Instead, their focus is on relatively small asteroids, those about the size of football stadiums, notable for their abundance as well as their ability to evade asteroid-hunting observatories. “Those are the ones that we tend to worry more about because they could come out of nowhere,” Dr. Bruck Syal said.

Such a diminutive asteroid may not sound like much of a danger compared to the 6.2-mile colossus that slammed into Earth 66 million years ago with apocalyptic results. But a meteor that exploded over Siberia back in 1908 was only about 200 feet across — and the blast’s shock wave leveled 800 square miles of forest. “That’s the size of the whole Washington D.C. metro area,” said Dr. Bruck Syal.

Using high-fidelity simulations, scientists reported in a study published earlier this month that a stealthy asteroid as long as 330 feet could be annihilated by a one-megaton nuclear device, with 99.9 percent of its mass being blasted out of Earth’s way, if the asteroid is attacked at least two months before impact.