As this is a skeptic thread, I am unaware of any credible evidence that earth has been visited by aliens.
http://calgaryherald.com/news/local-...ent-conspiracy
http://skepdic.com/ufos_ets.html
"Choose the nearest star; decide how long you're willing to travel, how fast you will need to go to get there in that time, what you will have to take with you, and how many should be in the crew. Make it a one-way suicide mission if you wish. As a final step, calculate the kinetic energy that must be imparted to the spaceship to get you there in that time (one half the mass times the velocity squared.) I suggest you stay away from the relativistic limit; it complicates the calculation and won't help you anyway. The good news is that you will then sleep secure in the knowledge that UFOs from elsewhere in the galaxy are not subjecting humans to hideous experiments." --Bob Park