JPL Small-Body Database Browser.
21 metre wide space-rock 2009 KR21 whizzed past the Earth and Moon at a distance of 285,000 and 150,000 km respectively on June 1st. If it had been but 0.0019 AU or 0.001 AU closer to either, then it would’ve collided. The relative velocity was 12.87 km/s and 13.87 km/s respectively, so it would’ve been a decent collision speed after acceleration in the gravity field of either.
It would’ve entered Earth’s atmosphere at 17 km/s and probably burst quite high in the air from aerodynamic forces, liberating about 168 kilotons of TNT equivalent energy… quite a bang indeed. On the Moon it would’ve struck at 14.07 km/s and exploded with 114 kilotons, digging a decent hole and producing a brief flash that some amateur could spot with a patiently watching automated telescope.
That assumes it’s low density porous space-rock with an average density of water at ~60% porosity. If it was more compact iron/nickel then the explosive punch would be about 8 times higher – a megaton explosion on Earth and the Moon. Iron/nickel meteorites are mechanically very strong and impact the surface with most of their space-velocity intact.
But what if it was something stranger? One possibility is that a significant number of Shadow Matter meteoroids exist. But what is ‘Shadow Matter’ you ask?
see Part 2