I know I've been on the same thing for the past few weeks - about the possibilities of star travel - but it's a subject that keeps on giving. There are many things I want to say about this, but there are so many facts I've had to dedicate several columns to the subject, due to space considerations, as well as my enjoyment about discussing the possibilities involved in interstellar exploration - if it ever comes about.

This time, I'm going to tackle a question asked by many since it was determined that we are but one planetary system among many in this vast galaxy - namely if interstellar travel is possibly, why haven't we been visited by other civilizations from other solar systems who discovered it earlier?

This is a legitimate questions, and I'll address a number of the explanations that have been put forth from various sources through the years.

1. The reason we haven't been visited is because the speed of light really is the absolutely limit on how fast anything can travel in the universe. As a result our only possible contact would be restricted to worlds within a few light years of the earth, if any intelligent life exits out there. If this is the case, then we are stuck. We will have to content ourselves with living and surviving on earth, developing possible resources and living space from the moon, Mars, and other limited worlds, and possibly mounting expeditions to any habitable worlds that may be discovered within approximately 20 light years from earth.

2. We haven't been visited yet, because the civilizations which have discovered star travel are waiting until the human race is "ready" for such knowledge. There may be something to this. It's been demonstrated on several occasions that when a more advanced civilization on earth has had contact with a primitive one, the people of the less advanced civilization often go through a phenomenon known as "cultural shock." Some cultures have been severely damaged or even destroyed by this effect, and it could very well threaten the people of an entire world under certain circumstances.

3. Other alien races with star travel capabilities have looked us over, and decided they don't want to have anything to do with us. Well, I admit there's a possibility to this one, but I'll leave the arguments to the cynics of the world.

4. The solar system exists on a galactic "backwater" and as a result we haven't been discovered by any star-faring race yet. Again, a possibility. We have to remember that our galaxy, the Milky Way, is made up of billions and billions of stars, stretching out over hundreds of millions of light years. Even if faster-than-light travel is possible, its quite logical that other, more advanced civilizations, would be so far away, or we would be so far off the beaten path, that it will still be a while before we're discovered, or we discover them.

5. Intelligent life, capable of developing star travel, may be an extremely rare thing indeed in the universe. I have my own theory about this, and I'm very proud to share it here, because I have never seen this idea stated anywhere else, and like to think I'm the first one who has considered it.

It's true there are billions of stars out there, and no doubt hundreds of millions of planets. Of those hundreds of millions, it's a safe bet that there should millions of planets capable of supporting life as we know it. Yet, I'm certain that intelligent life - life capable of reaching out to the stars - is a very rare phenomenon. And I base my contention on the fact that the earth possesses something which I believe is a very rare item indeed on a cosmic scale. The moon.

Now, I know quite of few will say "Lots of planets have moons." True. But the earth is unique in the solar system in the size of its moon. Although two other moons - Ganymede at Jupiter, and Titan at Saturn - are bigger, if you consider the size of the moon in proportion to the earth, then we hold the record. In fact many scientists almost classify the earth and the moon as a double planet because of the size ratio.

And the odds for such thing occurring, I believe, is very small. It can happen, of course, as we so neatly demonstrate, but I get the feeling a situation like the earth and the moon is an astronomically small occurrence.

So how does this relate to the development of intelligent life? Well, scientists have demonstrated that planets continually shift as they spin on their axis. Observations have shown that these shifts cause tremendous climatic changes on the planetary surfaces, changes which could have an adverse effect on earth, if it weren't for the moon.

The ratio between the earth and the moon is so delicately balanced that the moon actually holds things in place, preventing all but the tiniest of shifts of the earth's axis. As a result, the dramatic and often violent climate changes associated with axis shifts typical of other planets in the solar system are much milder and almost unknown on the earth.

Although we are still investigating and exploring the causes of the evolutionary development of life, we do know that for such things to occur, very long periods of environmental stability are required. I'm firmly convinced that - without out the moon - those period of environmental stability would not have existed, and as a result higher orders of life would probably not have developed and the human race would not have come into existence.

So, while there are billions of stars in the galaxy, with no doubt millions of planets, the odds of a planet capable of supporting life having a satellite large enough to keep it very stable on its axis would be an extremely rare thing indeed. As a result, it's not impossible to contend we may be the first race in the galaxy capable developing the technology necessary to travel to the stars. If this is the case, then a realm of infinite possibilities await us, and I believe we should use every means at our disposal to reach for them.