Aucbvax.6165 fa.space utcsrgv!utzoo!decvax!ucbvax!space Fri Feb 12 04:07:37 1982 SPACE Digest V2 #105 >From OTA@S1-A Fri Feb 12 03:06:10 1982 SPACE Digest Volume 2 : Issue 105 Today's Topics: Mooning Around Lunar synchronous sites VP Bush's view on budget Eclipses in geostationary orbit ---------------------------------------------------------------------- Date: 11 February 1982 06:33-EST From: Robert Elton Maas Subject: Mooning Around To: ucbvax!decvax!watmath!pcmcgeer at UCB-C70 cc: SPACE at MIT-MC What makes you think a satellite in GEO is in the Earth's shadow half the time?? The shadow is only 8000miles in diameter (the umbra is slightly smaller, tapering to zero at four times lunar distance, the penumbra is slightly larger, tapering larger at the same rate, at geosync it's almost all umbra and rather close to 8000 miles in diameter), while the circumference of the orbit is 25,000 * 2 * PI which is about 150,000 miles. Thus the satellite spends only 8/150 of its time in Earth-shadow (about 75 minutes a day) on those days in March and September when the Sun and equator are nearly inline, less time in shadow when they aren't in line, zero when the sun is so far north or south of the equator that the far-night point of the orbit is more than 4000 miles from the center of the shadow. If major industries are willing to close down for an hour when the SPS is in shadow one satellite should do fine. If it's over the USA (actually over South America, with beam aimed at Arizona/NM), this time should be near local midnight when most industries are quite willing to shut down anyway. ------------------------------ Date: 11 February 1982 08:32-EST From: Hans P. Moravec Subject: Lunar synchronous sites To: SPACE at MIT-MC Lunar synchronous satellites are possible at L4 and L5, which are stable equilibria. They are also possible at L1 between the earth and the moon, 58,000 km from the moon's surface and at L2 on the lunar farside, 64,500 km from the farside. These are unstable equilibria and would require a small amount of station keeping (so is and does geosynch orbit). The station keeping can be done away with if you use a very thin thread that anchors the satellite to the lunar surface! This would be a very minimal lunar skyhook, made of a small amount of a conventional material like fiberglass. It would be just strong enough to exert the tiny force needed to keep the satellite from flying outwards when the satellite is placed a (relative) gnat's eyebrow farther from the lunar surface than L1 or L2. ref - Jerome Pearson, "Anchored Lunar Satellites for Cislunar Transportation and Communication" J. Astronautical Sciences ca. October 1977 (presented at the European Conf. on Space Settlements and Space Industries, London, 20 Sept. 1977) ------------------------------ Date: 11 February 1982 21:02-EST From: Robert Elton Maas Subject: VP Bush's view on budget To: SPACE at MIT-MC I got a reply to my postcard to VP Bush about space budget. He seems to think the shuttle is the primary thing to spend money on now because it's needed before we can build LEO station. I'll have to explain to him that we need to start designing LEO station now so we can start building it soon and have it ready to put up when the shuttle is operational. [My letter was actually signed, and presumably written, by William D. Eckert, Lt.Col USAF, Military assistant/aide to the VP.] Anybody want me to type the full 3.5 paragraphs of the letter and send it to SPACE? ------------------------------ Date: 11 Feb 1982 18:58:40-PST From: ihnss!karn at Berkeley To: ucbvax!space@Berkeley Subject: Eclipses in geostationary orbit This is a topic that is well understood in the communications satellite business. Solar eclipses for spacecraft in geostationary orbit occur only during the spring and fall seasons (equinoxes) when the sun appears to pass behind the earth. During the summer and winter seasons (for the Northern hemisphere), the sun appears to pass above the north and south poles at local midnight, respectively, and there is no eclipse. The longest eclipses occur at midnight on the equinoxes, where they are 72 minutes long. Eclipse durations on days preceeding and following equinoxes decrease to zero in about + or - 20 days. A common practice in communications satellite operations is to place them to the west of the areas they serve. This causes eclipses to occur after midnight in the service area, when the traffic has fallen to lower levels. A more serious problem for communications are sun transits, in which the sun passes behind the satellite. The radio noise from the sun blots out the satellite signal. Of course, this would not be a problem for the SPS; the sun would even contribute (very slightly) to the received microwave energy. Phil Karn Bell Labs Indian Hill ------------------------------ End of SPACE Digest ******************* ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.