It’s not really a question of whether a massive asteroid—one big enough to wipe out human civilization—will fall into a collision course with Earth. It’s a question of when.

NASA estimates a 1,000-foot-wide asteroid, big enough to flatten a city, hits our planet every 70,000 years or so. Half-mile-wide rocks, capable of killing much of the life on Earth, strike every 700,000 years.

Anticipating the inevitability of a catastrophic impact, scientists are scrambling to devise methods of detecting—then destroying or deflecting—an incoming space rock. The best solution might still be the most obvious: nuke it. Just like Bruce Willis’s miner-turned-astronaut did in the 1998 movie Armageddon.

But when we nuke a planet-killing rock matters, according to one Hungarian-Ukrainian science team. For the biggest and most dangerous incoming asteroids, it’s important to wait until the proverbial last second—just a couple of years ahead of impact—before blowing them up, according to Zsolt Regály, Viktória Fröhlich, Peter Berczik, respectively from the Konkoly Observatory and Eötvös Loránd University in Hungary and the National Academy of Sciences of Ukraine.

Their math is complicated: it takes into account an asteroid’s likely path through the solar system and the effects of gravity from the sun, Earth and neighboring planets. But their conclusion is simple: Shatter a space rock too soon, and it might just turn into a bunch of smaller rocks that would still hit Earth like a blast of shotgun pellets instead of a single big bullet.

“The number of fragments hitting the Earth is strongly influenced by the orbit of the impactor and the time of interception,” Regály, Fröhlich and Berczik wrote in their peer-reviewed study, which appeared in Astronomy & Astrophysics in early August. “To minimize the lethal consequences of an… impact, a well-constrained interception timing is necessary.”

There are two broad categories of asteroids whose oblong orbits around the sun