In fact, under mutual gravitational attraction, two particles attracted towards one another will approach in orbits described by one of the conic sections (circle, ellipse, parabola, hyperbola or straight line); they will interact, and then continue on their way without touching. As two masses accelerate towards one another, they will convert their potential energy to kinetic, but the sum of the two will always be a constant. If the final kinetic energy exceeds that due to escape velocity, the two will approach in hyperbolic orbits. If it is not enough, you get an ellipse. Just in between you have a parabola, which is just a special case of an ellipse. When cometary orbits are first calculated, resultant hyperbolas are the result of either the comet’s interaction with a third body, or it is a visitor from interstellar space. Most comets are in parabolic or elliptical orbits. (A very skinny ellipse is difficult to distinguish from a parabola.)

It turns out that their are only two ways two separated masses will collide, or orbit one another. First, a third body interacts with the pair adding or removing sufficient energy to the system to make the collision dynamically possible. The other is that the two bodies are initially floating in space with no residual motion away from the line-of-sight between them. This is highly unlikely, unless some conscious entity deliberately and carefully places them that way. As a rule, everything is in motion relative to everything else, and this residual motion will divert the bodies from ever actually touching; they will whip around each other and head back off into space. Even the creation of an orbiting pair is highly unlikely, a third body has to add or remove just enough energy and momentum for the equations to balance out; everything has to be balanced just right. This is why spacecraft must undergo a powered maneuver to go into orbit around another world. They are not “captured”, they slow down and lose kinetic energy.

At first, these remarks seem to clash with common sense. We can see that collisions were extremely common in the solar system, and stable elliptical orbits seem to be the rule, not the exception. But the former is explained by the fact the system was formed of many bodies interacting with each other, exchanging energy and transferring momentum. The latter is a consequence of the accretion process that formed the original solar nebula: it tends to result in objects revolving AND rotating in the same direction. Collisions and encounters tend to remove those planetesimals that don’t behave.

As short a time ago as my own youth, it was not understood how the solar system had formed. I remember one theory of the formation of the solar system as the result of the tidal debris extracted from and by the sun in a chance encounter with another star, one in which the mutual orbits and masses of both stars had to be just right. I had one high school teacher actually use this argument to prove the existence of god. This would make solar systems exceedingly rare in the universe. Astronomers knew that the total angular momentum of the solar system was simply too small, it could not possibly have formed from a nebula. Much later, we came to realize that excess angular momentum was dissipated magnetically in the primordial cloud, with just enough left over to yield the current organized dynamic behavior of the sun, planets and satellites. We went, almost overnight, from a solar system being a very rare chance encounter, perhaps one-of-a-kind, to a necessary by-product of star formation. It was the first blow to Fermi’s Paradox.

From a meditation on collisions, we arrive through the back door at a profound metaphysical conclusion. The clockwork regularity of the solar system is not a result of conscious, intelligent design, it is the inevitable result of blind, random chaos acting on a substrate of simple natural laws. Newton, like Darwin and Adam Smith, teach us the world is the way it is not because it was made that way, but because it has no choice but to be that way.