2019OK energy at impact 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?
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)