Friday, February 22, 2013

Putting the Russian meteorite in perspective

Friday morning, a large meteor entered the atmosphere over the southern Ural area of Russia, detonating with enough force to shatter windows in nearby towns and injure over 1000 people.  Preliminary estimates suggest an impactor traveling at 15 to 20 km/s, and weighing 8000 to 10,000 tons, exploding at an altitude of 20-30 km with the force of a nuclear weapon.

These are hard numbers to wrap one’s head around.

Let’s start with the size. There are numerous reports around on the bolide being “bus sized.”  But buses are not made of solid rock, so this is deceptive. In this situation, mass is more important than dimensions. A bus weighs about 15-20 tons. That’s a lot less than 8000-10,000.  For example, 8000-10,000 tons is the approximate size of the naval destroyer USS Cole, which was famously attacked by Al Qaeda in Yemen in 2000. It’s a lot bigger than a bus. Of course, that ship doesn't fly in space.  Rather, it sails in the ocean at about 50 km/ hour, thousands of times slower than 20 km/ second. The International Space Station is about 450 tons.

Orbital velocity for a low earth orbit is about 8 km/second, and reentry speeds returning from low earth orbit are similar.  So this meteor was traveling at about twice orbital speed when it hit the atmosphere.  This is substantially faster than the 11 km/s reentry of the Apollo missions returning from the moon, and about twice as fast as the space shuttles (and other low earth orbit spacecraft) re-enter.  It is about 50 times faster than a handgun bullet.

The total energy released, between a quarter and a half a megaton, was similar to a modern H-bomb.  However, it was more dispersed, and released high in the atmosphere. Because the impactor was traveling at twice orbital speed, the energy would be equivalent to an orbital object of four times the mass re-entering.  32,000 to 40,000 tons is about the size of the Titanic, or a WWII battleship. 

Something similar to this has been imagined.  Below is a model of CV-6, the famous 20,000 ton WWII aircraft carrier Enterprise.
Compare that to the fictional NCC 1701 spaceship enterprise, at the same scale.
The internet gives a spaceship mass of 10 times the aircraft carrier, which seems way to heavy to be sensible. 
If we say the spaceship is twice the mass of the aircraft carrier (it is bigger, after all, even if it is also probably made from a lighter & stronger material than steel), then it would have about the same energy on re-entry as the Chelyabinsk bolide.

We can compare the videos:

Star Trek III

Chelyabinsk Friday morning:

Reality is still far more gripping than imagination. 

Finally, here is what the Earth looked like from the asteroid’s point of view an hour before impact.  A few things to note:
First, the Earth is almost full.  As a result, the side of Earth facing the asteroid was in day, so it would have been hard to spot, as the sun was behind it.  However, the US space junk tracking radars in Hawaii should have been able to pick it up.  I wonder if they did, if they passed any sort of a warning on, or even are they allowed to?  It would be a shame if the 1200 injuries that occurred were preventable, but for American government red tape.