It seems like every month there is at least one news article talking about how an asteroid is coming our way. In fact, as I’m writing this, news articles are being written about how the asteroid titled “2002 PZ39” is making a close approach on February 15th.
Before we start freaking out and rioting in the streets, though, let’s try to understand just how much trouble we’re in. How hazardous is “hazardous”? How close is “close”? Turns out, NASA and other astronomers have a system to how they deem a near-earth object as “potentially hazardous”. NASA’s Center for Near-Earth Object Studies has all its data publicly available, and conveniently labels all the stuff it thinks is “potentially hazardous”.
To be classified as hazardous, it has to have a Minimum Orbit Intersection Distance (MOID) of 0.05 au or less, and an Absolute Magnitude (H) of 22 or less.
Let me explain what that means. Minimum Orbit Intersection Distance is the closest distance between the two rings that make up earth’s orbit and the object’s orbit. It is the closest we can theoretically get to each other. 0.05 au doesn’t SOUND like a lot, but one “astronomical unit” is the distance between the earth and the sun… which is to say, a lot. 0.05 au is nearly twenty times further out than our moon is!
Absolute Magnitude is a bit trickier to understand, but essentially, it’s a measure of how bright it is for how far away it is. It also happens to be a good way to estimate how big an asteroid is. A smaller absolute magnitude means a bigger asteroid. An absolute magnitude of 22 means an asteroid, at most, 0.24 km (0.15 miles) across. Anything smaller than that is apparently not a threat and will probably just burn up in entry.
If it sounds like these numbers might be a bit big to justify being called “hazardous”, you’d be right. This simply because of orbital uncertainty. It takes work to calculate with higher accuracy just where that asteroid will end up, and so our threshold for being “hazardous” is high to account for this potential uncertainty.
It turns out that there is also a number for how uncertain we are of an orbit, ranging from zero to nine. So, the question you might be asking is how much work are we putting into analyzing these “hazardous” asteroids?
As you can see, the majority of asteroids we deem hazardous have an orbital uncertainty of zero, which means we can calculate with great certainty whether or not it’s going to hit us decades, even perhaps CENTURIES in advance! Now, there is still work to be done on some of them, but it appears that we have paid a lot more attention to the asteroids that scare us just a bit. The label of “potentially hazardous” is, apparently, less a label for asteroids that will kill us all, and more being used as a sort of triage system we use on the asteroids that may kill us all, so we can figure out if it actually will.