[Stuart]

Well, I am no physicist, but AFAIK the only times you need propulsion in space is if you need to change speed or direction. In fact, I did read that if you went into orbit and were able to fire a handgun toward the sun, the bullet would eventually reach the sun (assuming it didn't get too near any other objects).

I personally do believe in Aliens, but I don't know if we have been visited by UFOS.

One interesting thing to note is that a lot of sightings are apparently near military bases, so it's entirely possible that some sightings are actually of experiemental military aircraft (like the aforemention Stealth bomber).