Feat happens in Showdown in Space, in Astro Boy volume 1. Astro Boy ends up causing an explosion to destroy the country of Fidora, the lost kingdom of Moon People who destroyed themselves when their desire for diamonds ended up causing them to nuke the entire moon with asteroids, and he survives the resulting explosion.
This story was about a dog.
Anyway.
The crater is 845 pixels while the explosion is 597 pixels
845 / 597 = 1.415
Cratere Of Copernic has a diameter of 93 km
93 km / 1.415 = 65.72 km
65.72 / 2 = 32.86 km
32.86 Y^(1/3)*0.28
Y = 1,616,324.601676 KT
1.616 Gigatons
Small Island level