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Moons of the Giants

One of the great joys of astronomy is that whatever you learned will soon be replaced by a something better yet. Nowhere has this been more spectacular than what we are learning about the moons of Jupiter and Saturn. There are more of them than we have been able to count yet! I remember that not so long ago I would confidently announce that Jupiter had such and such a number of discovered moons. All I had to do was keep up with the reported number in the International Astronomical Union's database once in a while. Now this number changes almost weekly. Dozens of moons became scores of moons and now Jupiter is moving towards perhaps hundreds of moons. These range up to fairly large bodies, greater in size than an earthly mountain. They follow Jupiter and Saturn in bands of bodies moving in the same direction in what vaguely is similar to an asteroid field.

For Jupiter, there are two very distinct belts, both far outside the orbits of the great Galilean moons we see in our telescope. The inner of these two belts turns in the same direction as Jupiter spins and the outer belt spins in the opposite direction. Detecting these moons is fairly simple if you happen to have a Hubble Space Telescope and nearly impossible if you don't. Basically you pick an area near Jupiter but not so near that you are looking at the planet. You then spin the telescope so that it tracks at the same speed as Jupiter which is a little different than the speed of the stars. After a while you get a picture where the stars leave little trails but a few star like objects won't. The point like objects, the new moons of Jupiter, dragged along by their great neighbor don't have their images smeared. After tracking such objects for a number of weeks, we can establish their orbit about Jupiter and the astronomer has the right to claim a new find. Much the same conditions apply to Saturn but Saturn's much greater distance from the Sun makes each of Saturn's distant moons harder to detect at all. In all likelihood, Neptune and Uranus may have some distant unseen moons.

How these moons ever managed to become attached to their host planets is a major source of confusion to astronomers right now. The obvious answer is that they are probably asteroids (or possibly comets) [a "body"] captured by the host planet. Unfortunately that simple answer leaves a great mystery. When a body falls from an "infinite" distance towards a planet, it arrives at the planet traveling with enough velocity to swing away from the planet. This is called the escape velocity. However, although bodies do not fall infinitely far, they always have an initial velocity (from traveling around the Sun) which more than makes up the difference. These bodies swing past host planets with far more velocity than the planet's gravity can cancel and so the escape.

In order for a host planet to capture a body as a new moon, somehow the excess velocity must be shed. We know of only three mechanisms which will allow this velocity to be shed. (1) A retro rocket can be fired. (2) The body can swing by a great existing moon in a complex orbit called a slingshot, trading some of its excess velocity by actually speeding the great moon slightly. (3) The body can dive into the atmosphere of the host planet converting velocity to an incandescent fireball.

Natural bodies lack retro rocket or their equivalents. Slingshot orbits are so exquisitely precise than they are the most complex orbits in the solar systems. Slingshot orbits cannot happen accidentally. The atmospheres of planets are so close to the bulk of the planets that any natural body attempting to shed velocity by diving into the planet's outer layer will almost certainly suffer the fate of Shoemaker-Levy 9 - death in fiery balls. Only carefully planned space shots can shed velocity by diving into atmospheres. In any case, all these result in a moon close to the host planet, not one in a very distant belt.

The only method which seems to make sense is to assume that in the early solar system, that the atmospheres of the planets extended tens of millions of miles outwards, rather than their current tens of thousands. The large atmosphere would allow bodies to hit the air without hitting the planets bulk. Jupiter seen from Earth would have been a fuzzy ball larger and brighter than the Moon. The more distant gas giants would have been bright comet like bodies easily seen with ours eyes. What a spectacular sight this would have been.

Leslie Coleman
Leslie Coleman
Entry Date:
Jan 1, 2004
Published Under:
Leslie Coleman's Columns
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