Asteroid Apophis Scared Scientists—Until It Didn't

[00:00:00]We think an asteroid this large comes this close to Earth once per thousand years, once per millennium, or maybe even less often.

[00:00:12]It's incredibly rare, thankfully. This is the good news. It's incredibly rare that an asteroid this large comes this close, which means it's even more rare that an asteroid that large would ever hit the Earth. That's on time scales maybe over geologic time scales of millions of years.

[00:00:32]Astronomers have this challenge. We discover an asteroid and we see only a little tiny piece of its orbit. And then we want to extrapolate that orbit forward for decades. And it's kind of like going to a baseball game and and the batter swings the bat and the ball comes off the bat. Now, tell me where it's going to land.

[00:00:55]Well, all you've seen is a little piece of the the the trajectory of the ball off the bat. And then maybe you say, "Oh, it's going to be a fly ball."

[00:01:04]Where? Right? And you have to keep watching it. You know, so astronomers have to do this until we keep watching the orbit the the path of an asteroid across the sky. This can take months or years.

[00:01:16]We don't know precisely where it's going to be decades in the future.

[00:01:21]And it's interesting when we discovered Apophis and we tracked it um over a few months, we had just enough information to say, "Oh my gosh, in 2029 it's going to come close to the Earth."

[00:01:34]But we couldn't say how close. We didn't know it was going to be out beyond the moon, it was going to intersect with the Earth. We didn't know, we just didn't have enough information. All we knew it was going to come close. Uh it simply took a while to get enough information to say, "Oh yes, we can pin down its orbit and we know it's it's we know it's going to miss the Earth and we could calculate its miss distance.

[00:01:58]Um and then even if it's going to miss in 2029, we said, "Okay, there's a chance it could loop around the you know, get a little gravitational slingshot. If it hits just the right spot, it gets a gravitational slingshot around the Earth to go around the Sun a few times and come back and get us." Anyway, and it's even missing the keyhole. So, not only do you want to make sure the asteroid misses you on that first pass, you want to make sure it doesn't hit one of these keyholes that that ends up letting it come back and get you get you again at some future point. And we now know Apophis so well that we know it's missing Earth in 2029 2029. It's missing any keyhole. There is no future encounter with Apophis impacting the Earth that we can calculate that that we can foresee for centuries. We know the orbit of Apophis so well that for the time scale of centuries, we know it's not a threat.