Posted to the UseNet Newsgroup "alt.astronomy"

                     "TUG OF WAR"

Ref.:                _Asimov on Astronomy_

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

(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

   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

                                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

                                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--


(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,

 P.S. Thank YOU for reading!

    P.P.S. Some secret sites (shh)...