|
Posted to the UseNet Newsgroup "alt.astronomy" "TUG OF WAR" Ref.: _Asimov on Astronomy_ http://tinyurl.com/bxdgs At the above reference you'll find one of several hundred books by Isaac Asimov. This particular book contains his essay titled "Just Mooning Around". This is where you will find several of these reasons explained by him. And it's also where you can find more detail on this final reason for your seeing the Moon as "planet Selene", a full-fledged major planet in its own right... The Moon's orbit is far too distant from Earth to be a true satellite! (Copyright law allows us to reprint brief excerpts of works without infringing upon the rights of the author. This post is designed more to "wet your whistle" and get you even more interested and in love with astronomy. I'm just touching on the highlights of this idea, so let me suggest that you read "Just Mooning Around". That way, you can get all of Asimov's details about this reason.) In this essay, Asimov came up with a fascinating idea. He reasoned that since the Sun is much more massive than the rest of our Solar System (everything... all the planets, and satellites, planetoids, comets, meteoric matter, all of it put together is only 1/750th the mass of the Sun), then the Sun must be in a "tug of war" with each planet that has satellites. The Sun must pull considerably on those moons, but just *how much* is "considerably"? To find out, Asimov came up with what he called the "Tug- of-War value". He used the mass of the Sun and the mass of each planet calculated with the distance of each satellite from its planet, and from the Sun. It is obvious that the Sun would win the tug of war when only mass is figured, but since the satellites are so much closer to their primary planets than they are to the Sun, we can expect the Sun to ultimately lose out in the tug of war. So... let's check it out. We'll look at satellites of Neptune first, Triton and Nereid. Here is a table of Tug-of-War values... PLANET NEPTUNE SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Triton 8,400 Nereid 34 So Neptune's hold on Triton is very strong, but it pulls on Nereid only 34 times as strongly as the Sun does. Keep in mind that these are average figures. Nereid's orbit about Neptune is highly eccentric. It comes as close as 800,000 miles at one end of its orbit, but at the other end it gets as far away from Neptune as 6 million miles! So when Nereid is that far away from its primary, the Tug-of-War value is only 11. Astronomers feel that Nereid may only be a temporary moon of Neptune. Perhaps the Sun's influence might help snatch it away one day. Asimov figured the Tug-of-War value for several other satellites, and i've listed a few of them here... PLANET URANUS SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Miranda 24,600 Ariel 9,850 Umbriel 4,750 Titania 1,750 Oberon 1,050 We can see that all of Uranus' moons are securely tied to their orbits. PLANET SATURN SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Janus 23,000 Rhea 2,000 Titan 380 Iapetus 45 Phoebe 3.5 As massive as Saturn is, and as far away from the Sun as Phoebe is, we can see that the Sun still doesn't lose this tug of war by much. Saturn's pull on Phoebe is only 3½ times stronger than the Sun's. PLANET JUPITER SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Amaltheia 18,200 Europa 1,260 Callisto 160 X 4.3 XII 1.3 VIII 1.03 Jupiter's grip on its outer satellites is feeble indeed. PLANET MARS SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Phobos 195 Deimos 32 Even Mars, much smaller than Earth, holds on to its moons and wins the tug-of-war. Mars' hold on Deimos is 32 times stronger than the Sun's pull on Deimos. From here, Asimov talks about two types of satellites... "true" satellites and "captured" satellites. The true ones are believed to have formed along with their primary planets way back when the Solar System was very young. And the captured ones were, well, captured. At some point in the history of our Solar System, they were passing near a planet and were caught in the huge gravitational field. And they became part of the planet's satellite system. Up to now, i've left out the Tug-of-War value where our Moon is concerned. You'll see why in a few moments. And now i'd like to show you an excerpt from Asimov's "Just Mooning Around". Keep in mind that while some astronomers think that Mars' moons, Phobos and Deimos, might be captured asteroids, they are presently listed officially as "true" satellites. Asimov writes... Among the true satellites the lowest Tug-of-War value is that of Deimos, 32. On the other hand, among the satellites listed as captured, the highest Tug-of-War value is that of Nereid with an average of 34. Let us accept this state of affairs and assume that the Tug-of-War figure 30 is a reasonable minimum for a true satellite and that any satellite with a lower figure is, in all likelihood, a captured and probably temporary member of the planet's family. Knowing the mass of a planet and its distance from the Sun, we can calculate the distance from the planet's center at which this Tug-of-War value will be found. That will be the maximum distance at which we can expect to find a true satellite. (Note that Asimov left planet Pluto out of all this, because very little detail was known about Pluto at the time he wrote it.) We can also set a minimum distance at which we can expect a true satellite; or, at least, a