Have aliens ever visited Earth?

Chivalry Augustus wrote:But the physiological effects of travelling at the speed of light could nevertheless prevent humans from making the supposed journey. Light-speed travel is not a formality by any stretch of the imagination - no progress has been made towards it in 40-odd years of space exploration, so to presume that in 400 or 4000 years we will be capable of attaining something that might not even be possible (from a practical perspective as opposed to a factual perspective - light-speed obviously is possible as light travels at light-speed) is presumptuous. It would also take a couple of geniuses to take us there, and given that NASA is increasingly under-cut and under pressure from the US Government, you have to question the likelihood of them ever having the money or determination to get that far. The better hope probably lies with the Chinese longer-term. Irrespective of all that, you have to consider what I said - how would aliens find us? How likely is it that another intelligent life-form, even if they CAN attain light-speed travel, is at a suitable distance that they can reach us? And furthermore, what is the likelihood that they would set out in the direction of Earth, a planet that was until the early 1900s devoid of any signal indicating that it had life upon it? Considering that the three-dimensional dynamics of space mean that you would have to set off in a constant line for earth to actually reach it unless you made it by an absolute fluke (it's very difficult to start altering your direction once you've got going) - if you began one millimetre to the left you would bypass it by light years. So, whilst statistically extra terrestrial intelligent life seems likely to me, first contact is improbable unless it is by design.

Have aliens ever visited Earth?