When a 4.5 billion year old relic from the dawn of the solar system hurtled through the skies above the Russia in 2013, we all heard about it. Now after over a year of poking through bushes for lost fragments and analyzing data, we've got a good idea of what happened.


Top image: Meteor over Chelyabinsk, Russia on 15 February 2013. Credit: M. Ahmetvaleev/JPLPIA16828

David A. Kring, scientist with the Lunar and Planetary Institute, and Mark Boslough, a physicist at Sandia National Laboratories, traced out everything we now know about the rock that disintegrated 100 kilometers above the Russia-Kazakhstan border for Physics Today.

The rock's entry speed gives away its original orbit, out in the asteroid belt between Mars and Jupiter. Similar to how scientists have reconstructed the collisional history of the meteorite that crashed into a Novato, California home in 2012, researchers have pieced together the history of the Chelyabinsk meteor. It was rocky, metal-poor, and worse-for-wear from rattling around the solar system so long.

Once the rock started feeling the gravitational influence of the Earth is where things get really interesting. Kring and Boslough write:


The Chelyabinsk asteroid first felt the presence of Earth's atmosphere when it was thousands of kilometers above the Pacific Ocean. For the next dozen minutes, the 10 000-ton rock fell swiftly, silently, and unseen, passing at a shallow angle through the rarefied exosphere where the molecular mean free path is much greater than the 20-m diameter of the rock. Collisions with molecules did nothing to slow the gravitational acceleration as it descended over China and Kazakhstan. When it crossed over the border into Russia at 3:20:20 UT and was 100 km above the ground, 99.99997% of the atmosphere was still beneath it.

Because the asteroid was moving much faster than air molecules could get out of its way, the molecules began to pile up into a compressed layer of high-temperature plasma pushing a shock wave forward. Atmospheric density increases exponentially with depth, so as the asteroid plunged, the plasma layer thickened and its optical opacity rapidly increased. About one second later, at 95 km above the surface, it became bright enough to be seen from the ground. That was the first warning that something big was about to happen.

They trace out those next fifteen seconds, investigating the kinematics of flight, impact of stress on the rock's integrity, and what this rock was doing to the air as it ripped through. From the physics of bow shocks and explosions, the mechanics of the iridescent trailing clouds, and the transition to dark flight, the piece is well worth the read.