When it comes to UFOs, I am a complete sceptic. My personal opinion is that all the people who claim to have seen them were either:
1. Mistaken - what they saw has a simple explanation
2. On drugs
3. Lying in order to get publicity
4. Bonkers
After all the thousands of “incidents”, there has never been even a single decent photograph, no physical evidence and nothing that couldn’t be explained by something far more mundane. In the absence of hard evidence, Occam’s Razor makes the above 4 possibilities the most likely ones. Two explanations are often proffered for these “visitations”:
A. They are time travellers from our own future
B. They are aliens from another planet
Explanation A. is exceedingly problematic physically since this would require the invention of a time machine at some point in the future that could travel back into the past, I.e. our present. The astro-physicists and cosmologists who have thought about this the most reckon that if (and that’s a HUGE if) such a machine ever gets invented, then travel back to before it was invented would be impossible. I.e. it could only be used in it’s own future. But the building of such a device is enormously problematic anyway. Any means of doing so either involves travelling faster than the speed of light (disallowed by Einstein‘s Special Relativity), or the consumption of amounts of energy so utterly massive that it becomes infeasible.
Explanation B. is similarly problematic. This requires the existence of aliens at a higher level of development than us to exist, and to exist at the same time as us, and within reasonable travelling distance of us. They would also have to be able to build ships that were invisible to us whilst they travelled to earth, and which then became briefly visible for short periods whilst they got here, to tiny numbers of people at a time. Whilst not impossible, all these factors make repeated alien visitations astonishingly improbable.
I know lots of people really “want to believe”, but we all need to face a basic truth here: the odds are that there are aliens somewhere in the universe, but our chance of meeting them is almost vanishingly small. I’m sorry if anyone finds that thought distressing, but that’s how it is.
Here is the nice song by Steve Hillage where my title comes from:
http://www.youtube.com/watch?v=cjTGKkJB7Jo
Cheers, Tom.
jenray
Pro 
Re the first option...I'm non-committal on both options, but would prefer to opt for the first if given a choice...I'm really not sure whether the scientists of this day have sufficient knowledge yet to say that it is very unlikely that we could travel back in time...how about the theory that if we can imagine it, it's possible...we cannot imagine anything that doesn't exist anywhere...in other words, the mind has to have some reality upon which to fix before it can surmise such and such a thing is possible even before it exists in his or her reality...doubt if the scientists of the thirteenth century believed it was possible to land on the planet Mars and send pictures back to Earth...LOL...great big hugs...