Last week, the world was obsessed with an asteroid that had a 3% probability of hitting us in 2032. Now NASA says it doesn’t have a chance in 50,000. What happened?
How are impact probabilities calculated?
I wrote last year about how potentially hazardous asteroids are detected. One aspect I neglected is what that “impact probability” actually means. In the wake of the news about 2024 YR4 and its rapid upgrading and downgrading of threat, I’ve gotten a lot of questions about how reliable our asteroid observations actually are.
The answer: very. When asteroid hunters first detect that little moving dot against a stationary sky, they have only a handful of data points. Uncertainty will mar any measurements they make of the asteroid’s position and velocity.
It’s that uncertainty that defines impact probability. Astronomers calculate the trajectory of the observed asteroid, allowing room for error. If that trajectory passes anywhere near the Earth, then as long as the error bars are substantial, the impact probability is as well.
Why did the impact probability go up first?
The evaporating fragments of comet Shoemaker-Levy 9 approach Jupiter. Photo: NASA/ESA/H. Weaver/E. Smith (STScI)
Once asteroid hunters spot a potentially hazardous asteroid, telescopes around the world jump to observe it, reducing the uncertainty about its trajectory.
But if the asteroid is still heading anywhere near the Earth, the smaller error bars increase the probability of an impact. That’s what brought YR4 into the public eye.
This is a pattern with asteroid impact probabilities. An asteroid starts with a moderate possibility of impact. Astronomers jump to observe it, decreasing the uncertainty but not yet ruling out a collision with the Earth. The impact probability jumps up.
Then the error bars on the orbit shrink even further, and suddenly they no longer encompass the Earth. The impact probability drops to pretty much zero.
Asteroid 2024 YR4 hasn’t suddenly started behaving less chaotically. We just understand it better, and know exactly where it’s going to go. And that’s not the Earth.
