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The asteroid 2019 OK recently passed at 77.000km of the Earth, with an estimated size of 50-130m.

But if this asteroid had crashed onto the Earth, what would have been its kinetic energy on impact? And how would its mass have been affected by the atmosphere?

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You can use the Purdue 'Impact Earth!' website to estimate the effects of impacts of objects of various size, speed and composition. As detailed in the accompanying documentation, which was published in Meteoritics and Planetary Science, the energy calculation and atmospheric drag during entry is relatively straightforward physics; things like crater sizes, overpressure and thermal effects are scaled from nuclear tests.

Plugging the numbers in for 2019 OK involves some assumptions since there is considerable uncertainty (~50%) on the diameter since we don't have an albedo and this leads to a large uncertainty in the volume. In addition, since nobody got a spectrum during the close approach, we don't have a good idea on the NEO's taxonomy and therefore what it's made of. This makes the mass, and therefore the energy of the impact, somewhat uncertain (since $Energy\sim\frac{1}{2}mass \times velocity^2$).

If we assume 80m diameter and density of 2000 kg/m$^{-3}$, which is mid-way between that of C- and S-type asteroids (most common types among NEOs, making up ~65% of NEOs; Binzel et al. 2015, in Asteroids IV), and an encounter velocity of ~25 km/s (rounded up from JPL Small Body DB close approach data) gives ~40 Megatons of TNT equivalent. ImpactEarth! also reports:

The projectile begins to breakup at an altitude of 71800 meters = 235000 ft The projectile bursts into a cloud of fragments at an altitude of 3860 meters = 12600 ft. The residual velocity of the projectile fragments after the burst is 4.94 km/s = 3.06 miles/s. The energy of the airburst is 1.61 x 10^17 Joules = 3.85 x 10^1 MegaTons. No crater is formed, although large fragments may strike the surface.

If you put in a distance from the impact point, it will calculate the effects of the airblast for you, which will be the main effect (as it was for the ~0.5 Megaton 2013 Chelyabinsk impact)

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The size uncertainty makes it impossible to estimate kinetic energy precisely, but it would have been pretty large, of the order of several megatons. A megaton is an energy unit equivalent to a million tons of TNT, and if you can imagine that every ton of meteorite turns into a ton of high explosive on impact, that will give you some idea of the energy involved. Asteroids arrive at speeds of around 20 to 25 km per sec, and the larger they are, the less they are slowed by the atmosphere. This one would not have been slowed very much, so would have struck the ground with almost full force and vaporised. In the case of the largest size estimate quoted, the explosion would have been similar to one of the largest hydrogen bombs ever tested.

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For a very ballpark estimate, this impactor would have been comparable to the Tunguska event, where the effects are quite well documented. It's impossible to know the size of that impactor with accuracy but the estimate is fairly similar, about 50-190 meters across.

Tunguska event

The Tunguska impactor didn't leave a crater in the ground. It's damage to the ground came from an air-burst, which still flatted a blast area about 30 miles across (15 mile radius) and the Tunguska shockwave is estimated to have caused the equivalent of a 5.0 Earthquake on the Richter scale.

As a sidebar, most of the injuries from the much more recent (and caught on film), Chelyabinsk meteor impact were from people looking out their window after the impact and the shockwave blew the windows into their faces. (Not fun), so meteor safety 101 - you can watch it fly through the sky, though the flash may be bright enough to hurt your eyes, so maybe film but don't watch if it's a big one, but after you've seen it, find cover and step away from anywhere that a window can blast into you.

Meteor impacts can vary significantly, based on the material the meteor is made of, the velocity and even the angle of impact, so Tunguska is a ballpark estimate with perhaps as much as an order of magnitude error, but it's not too far off.

I read somewhere that Tunguska events are expected to happen about once every 300 years, so near misses the size of 2019 OK and distance, 45,000 miles or less, a little math puts events like that about once every 2-3 years. If this was golf and the meteor was a Tiger Woods putt, the putted ball would have passed by the hole never getting any closer than about 2 feet. That's kinda close, I suppose, but hardly a graze. I look forward to the next big meteor that flies past just a few thousand miles from our surface, perhaps even viewable to the naked eye, still a miss, but a closer miss. Events that close are much more rare. But, I digress.

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