Scientists would need at least six years' notice to prevent a small asteroid from smashing into the Earth using existing technology.
But anything more substantial than 200m across or on a trajectory that evades detection until just a few years before impact would leave humankind virtually helpless to defend itself.
Experts at the first planetary defence conference, held this week in the US, issued the warning after starting work on preliminary plans to fend off cosmic catastrophe.
They have been challenged to devise detailed schemes to deal with four scenarios devised to represent threats that the Earth might face.
A study by specialists at the Aerospace Corporation and organisers of the Los Angeles conference suggests that only one of the scenarios could be defended against at present.
This involves a 200m asteroid called Athos crashing into the ocean off the coast of California in 2016, 11 years after being detected.
The $12 billion (£6.3 billion) plan involves two salvoes of at least six nuclear warhead-carrying spacecraft deflecting the asteroid from its course.
The Earth has suffered many collisions in the past. Scientists have debated the risks of future catastrophes and have started plotting the courses of nearby asteroids.
The planetary defence conference attracted engineers, astronomers, policy-makers and even psychologists from around the world.
A delegation from the European Space Agency, led by David Southwood, director of science, pulled out of the conference at the last minute to attend the rescheduled launch of the Rosetta comet probe.
William Ailor, the conference chairman, said it was vital to start practical mission planning now. "We are getting to the point where we could carry out a real mission to deflect an asteroid," he said.
Dr Ailor said the key problem of the uncertainty surrounding asteroid trajectories could be improved through better observation programmes.
The Aerospace Corporation plan to deal with Athos relies on action being taken when only a slim chance of a collision is predicted.