true satellite in the usual form. It has been calculated that if a true satellite is closer to its primary than a certain distance, tidal forces will break it up into fragments. Conversely, if fragments already exist at such a distance, they will not coalesce into a single body. This limit of distance is called the "Roche limit" and is named for the astronomer Édouard Albert Roche (1820 - 1883), who worked it out in 1849. The Roche limit is a distance from a planetary center equal to 2.44 times the planet's radius. So, sparing you the actual calculations, here are the results for the four outer planets: DISTANCE OF TRUE SATELLITE (MILES FROM THE CENTER OF THE PRIMARY) MAXIMUM MINIMUM PLANET (TUG-OF-WAR=30) (ROCHE LIMIT) ---------- ---------------------- ------------------ Neptune 3,700,000 38,000 Uranus 2,200,000 39,000 Saturn 2,700,000 87,000 Jupiter 2,700,000 106,000 As you see, each of these outer planets, with huge masses and far distant from the competing Sun, has ample room for large and complicated satellite systems within these generous limits, and the 22 true satellites all fall within them. Next we can try to do the same thing for the inner planets. Since the inner planets are, one and all, much less massive than the outer ones and much closer to the competing Sun, we might guess that the range of distances open to true satellite formation would be more limited, and we would be right. Here are the actual figures as I have calculated them: DISTANCE OF TRUE SATELLITE (MILES FROM THE CENTER OF THE PRIMARY) MAXIMUM MINIMUM PLANET (TUG-OF-WAR=30) (ROCHE LIMIT) ---------- ---------------------- ------------------ Mars 15,000 5,150 Earth 29,000 9,600 Venus 19,000 9,200 Mercury 1,300 3,800 Thus, you see, where each of the outer planets has a range of two million miles or more within which true satellites could form, the situation is far more restricted for the inner planets. Mars and Venus have a permissible range of but 10,000 miles. Earth does a little better, with 20,000 miles. Mercury is the most interesting case. The maximum distance at which it can expect to form a natural satellite against the overwhelming competition of the nearby Sun is well within the Roche limit. It follows from that, if my reasoning is correct, that Mercury CANNOT have a true satellite, and that anything more than a possible spattering of gravel is not to be expected. Venus, Earth, and Mars are better off than Mercury and do have a little room for true satellites beyond the Roche limit. It is not much room, however, and the chances of gathering enough material over so small a volume of space to make anything but a very tiny satellite is minute. And, as it happens, neither Venus nor Earth has any satellite at all (barring possible minute chunks of gravel) within the indicated limits, and Mars has two small satellites, each less than 20 miles across, which scarcely deserve the name. It is amazing, and very gratifying to me, to note how all this makes such delightful sense, and how well I can reason out the details of the satellite systems of the various planets. It is such a shame that one small thing remains unaccounted for; one trifling thing I have ignored so far, but-- WHAT IN BLAZES IS OUR OWN MOON DOING WAY OUT THERE? (Note... Selene's, the Moon's, mean distance from Earth is 239,000 miles. This is fully 210,000 miles more distant than Asimov's maximum Tug-of-War distance .) Again writes Isaac Asimov... It's too far out to be a true satellite of the Earth, if we go by my beautiful chain of reasoning--which is too beautiful for me to abandon. It's too big to have been captured by the Earth. The chances of such a capture having been effected and the Moon then having taken up a nearly circular orbit about the Earth are too small to make such an eventuality credible. There are theories, of course, to the effect that the Moon was once much closer to the Earth (within my permitted limits for a true satellite) and then gradually moved away as a result of tidal action. Well, I have an objection to that. If the Moon were a true satellite that originally had circled Earth at a distance of, say, 20,000 miles, it would almost certainly be orbiting in the plane of Earth's equator and it isn't. But, then, if the Moon is neither a true satellite of the Earth nor a captured one, what is it? This may surprise you, but I have an answer; and to explain what that answer is, let's get back to my Tug-of-War determinations. There is, after all, one satellite for which I have not calculated it, and that is our Moon. We'll do that now: SATELLITE TUG-OF-WAR VALUE ------------- -------------------------- Moon 0.46 The Moon, in other words, is unique among the satellites of the Solar System in that its primary (Earth) LOSES the tug of war with the Sun. The Sun attracts the Moon a little bit more than TWICE as strongly as Earth does. We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth. To be sure, from within the Earth-Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always "falling toward" the Sun. All the other satellites, without exception, "fall away" from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets--but not the Moon. No sirreee! Not Earth's sister planet, Selene! happy days and... starry starry nights! -- Indelibly yours, Paine P.S. Thank YOU for reading! P.P.S. Some secret sites (shh)... http://painellsworth.net http://savethechildren.org http://eBook-eDen.secretsgolden.com |