Worried about asteroids? This is the scale NASA uses to gauge how much danger they pose.

NASA raised eyebrows earlier this month when it announced that astronomers were tracking a massive asteroid that poses a small, but troubling, chance of striking Earth in the coming years.

The space agency’s Center for Near-Earth Object Studies reported that an object between 130 and 300 feet across, which had been labeled 2024 YR4, was moving along a path that gave it as high as a 3% chance of colliding with our planet on Dec. 22, 2032. An asteroid of that size isn’t big enough to threaten life on Earth, like the 6-mile-wide asteroid that killed the dinosaurs, but its impact would be enough to cause millions of deaths if it struck a major city.

Thankfully, further observations have led NASA’s team to reduce the odds that 2024 YR4 will hit Earth down to 0.004% as of Sunday, low enough that it’s considered to have “no significant potential” to crash into our planet.

How astronomers rank impact threats

Our planet is constantly being bombarded. It’s estimated that roughly 100,000 pounds of space rock falls on Earth every day. Nearly all of that gets burned up in the atmosphere. Objects big enough to pose any actual risk to humans, let alone cause any major damage, are very rare.

For more than 20 years, the Center for Near-Earth Object Studies has monitored outer space for asteroids that could intersect with Earth’s orbit and are large enough to cause significant destruction if they do.

They then use those two factors — likelihood of a collision and the potential devastation that would result — to rank an asteroid on a 0-10 scale that ranges from no threat at all to all-but-guaranteed to end life on Earth as we know it if humans don’t somehow intervene.

The Torino Impact Hazard Scale, named after the Italian city where it was first presented to the space-monitoring community in 1999, categorizes asteroids by both number and color. At its peak, 2024 YR4 reached a level 3 on the scale. That’s in the yellow zone of objects that merit attention from astronomers but have a high likelihood of being reduced to level 0 upon further observation — which is exactly what happened in this case.

Asteroids in the yellow zone can be big enough to create substantial destruction, enough to devastate a city or even a whole region of the world, but their odds of colliding with Earth are so low that there’s no major reason for concern.

Things get scarier in the orange zone, where the likelihood of impact is significantly greater, though not yet guaranteed. Asteroids in the orange zone are also large enough to cause “regional devastation” or even “global catastrophe.” At this level of threat, astronomers advise governments and scientists to enact contingency plans for how to prevent the object from colliding with Earth.

The red zone is reserved for the greatest threats — massive asteroids that are definitely going to strike the planet. The levels within the red zone are based on the size of the object. Level 8 is big enough to destroy a city. Level 9 could devastate an entire region. Level 10 could “threaten the future of civilization as we know it.”

While asteroids at every level — including level 10 — have hit the Earth in the billions of years since it was formed, those events are extraordinarily rare. Asteroid 2024 YR4 is noteworthy because it’s uncommon for any object to rise to level 3. In fact, no incoming asteroid has ever been rated above level 4 since the Torino scale was developed. NASA keeps a list of 33 potential future Earth impact events that could occur over the next 100-plus years. All of them are currently at level 0.

Still, NASA isn’t content to sit back and hope our luck doesn’t run out. In 2022, the agency successfully changed the orbit of an asteroid for the first time by slamming a spacecraft into it at high speed. The test, which altered the asteroid’s path far more than initially hoped, was a major breakthrough in the rapidly advancing scientific field of planetary defense.

“All of us have a responsibility to protect our home planet,” NASA Administrator Bill Nelson said at the time. “After all, it’s the only one we have.”