Recently we had a short discussion concerning objects being ejected from planetary systems such as ours. I was toying with the Planets program that I stumbled upon when testing Linux. That program is very interesting and it is easy to fiddle away hours of time just observing how planets interact in a two dimensional stellar system. I recommend that anyone desiring to be fascinated give it a try!
One interesting observation that caught my attention was that so many objects appeared to be ejected from the system as they interacted with others orbiting the star. My concern is that this suggested that perhaps millions of these large and dangerous objects would be drifting in open space and some might actually find their way to earth. I made a short post to the list and some of the vorts suggested that the reason for the apparent onslaught was rounding off errors in the math calculating the inverse square law interactions. I looked into the issue further and come away believing that the effect is real and they are out there. In further playing with these types of systems, I generated some with two similar sized star equivalents in orbit around themselves. Each one was orbiting the center of mass of the system at a fairly fast rate. When I introduced planets to the system, it was brutal. Almost every one of the planets that passed near the stars was immediately sent rapidly exiting the system or found itself being destroyed by one of the pair. This effect was so pronounced that I realized that there must be something that demonstrated its powerful influence under these conditions. I reviewed planetary orbits and found the reason fairly quickly. The escape velocity associated with a star or planet is only the square root of two times larger than the orbital velocity at that radius. Also, gravity assist is used to send space probes into and beyond the outer reaches of the solar system all the time and is quite effective. A typical probe can gain the orbital velocity of the object that it encounters by a moderately close fly by. With this process available, an object that begins at a position that is for example located at the distance of the Earth from the sun can fly by Mercury and gain sufficient speed to escape the suns gravity entirely. This same effect would result in the ejection of many random objects that find their way near to inner planets. This would likely happen often and the program appears to demonstrate this nicely. To add some numbers to the words: The escape velocity for an object at the distance of Mercury from the Sun is 67.7 kilometers/second. The same figure for an Earth distance object is 42.1 kilometers/second. So our object orbiting at the Earth's distance from the Sun would be moving at 42.1 km/sec / 1.1414 or 29.77 km/sec. Now, if we direct the object toward Mercury and pick up its orbital velocity of 67.7 km/sec / 1.414 which is 47.87 km/sec , then it is evident that it will have sufficient velocity to escape the Sun's influence. For these reasons, I believe that empty space between stars is more than likely not empty at all and that many large objects are passing through the solar system every year as they wander about. The probability of collisions is small, but not zero due to the enormous dimensions of space and one day we might observe a close encounter of the nasty kind. I would not be surprised if it eventually is determined that hoards of these asteroids and comets have impacted us in the past and come in groups as each of the nearby star ejection events during their births reach our distance. Since planetary systems stabilize in a few million years from what I have read, the ejections and resulting collisions should occur during relatively short historical time frames. It would be interesting to calculate how far one of these hoards reaches in a billion years and compare that to the distance between forming stars. Perhaps someone needs a hobby? :-) Dave
