I found this on the BBC website....
Are We Really Alone?

In the past, probes have been sent into space with engravings of human beings on board and diagrams showing the location of our planet.

Radio beams have been fired into space in the hope of reaching alien civilisations.

“It is 'perfectly rational' to believe in aliens,” said renowned astrophysicist Professor Stephen Hawking. " To my mathematical brain, the numbers alone make thinking about aliens perfectly rational. The real challenge is to work out what aliens might actually be like. Aliens almost certainly exist, but humans should avoid making contact.”

In a series for the Discovery Channel the astrophysicist said it was "perfectly rational" to assume intelligent life exists elsewhere.

Let us just prey that we never meet. For he added, "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans."

Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact.

I agree. To paraphrase what the renowned scientist himself explained: "We only have to look at ourselves to see how intelligent life might develop into something they – (the Aliens) - wouldn't want to meet."