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 million years ago by an asteroid that struck what is now the Gulf of Mexico.

Dr Nuth, speaking at Nasa’s Goddard Space Flight Centre in San Francisco, said that if a potentially dangerous object was on a crash-course with our planet “there’s not a hell of a lot we can do about it at the moment”, according to The Guardian.

He said: “They are the extinction-level events, things like dinosaur killers, they’re 50 to 60 million years apart, essentially. You could say, of course, we’re due.”

Earth had a “close encounter” with a comet in 1996 and again in 2014, when one passed “within cosmic spitting distance of Mars”, he said.

Scientists had only 22 months’ warning time for the second pass, less than half the time currently needed to get a craft capable of deflecting such an object into space, Dr Nuth said.

Dr Nuth said he had recommended Nasa build an interceptor craft and keep it in storage in order to cut down on the time it takes to organise such a mission.