Have aliens visited Earth?
Aliens are an interesting topic, mainly because nobody knows much about them(not including the countless number or rednecks who claim they saw a "fire in the sky"). Besides all the rednecks there are people with plausible theories and sightings. Whether they are true or false we may never know, but we'll let you decide what you think for yourself. Very few theories are logical or plausible, so we've tried to come up with a plausible theory of our own that supports the theory that aliens have visited us and one plausible theory that shows they have not visited us yet.
In favor of aliens visiting us:
Aliens could be much more advanced than us, they might have sort of "speed of light" travel, better technology and so on. They observe us from a distance so that they can study our planet and it's inhabitants but they do not interfere with us so that natural evolution can take place. If the aliens were to come down here and walk into NASA and say "You know that Orion Deep Space Capsule your developing, it's terrible. Look here's about 10 thousand blueprints and schematics that show you how to time travel and make a warp drive". That would put humanity way up "The Galactic Supremacy chart" and skip hundreds of years of science and engineering. If they wanted to observe natural evolution on our planet they certainly would not be allowing us to skip hundreds maybe even thousands of years of science. They might have visited our planet without us knowing, they might just be watching.
Not in favor of aliens visiting us:
The distance between our sun and (for example) Gliese 581c, which is a potentially habitable planet, is 20 light years. This means that if you were travelling at the speed of light it would take 20 years to get there. For us humans, that is currently impossible. For really advanced aliens it might be possible but is likely not. Lets just say that they can travel at 99.9999999999999% the speed of light. They would need a lot of food(or produce a lot of food on board). They would need a lot of water and some way to filter used water and make it reusable. They would need a large crew at this point with all of the above on their ship and they would need a place to let the crew sleep, relax, eat, and bathrooms. If they haven't invented artificial gravity then they would need to be exercising for a few hours every day(like we do on the space station). They would need some sort of propulsion(and before you ask, no. Droping nukes out the end of your ship is not propulsion). So they would need all of the above facilities and all of them require power. Wind, wave, hydro etc. are not going to work in space. They could use solar, but when you get far from your sun, you wont be generating much energy. Nuclear is probably the only form of energy for a ship like this, but then there is the risk of an explosion and you need more water as a coolant. So they fly to us on a huge nuclear missile that can produce food, water and allow aliens to live in it, but how do they know we are here. There are thousands of other habitable exo-planets around them, they would have to decide which one to pick first and it might not be ours. So the odds are really slim. If they did manage to accomplish all of the above things, they might just die out before they reach us.