With billions of planets capable of supporting intelligent life, why no visits to earth? This is "The Fermi Paradox." Any civilization, with modest rocket tech & immodest funds, could rapidly colonize the Milky Way...
R. Raju, D. Dhivya, R. Saranya, and S. Abbinaya. International Journal of Innovative Science and Modern Engineering (IJISME), 1 (3):
72-75(February 2013)