If you ever needed reminding of how very fragile our existence is on our rocky little planet, consider this: on December 18, 2018, at around noon local time for the Bering Sea, a meteor crashed through Earth’s atmosphere traveling roughly 32km/s (20 miles per second), on a steep trajectory of seven degrees.
The cosmic interloper, measuring about 10 meters (32 feet) in diameter and weighing some 1,400 tons, exploded 25.6km (15 miles) above our planet’s surface with an impact energy of 173 kilotons.
“That was 40% the energy release of the Chelyabinsk fireball, but it was over the Bering Sea, so it didn’t have the same type of effect or show up in the news,” said Kelly Fast, Near-Earth Objects Observations Program Manager of NASA.
For reference, the Chelyabinsk fireball, which took place in 2013 and will be remembered by some for the stunning dash-cam footage that appeared all over social media at the time, was accompanied by a shockwave that caused roughly 1,200 injuries through shattered windows and other damage to surrounding buildings.
Chelyabinsk was the largest object to hit our planet since the Tunguska Event of 1908 that destroyed 2,000 square kilometers of Siberian forest.
Putting the Bering Sea event into perspective: this release of energy, which amounts to about 724 trillion joules, is equal to 724,000 average lightning strikes.
And while that may sound like a lot, keep in mind that in an average hurricane, that much energy is released through cloud and rain formation every single second. Still, this was comparable to a release of energy 10 times that of the atomic bomb that was dropped on Hiroshima.
But what exactly causes an atmospheric meteor explosion?
Air-burst events for meteors of a certain size are actually fairly common. If the meteor comes in at the right trajectory, enough air can become compressed in front of it, causing it to super-heat.
As this happens, the outer layers of the space rock begin to ignite, and thus, a fireball is born. As the pressure increases on the front edge to a critical point, the strength and durability of the rocky material is overcome and is broken so rapidly that a huge amount of kinetic energy is released.
If a meteor is too large, it will still ignite a little, but won’t likely shatter. Rather, it will impact on the surface of the planet, instead.
If, however, the meteor is too small, it will simply burn up completely in the atmosphere. This is similar to what happens during the regular annual meteor showers we see every few months.
The December 2018 explosion went largely unnoticed until now because it violently disintegrated over the Bering Sea, just off Russia’s Kamchatka Peninsula. Military satellites picked up the blast and NASA was notified of the event by the US Air Force.
Lindley Johnson, Planetary Defense Officer at NASA, told BBC News that a fireball this big is only expected about two or three times every century. Dr Johnson went on to tell BBC News that the space rock was close to routes used by commercial flights and NASA researchers have contacted airlines that fly there to find out if any pilots spotted the explosion.
Blasts that are considered to be “problems without passports” because of the devastation they could cause to entire regions if they come into contact with Earth’s atmosphere, are from asteroids or meteors that are 140 meters or larger in size. For reference, the Chicxulub impactor that is thought to have wiped out the dinosaurs, was 180 kilometers (112 miles) across.
But not to worry, in 2005, Congress requested that NASA work to find 90% of the near-Earth asteroids that could cause catastrophe, and NASA has been working diligently with the US Government to achieve this goal.
In the meantime, if you want to spot the next fireball that comes our way, look up! Because you never know what you’ll see, if you just keep your eyes to the skies.