five centuries ? that's optimistic Steven.. I'd give us about 40 years to get out and establish self perpetuating colonies in the solar system and then 100 to get out to Alpha Centauri. I think we'll find most planetary systems worth having will already be inhabited. I mean what's the chance of finding an Earth like planet .....BUT WITHOUT Millions of species.

I also really like Mr Hawking. I've worked on a series all about his work and I've read some of his writings for laypeople. His slightly ridiculous view that we should keep quiet or bad aliens will come and get us is a bit childish. A) they know we're here. B) we're still alive C) if you're a space faring race, you probably need to develop ethics for visiting planets (like they have in Star Trek) .. even our smartest can't reach escape velocity of the human imagination limit. ie we always think about the unknown couched in our own little relatively inexperienced terms.. speak to a good sci-fi writer. they can see round corners where light can't bend !