He feared aliens might hate us. He worried that we were wrecking our environment so much that we'd have to leave Earth within 100 years. He had a strong suspicion that President Trump would turn our planet into Venus. But in Brief Answers to Brief Questions, a collection of his articles and essays to be published on Tuesday, he offered an even greater -- and perhaps more immediate -- fear. As excerpts in the UK's Sunday Times reveal, the famed physicist and general media star worried about humans using genetic engineering to turn themselves into "superhumans."