keithcurtis Posted January 2, 2004 Report Share Posted January 2, 2004 Originally posted by Fuzzy Gnome Prior to this thread, I never heard of Mars's moons being retrograde. Does anybody have any sources that say they are? Actually, this was news to me, too. McCoy sounded pretty authoritative, so I skimmed past it Doing some research, I think I see where the assumption might have come from. Phobos is below synchronous orbit, meaning it will eventually impace the surface. This is the fate of retrograde moons. Our own moon, being prograde, is gradually getting farther away. But it is already beyond geosynch. Keith "Fear and Terror" Curtis Quote Link to comment Share on other sites More sharing options...
Dr. Anomaly Posted January 3, 2004 Report Share Posted January 3, 2004 Originally posted by McCoy No. The center of mass of the elevator needs to be standing still relative to the ground, geosyncronous orbit, or Clarke orbit (after Sir Arthur C Clarke, who was the first to point out that a geosyncronous orbit would be a useful place to put a communications sattelite). From that center of mass you build up and down at the same time. The length from Clarke orbit to the ground is fixed, the upper limb can be longer or shorter, as long as it has the same mass. Rather than acored to the ground and being under tension, the entire length of the elevator is in orbit. I know who the "Clark Orbit" is named after. As for the rest of your post... (shrug) I dunno. It's been a while since I've done any serious reading on the subject, but I seem to recall all the most modern "serious" proposals being about a ballast asteroid positioned such that it had slightly higher than what orbital speed should be for its height to keep the cable under tension, and the cable being under tension is what kept the rock stationary above the same spot on the Earth's surface, when it's speed would want to have it fly off (move into a higher orbit). I could easily be wrong, though. Now you're going to make me go find references this weekend. Darn the luck! Darn it! [/Huntsman] Quote Link to comment Share on other sites More sharing options...
McCoy Posted January 3, 2004 Report Share Posted January 3, 2004 Originally posted by Fuzzy Gnome Prior to this thread, I never heard of Mars's moons being retrograde. Does anybody have any sources that say they are? Humm, my first source was Clarke, the climax of his first novel (The Sands of Mars) occured as Phobos rose in the West. I also seem to remember an Analog article that refers to the retrograde orbits of Phobos and Deimos as evidence that they did not form with the planet, but are relatively recently captured asteroids. But right now I can't find anything on the web that says if their orbits are prograde or retrograde. Did find that both are irregullary shaped, their long axies point toward Mars, and that their orbits are within a few degrees of the Martian equator. And that Phobos is near the Roche limit, and may (within the next 50 million years) break up and briefly form a ring around the planet. But it's also one thirty in the morning here, I'll see if I can find anything tomorrow. Quote Link to comment Share on other sites More sharing options...
Dr. Anomaly Posted January 3, 2004 Report Share Posted January 3, 2004 Originally posted by McCoy But right now I can't find anything on the web that says if their orbits are prograde or retrograde. Actually, it appears they are not retrograde after all...see this and this. Quote Link to comment Share on other sites More sharing options...
McCoy Posted January 3, 2004 Report Share Posted January 3, 2004 Looks like I was wrong. Phobos does indeed rise in the West as seen from the Martian surface, but this is an artifact of having an orbital period less than the planets rotational period. Phobos rises in the West, and sets in the East five hours later. Deimos, OTOH, rises in the East, and sets in the West some 60 hours later. Phobos crosses the sky twice a day, Deimos is up two and a half days at a time. http://www.msss.com/mars_images/moc/11_1_99_phobos/ Quote Link to comment Share on other sites More sharing options...
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