They will likely be sentient machines. Also, they're already contacting us. An example is the 2004 Nimitz encounter, which is undeniable -- the smoking gun. No longer the stuff of loons (and it never was), they seem to be very "curious" about our military capabilities. The former head of the Pentagon's secret "Advanced Aerospace Threat Identification Program" toured cable news outlets proclaiming that "they're here". The 1999 French Cometa report, in its conclusion, stated extraterrestrial visitation is "likely". If find the apparent interest in our military capabilities disconcerting. I believe Hawking is correct, and we shouldn't advertise our presence. Our first encounter with aliens may be our last, and the "historic event" may - likewise, end history.