a barrier would cast a cone-shaped penumbra that is very large, thus being a very inefficient way to block the sun. I suggest using thin lightsails much nearer to earth. They could be in low orbit, staying in front of the Earth half the time, and using their sailing capability to tack in orbit. Alternatively, use a static arrangement with two sails in equilibrium with Earth's gravity and solar pressure, which would be further from the Sun than the Earth, and one "sun blocking" sail in equilibrium in front of the Earth, like this: \ -->sunlight>------------------------------\ _/ \ | _/ |/ | (Earth) |\_ | \_ \ / --->sunlight>-----------------------------/ / --Marc ------------------------------ From: pfennige@uni2a.unige.ch Subject: Re: Lagrange point L2 and earth warming Date: 1 Oct 90 10:29:00 GMT >I think there's a fatal problem with a sunblock at the L2 point. The >sun is 1/2 degree in diameter. At the distance of the L2 point, a >barrier would cast a cone-shaped penumbra that is very large, thus being >a very inefficient way to block the sun. L2 is about 1.5 million km from earth. A dust particle at L2 would therefore (simple trigonometry) makes a penumbra disk about 13100 km wide, just slightly larger than the earth! So contrary to your statement, the sunblocking effect would be smoothly and efficiently distributed over the whole earth. A disk of diameter D would just produce a penumbra disk larger by D. For blocking 1% of light, a disk about one tenth of the earth diameter would be necessary, which is still a very large structure. Supposing this is made of a material 1 micron thick, this makes about one cubic meter of stuff, at most a few tons of mass. Daniel Pfenniger, Geneva Observatory ------------------------------ Date: Mon, 1 Oct 1990 14:13-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: PFENNIGE%uni2a.unige.ch@VMA.CC.CMU.EDU Subject: Re: Lagrange point L2 and earth warming Way cool! I forgot that L2 would be so close to the Earth. That works out great. However, the sunblock would have to balance the light pressure of the sun somehow. It might have to be more massive and closer to the sun than you suggest, if it is to use the sun's gravity to counteract the light pressure. I guess it might help if the sunblock merely deflected the sunlight by a degree in some direction. A fresnel lens, maybe? --Marc ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) Subject: Re: Lagrange point L2 and earth warming Date: 1 Oct 90 13:20:51 GMT Reply-To: dietz@cs.rochester.edu (Paul Dietz) >A disk of diameter D would just produce a penumbra disk larger by D. >For blocking 1% of light, a disk about one tenth of the earth diameter >would be necessary, which is still a very large structure. >Supposing this is made of a material 1 micron thick, this makes about >one cubic meter of stuff, at most a few tons of mass. You dropped a decimal point, David. 6.4e5 m^2 x pi x 1e-6 m is about 1.2e6 cubic meters. Light pressure would exert perhaps 6e6 newtons of force on the shade. If its mass is 3e9 kg, it experiences an acceleration of 2e-4 gees, about 3% of the acceleration due to the sun's gravity. If one could make the shade out of <~.1 micron thick aluminum, its mass would be about 3e5 tons, which, at $100/lb, would cost $60 B to put into space. This is small compared to the cost of abandoning fossil fuels. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Tue, 2 Oct 90 09:45 N From: PFENNIGE%uni2a.unige.ch@VMA.CC.CMU.EDU Subject: Re: Lagrange point L2 and earth warming To: Marc.Ringuette%DAISY.LEARNING.CS.CMU.EDU@VMA.CC.CMU.EDU >However, the sunblock would have to balance the light pressure of the >sun somehow. It might have to be more massive and closer to the sun >than you suggest, if it is to use the sun's gravity to counteract the >light pressure. My guess is that if radiation pressure is a small perturbation of gravitational forces, the shift toward the sun for compensating the radiation pressure will also be small. But you are right that a screen should be more massive than I wrongly estimated, the mass involved would be rather a few 100000 tons as indicated by dietz@cs.rochester.edu in a recent posting. >If light pressure is too much of a problem, I guess it might help if the >sunblock merely deflected the sunlight by a degree in some direction. A >fresnel lens, maybe? Yes a diffusive, slightly translucide material would disperse light over a wide solid angle, while a mirror absorbs the maximum momentum from the photons. But probably the shape should be as simple as possible, a big "lens" would be much more expensive than a rough material, while shifting the whole structure by a small fraction of 1e6 km sunwards shouldn't be very expensive. Daniel Pfenniger ------------------------------ [ end of excerpt from sci.space ] ------------------------------ Date: Fri, 5 Oct 90 15:07:42 -0500 From: Fraering Philip To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Cc: space-tech@CS.CMU.EDU Subject: NASA's Commercial Experiment Carrier Has anyone on the list seen any of the proposals for NASA's microgravity experiment carrier? Just curious, Phil Fraering dlbres10@pc.usl.edu ------------------------------ End of Space-tech Digest #72 *******************