Be prepared to get extinct, anytime. Earth might get into an "extinction-level" event from the sky. And once we spot it, we will not even be able to do anything to protect ourselves, said a NASA scientist Monday.

As a San Francisco meeting, Dr. Joseph Nuth of NASA's Goddard Space Flight Center explained that huge asteroids and comets can wipe out civilizations anytime. Although they are not too common, they can hit the earth anytime. "But on the other hand 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, but it's a random course at that point."

So far, humans have tracked 5,000 "potentially hazardous objects," or PHOs, mostly asteroids in space. Even as more space rocks are found every night, the number of "PHOs" is now more than 700,000, said Nuth.

He added: "The biggest problem, basically, is there's not a hell of a lot we can do about it at the moment."

Nuth actually drew a parallel with events in 1996, when a comet flew into Jupiter, and 2014, when one came close to Mars. When a comet came close to the Red Planet, it was seen only 22 months before it came close. That doesn't leave anytime for earth to protect itself, says Nuth.

One option is to build a rocket that can be kept in deep storage and sent whenever we feel that a huge strike might take place anytime. If we keep it tuned, we could be ready to launch within a year, he said.

It "could mitigate the possibility of a sneaky asteroid coming in from a place that's hard to observe, like from the sun," he added.

Dr Cathy Plesko, a scientist at Los Alamos National Laboratory, said humans can deflect an asteroid with the help of a nuclear warhead or a "kinetic impactor, which is basically a giant cannonball".

"Cannonball technology is actually very good technology, intercepting an object at high speed actually ends up being more effective than high explosives," she said.