Scientists around the world held what they billed as the first annual Asteroid Day on Tuesday, giving lectures and hosting panel discussions promoting projects designed to detect threats from space before it’s too late.

The scientists who organized the events said humans must do a better job searching for asteroids because even a relatively small one — about the size of a house — could level a city and kill millions of people.

As it orbits the sun, the Earth encounters tons of tiny space rocks every day. Most burn up in the atmosphere, but larger objects occasionally make it to the surface or near it.

“What we’re trying to do with planetary defense is, at a minimum, we can evacuate people” from the potential impact area, Rusty Schweickart, an Apollo 9 astronaut who has pushed for better preparedness, said at an Asteroid Day event at the California Academy of Sciences in San Francisco.

Potentially civilization-ending asteroids — miles-wide ones that could scorch continents and blast dust into the sky, blotting out the sun for decades — hit only once every tens of millions of years, he said, noting the 7-mile-wide object whose impact is thought to have caused the extinction of the dinosaurs 66 million years ago, starving them beneath a pall of choking ash.

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