Yesterday's Earth-Skimming Asteroid Was Unusually Large And Exceptionally Close

Asteroids large enough to be detected pass the Earth roughly weekly, but a recent visitor was disturbingly large. Dima Zel/Shutterstock

Earth has close encounters with asteroids large enough to do damage on a fairly frequent basis, but yesterday one came considerably closer than most.  Although not in the league of the “dinosaur killer,” this visitor was also large by the standards of those that have snuck up without us noticing until the last minute.

If you’re one of many who thinks a year marked by heartbreaking wars, climate catastrophes and more than usually stupid politics is not a good one, there’s a certain irony that the asteroid that nearly made things considerably worse is named 2019 OK.

What makes 2019 OK unusual is the combination of size and closeness. Anything inside the Moon’s orbit is considered a close approach and so far this year we’ve spotted 24 asteroids doing that, almost one a week.

On July 25, the International Asteroid Warning Network reports 2019 OK got to 0.19 lunar orbits from the Earth’s center; considerably closer than most, but hardly the closest. Small asteroids burn up in the atmosphere frequently, with some dropping a hail of meteorites and creating dramatic explosions. In June 2019, MO buzzed so close it is thought to have encountered our atmosphere, and been responsible for a flash seen over the Caribbean.

2019 OK could have done a lot more than that. Estimated to have been 100 meters (330 feet) long, it would have released more energy than the largest nuclear bomb, easily wiping out any city it landed on. The more likely outcome of an ocean splashdown would have set off enormous tsunamis.

On the other hand, 2019 MO was only about 5 meters (16 feet) across, giving it less than a thousandth of 2019 OK’s mass. The largest impactor of recent years, which created the Chelyabinsk explosion that injured 1,000 people, is thought to have been 15-20 meters (50-70 feet) across. 2019 OK would have at least 100 times the mass.

That makes this exactly the sort of object we’d like to know about beforehand. In fact, the earliest observations of 2019 OK were almost a month ago in late June, but the orbit wasn’t calculated until a few hours before it rushed past. The problem was that 2019 OK didn’t play fair, sneaking up on us from the Sun side and therefore only visible at twilight.

2019 OK has a 2.7-year-long orbit – longer than Mars, but its path is quite elliptical, taking it inside the orbit of Venus at its closest approach to the Sun. That’s a bad strategy for long term survival. Unless a close approach radically changes its orbit, 2019 OK will eventually collide with one of Earth, Venus, or Mars. Now we know it’s orbit we’ve established it won’t be hitting us soon, but it serves as a reminder there are other things out there we really should be ready for.

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