A cosmic impact

Ken Tapping, February 27, 2013

In the sky this week…

  • Jupiter is high in the south after dark, moving into the west by midnight.
  • Saturn rises around 11 p.m.
  • The Moon will reach Last Quarter on the 4th.

It was about 9:20 a.m. on a typical Friday morning in the industrial city of Chelyabinsk, Russia. February 15 was shaping up to be an ordinary working day. But then things changed dramatically. Thanks to many video cameras being in use at the time, what followed must easily have been the most comprehensively recorded meteor impact in history.

The recordings show a glowing, moving dot appearing in the predawn sky. The dot brightened and started to leave a glowing trail. It got brighter and brighter, and the trail got longer. Finally the glare became brighter than the Sun, dazzling the cameras and onlookers, then, as it faded and finally disappeared below the horizon, a long, complicated trail became visible. Soon after, a series of explosions damaged buildings, blew out windows and sent over 1000 people to hospital.

The amazing videos of the event showed the arrival of a larger-than-usual lump of cosmic debris. NASA estimates at the time of writing suggest the body weighed about 10,000 tonnes, was roughly 10m across and moving at about 20 km/sec.

This makes it the largest object known to have hit the Earth since 1908, when something came into the atmosphere over Tunguska, Siberia and exploded, releasing an amount of energy equivalent to about 30 million tonnes (Megatons) of TNT. Trees were flattened for many kilometres and the blast rattled glasses in Paris. If this explosion had happened over a densely populated area, the disaster would have been indescribable. There was another meteoric impact in 1947, also in Russia, which was smaller than the 15 February event.

An object weighing 10,000 tonnes and moving at 20km/sec possesses a lot of kinetic energy, roughly equal to the energy released in the explosion of 500,000 tonnes (500 kilotons) of TNT – 20 times the power of the Hiroshima bomb. All of this has to be released on the way to the ground or on impact.

For an object moving that fast, more than 50 times the speed of sound, the atmosphere would have seemed almost solid. Pushing through it produced enormous temperatures, up to 10,000 degrees, and generated huge shock waves. Eventually the heat and rapid deceleration were too much, and the object exploded. It was these shock waves and explosions that caused the damage and injuries.

It could have been worse. This cosmic visitor came into the atmosphere at a shallow angle, so that most of its energy was lost before its fragments reached the ground. If it had come in more steeply, more of the energy would have been released at ground level or as a low-altitude, nearly 500 kiloton explosion. That would have been catastrophic.

We are getting better at spotting objects passing close to the Earth, such as Asteroid 2012 DA14, which passed us by at a safe distance of 28,000 kilometres the same day. However we are a long way from detecting bodies 10m or so in diameter before they bore into the atmosphere, causing the drama that happened in the skies over Russia on Friday. It’s not clear we will be able to prevent them from hitting us any time in the foreseeable future.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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