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-----------------------------1900 - Moons in our Solar System
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- The Earth has only one Moon. Mars has two, Phobos and Diemos. Mercury and Venus, our two inner planets, have none. So, there are only 3 moons orbiting the four rocky planets.
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- It is a whole different story for the 4 outer gaseous planets and beyond:
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---------------------------- Jupiter -- 67 known moons.
---------------------------- Saturn -- 53 known moons.
---------------------------- Uranus -- 27 known moons.
---------------------------- Neptune -- 14 known moons.
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---------------------------- TOTAL -- 161
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- Pluto has 5 moons.
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- So the definition of an orbiting moon needs more refinement. A definition adopted in 2006 has the count for the Solar System:
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----------------------------- TOTAL -- 173 know moons
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- If you include the Dwarf Planets, Pluto and beyond the count is:
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---------------------------- TOTAL -- 182 known moons.
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- It does not stop there:
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-------------------------- 76 known satellites around the asteroids in the Asteroid Belt.
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------------------------- 4 Trojan satellites sharing Jupiter’s orbit.
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-------------------------- 39 near -Earth objects with 2 satellites each.
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-------------------------- 14 Mars crossers
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------------------------- 84 Neptune crossers
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------------------------ 150 small moonlets in Saturn’s rings.
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- You count up all the known satellites orbiting other objects in the Solar System and the count becomes:
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--------------------------- 545 known satellites.
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- There are only 3 natural satellites in the Inner Solar System around the Rocky Planets. Theory has it that Mercury and Venus had moons in the past but they have long ago impacted on the surface.
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------------------------- Our Moon is 3,474 miles in diameter
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------------------------- 27.3% the size of the Earth. That is over one forth the size.
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------------------------ 1.23% the mass of the Earth. The Moon is monthly Earth’s crust.
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- What determines whether a moon will eventually crash into the planet or escape into free space? It is called the “ Roche Limit”.
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- If the moon’s orbit takes longer than a day the planet’s rotation slows down and the moon drifts further away to compensate due to the Conservation of Angular Momentum of the system.
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- If the moon’s orbit is faster than the planet’s rotation then the opposite occurs. The moon makes the planet rotate more quickly and the moon drifts closer to compensate. It will eventually impact the surface of the planet.
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- Our Moon has the first scenario. It is slowly drifting away from Earth at a rate of ½ inch per year.
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- Shortly after the Moon’s formation, 4.3 billion years ago, it was much closer and the Earth was spinning much faster. A day on the Earth was only 6 hours. And, the Moon’s orbit took only 17 days.
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- The Earth’s constant gravity pull eventually stopped the Moon’s rotation. Now, only one face always faces the Earth. And, the Moon’s constant gravity pull has been slowing the Earth’s rotation. To maintain the total angular momentum of the system the Moon has been slowly drifting away to compensate.
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- Today the Moon’s average distance is 238,855 miles. It is in a slightly elliptical orbit with its closest at 225,309 miles and its farthest at 251,904 miles. It takes a little over one second for reflected moonlight to reach the Earth. ( That is also the delay in radio communications).
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- If the Moon’s orbit actually gets close enough to reach the “ Roche Limit” the planet’s gravity will tear the body apart. It all depends on several factors: mass, size, density, composition. Our Earth - Moon limit is calculated to be 59,000 miles. So, we are safe from a Moon impact.
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- At 59,000 mile orbit the Earth’s gravity overwhelmingly over powers the gravity holding the Moon together near-side to far-side. At that point the Moon would turn into a ring of material orbiting the Earth. ( Like the rings of Saturn ). Eventually that orbiting material would crash into the Earth’s atmosphere and some to the surface. That would be one enormous meteor shower, and meteorites peppering the Earth’s surface.
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- For the average comet the Roche Limit is 11,185 miles orbiting Earth. That comet if orbiting the Sun would be torn apart at 808,000 miles.
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- Mars will experience the first moon landing with its moon Phobos. Phobos is orbiting more quickly that a Martian day. Phobos keeps drifting closer and closer to the planet. In a few million years it will be torn into a ring and all the pieces will begin crashing in to the surface.
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- Why don’t our own bonds of our body’s atoms get torn apart ? We are certainly inside the Roche Limit. What is holding us together? It is the electromagnetic forces of the atoms that are much, much stronger that the force of gravity.
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- Now, the same scenario would not work if out body approaches the gravity of a Blackhole. The intense gravity there below the Roche Limit would pull our body apart like a string of spaghetti.
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- All the moons in the Solar System are at least beyond the Roche Limit. Otherwise, they would have already broken up on the surface of their planet.
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- See Review #1901 to learn more about the many moons in our Solar System.
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----- 707-536-3272 ---------------- Tuesday, August 23, 2016 -----
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