Remember in February when there was this hullabaloo about a gigantic piece of rock flying through space that would just narrowly miss striking Earth by just this much? Well, if you remember correctly, as that rock zoomed past us, another, smaller one landing in Russia. Chelyabinsk, to be exact.
Well, that meteor was only about 20 m in diameter – just shy of the length of a basketball court – and it weighed 10 THOUSAND tons, making it heavier than the Eiffel Tower! And knowing the speed of the meteor when it exploded to be about 18.6 km/s, we can calculate the meteor’s kinetic energy to be
KE = ½·m·v2 = ½·(107 kg)·(18.6 × 103 m/s)2 ≈1.8 PJ
That amount of energy is 20 to 30 times the energy released by the atomic bomb dropped on Hiroshima in WWII. Luckily for Chelyabinsk residents, the meteor exploded about 15 miles above the Earth’s surface, so that energy didn’t cause as much damage as it had the potential to. Thousands were injured from windows that shattered by the exploding meteor’s sonic boom, but there were, thankfully, no casualties.
The Chelyabinsk meteor is indicative of a larger problem: we don’t know too much about small Near Earth Objects (or NEOs). NEOs are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that can cross the Earth’s orbit, meaning that they have the potential of hitting Earth under the right conditions. When it comes to larger NEOs (ones that are upwards of 500 m in diameter), we know of and track about 90% of them. But our technology is limited, and so we don’t know of the smaller ones until after they’ve already hit Earth. For example, we were able to look at seismographs after the Chelyabinsk meteor landed and find that it entered our atmosphere above Alaska about one minute before impact, but that knowledge was obtained only in hindsight.
This isn’t the first time this sort of impact has happened. Just over a century ago, in 1908, another meteor hit Tunguska, Russia. This meteor was 100 m in diameter – the length of a football field! – and it’s the largest NEO to hit Earth in recorded history. This impact leveled 830 square miles of land that was, fortunately, unoccupied, with the energy of 1000 Hiroshima bombs. It is enough energy to wipe out all of NYC, parts of NJ and Long Island, and a small part of Connecticut. Or, consider the K-T impact, which wiped out the dinosaurs.
So what exactly is the likelihood of an NEO hitting Earth? The truth is that they impact Earth in a fairly regular and predictable pattern, and so we can estimate how often they will impact and the amount of damage they might cause by their size. The following data is from NEOShield.net.
NEO diameter (m) larger than:
|Average interval between impacts (years)||Energy released (megatons of TNT)||Crater diameter (km)||Possible effects/comparable event|
|–||–||0.015||–||Hiroshima atomic bomb detonation.|
|30||300||2||–||Fireball, shock-wave, minor damage.|
|50||2000||10||≤1||Tunguska-type explosion or small crater.|
|100||10,000||80||2||Largest H-bomb detonation.|
|200||40,000||600||4||Destruction on national scale.|
|500||200,000||10,000||10||Destruction on continental scale.|
|1000||600,000||80,000||20||Many millions dead, global effects.|
|5000||20 million||10 million||100||Billions dead, global climate change.|
|10,000||100 million||80 million||200||Extinction of human civilization.|
While the chances of a larger NEO impacting Earth are relatively low, there are still a bunch of smaller asteroids that may potential enter Earth’s atmosphere at any moment, since we don’t currently have the technology to track them. CUNY CSI Professor Charles Liu estimates that it would cost something like $10,000,000 to build the necessary equipment to locate and observe these small rocks. If we broke that down to committing $1,000,000 each year over a decade, and divided that by the 7 billion inhabitants of our lovely planet, it would cost about $0.14 per person per year to fund such an apparatus! Of course, there are some people who would not be able to afford that, but to ask those that could spare even $1 per year to construct the technology that could save cities and millions of lives seems like it would be money well-spent.