There were many things that made the meteor that exploded over Chelyabinsk Russia this last February memorable. For most, the amount of camera footage of the event was the most staggering aspect. Indeed, it was the first impact of its kind to ever be caught on film. The scale of damage caused by the meteor was also noteworthy, bringing the dangers of space impacts out of science fiction and history books and into the modern world. However, the most notable aspect of the event for scientists, and what holds the greatest implication for humanity as a whole, is the fact that it came completely out of nowhere.
Since the original impact on February 15, teams of scientists have combed over Chelyabinsk and the surrounding countryside, visiting over fifty villages and recovering any remaining fragments of the meteor. Just last month, the largest piece to date, a 1,430 pound chunk of rock the size of a coffee table, was recovered by divers from a lake outside of the city. By mapping the locations of debris and collecting thousands of interviews and personal accounts, research teams have recently been able to create an accurate model of the meteor’s path and behavior once it entered Earth’s atmosphere.
As any video of the incident will show, much of the damage from the impact came from the powerful shockwave accompanying the meteor’s entry. This supersonic blast first began developing over 55 miles in the sky while the meteor traveled at over 40,000 miles per hour. At 18 miles, the meteor reached peak brightness (30 times brighter than the sun by some estimates), burning hot enough to give many people on the ground mild sunburns. At this point, the rock broke apart into about twenty smaller objects. At 15 miles, these pieces then shattered into countless smaller rocks. Together, these successive airbursts packed as much energy as 500 kilotons of TNT, and produced shockwaves powerful enough to cause damage 55 miles in every direction. By comparison, the nuclear blasts over Hiroshima and Nagasaki measured equal to about 16 kilotons each.
Based on these investigations, it has been estimated that the Chelyabinsk meteor was originally about 62 feet wide and weighed 13,000 tons. The vast majority of this mass was reduced to dust upon entry of the atmosphere though, with only about 0.05% of the original rock impacting the earth in the form of solid fragments. The friction caused by the atmosphere was enough to cause it to violently break apart in midair, a habit shared by most small to medium-sized meteor impacts. Instead, most of the energy reach earth in the form of heat or as massive shock waves.
Much is known about the impact today, but on February 15th, it took scientists completely by surprise. This is especially disturbing considering the tens of thousands of near-Earth objects (NEOS) believed to be in the vicinity of our planet, many of which have not even been catalogued. Analysis of the age and chemical content of the Chelyabinsk meteor indicates that it likely originated around 1.2 million years ago as a fragment of a much larger asteroid, which itself likely came from the inner asteroid belt. But what does its impact say about the likelihood of future impacts?
There have been two meteor airbursts in recent history similar to the event over Chelyabinsk. In 1908, the Tunguska meteor erupted over Eastern Siberia, releasing a 15,000 kiloton blast which flattened 80 million trees over an 830 square mile area. Then in 1963, another airburst, this time 1,500 kilotons, occurred over South Africa. Considering this recent impact, scientists now believe that events similar to these may be up to ten times more likely to occur than previously thought. Before, it had been thought that moderate meteor impacts occurred about once a century. Now, there is estimated to be a 13% chance of one every twenty years.
Realistically, the likelihood of a meteor causing any significant damage over the next 100 years is slim. The destruction and injuries seen in Chelyabinsk were incredibly rare, but nevertheless highlight the need to be more aware of the danger posed by meteors. The most important action to be taken now is to try and catalogue as many NEOs as possible in the hope of developing an early warning system. Following this vein, the United Nations announced earlier this month a proposed Asteroid Defense Plan. Space agencies from numerous countries would take part in cataloguing the size and trajectories of as many asteroids as possible. Considering what we know now, this would be an important step, since the next Chelyabinsk is certainly not an ‘if’, but a ‘when’. The only question is how ready humanity will be.