The risk of an asteroid hitting Earth
In less than twelve months, one asteroid flyby, one comet near-miss and one asteroid hitting Earth became news.
An asteroid hitting Earth in the news
2014 started with a blast when the Chelyabinsk meteor exploded spectacularly over Russia in February. At 20m/65ft in diameter, the former Near-Earth Asteroid (NEA) detonated well above the ground. Despite causing extensive damage, there were no fatalities, mostly injuries from flying glass or the shockwave. There were also reports of sunburn from the flash.
By October, the Robots of Mars were preparing to face their own catastrophe. Comet Siding Spring, an icy, asteroid-like object from the Oort Cloud was on it’s way to Mars. At first it was thought that the comet might collide with the Red Planet, but later it was determined to be a near-miss. The Martian orbiters ducked behind the planet to avoid being hit by comet fragments. Once the danger passed, everyone reported in, safe-and-sound.
In January 2015, a 300m/985ft asteroid with the catchy name 2004 BL86 performed a close flyby of Earth. It passed at about three times the distance from the Earth to the Moon. In addition to being fairly large, it was also notable for having it’s own tiny, orbiting moon.
How many Near-Earth Objects are a concern?
As of February 08, 2015, 12202 Near-Earth objects (NEO) have been discovered. NEO’s include comets and asteroids that will reach at least 1.3AU at closest approach to the Sun, close to the Earth’s 1AU orbit.
Of the NEO’s, nearly 10000 are Near Earth Asteroids (NEA). These will come within 0.983AU and 1.3AU of the Sun.
The NEA’s feature 867 are larger than 1km/0.62mi in size. Even at 1km, most of these are much smaller than the 10km/6.2mi dinosaur killer. Considering the damage from the 20m/65ft Chelyabinsk meteor, even a relatively small comet or asteroid hitting Earth could do a lot of damage.
NASA maintains a list of the NEO’s that will pass nearby over coming months. Despite stories about NEO’s being quite rare in the media, the list shows that at least one NEO passes the Earth almost every day of the year. Most of the NEO’s pass beyond the orbit of the Moon. Of the few that get really close, some are a similar size to the Chelyabinsk meteor, but most are smaller.
In total, 1546 NEO’s have been classified Potentially Hazardous Asteroids (PHAs). Reassuringly, there’s no significant risk in the immediate future. The few that could hit the Earth decades from now only have a slim chance of doing so.
But what about the objects that escape detection?
Should I worry about an asteroid hitting Earth?
Small asteroids and dust are constantly bombarding the Earth. Objects 4 meters/13 feet in diameter arrive about once a year. Up to 7 meters/20 feet every 5 years. Asteroids up to 50m/165ft in size every 1000 years.
These objects are travelling at enormous speeds when they arrive. Entering into the atmosphere generates a lot of heat. If they don’t burn up as falling stars, they often explode in the air. This is known as air-burst event. Between 1975 and 1992, military early warning satellites picked up 136 major explosions in the upper atmosphere that would otherwise have gone unnoticed.
While historical records report events that could be an asteroid hitting Earth and killing people, there are no confirmed reports of any deaths in modern times.
Larger objects that survive the atmosphere, or cause serious shockwaves when exploding, are quite rare. The Tunguska event, which levelled a forest in Siberia, Russia, was caused by an object 60m-190m/200-640ft exploding 5-10 kilometres/3-6 mi above the ground. This sort of event can occur once every 2000 to 36000 years.
In addition to protecting us from the Sun’s radiation, the Earth’s atmosphere is doing a fine job keeping us safe from the majority of small meteors heading our way.
The remaining risk is from the larger asteroids that could cause regional or worldwide devastation. Being so big, these are usually easier to detect. With sensing technology continually improving it will be increasing difficult for one of these nasties to sneak up on us without plenty of warning.
Should we one day discover a dinosaur killer (or any other extinction event) with our name on it, will we be able to work together as a species to do something about it? That worries me more than any rock falling to Earth.