Planet Earth is effectively defenceless in the face of a "dinosaur-killer" asteroid strike, a Nasa scientist has warned. And the planet is overdue for an extinction-level event involving a giant space object - such as an asteroid or comet - following a number of close encounters over the last 20 years, Dr Joseph Nuth said. Large objects from outer space that could cause mass extinctions have tended to hit Earth about 50 to 60 million years apart. The dinosaurs were wiped out 66...
