This article was originally published in June 2019. It was the first in an ongoing series by our very own Property Chronicle NASA astrophysicist, to make sure we don’t take real estate, economics and investing too seriously and to keep our daily lives in perspective.
An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a potentially hazardous asteroid (PHA). Specifically, all asteroids that come closer to Earth than 0.05 AU or about 8 million km, and diameters of at least 100m (330ft to 490ft) are considered PHAs. By January 2016, astronomers had discovered about 1,651 PHAs that presented a possible hazard to Earth including impacts. About 153 of these are believed to be larger than one
The largest known PHA is (53319) 1999 JM8 with a diameter of ~7km, but it is not currently at risk of any impacts. The asteroid Ida, located in the asteroid belt outside the orbit of Mars is shown in the image below and measures 60km x 25km by 18km gives you some idea of what these huge rocks look like!
The smaller an asteroid, the more numerous they are, is the general rule of thumb for our solar system. According to the best estimates, objects 3m across impact the Earth every year and deliver about 2 kilotons of TNT of energy. Objects 100m across collide with the Earth every few hundred years and deliver about 2 megatons of TNT equivalent. A 1km-sized object impacts the Earth every million years or so and delivers about 100,000 megatons of TNT.
Now, the good news is that the Earth’s atmosphere shields us from objects that are initially below about 100m in size because they break-up and evaporate before reaching the ground. Still, the famous Tunguska Event in 1908 was a 50m stony meteor which evaporated about 20km above the Earth, and still flattened trees in a 30km area. Its yield was about 10 megatons of TNT, and the frequency tables predict that such strikes should happen every 100 years or so. In 2013 we got lucky again!
On February 15,
For the ocean impacts of objects about 300m across, the tsunami tidal waves produce more damage than an equivalent impact on the land. Had the Tunguska Event happened over a populated city, the damage would have been equal to a major earthquake exceeding 7.0 with perhaps thousands of people killed by the concussion which would flatten poorly designed buildings and cause fires just below the impact. Fortunately, humans occupy so little of the surface of the Earth that although these impacts happen about once every century or so, in the past no one has been around to see them.
Ocean impacts of bodies in the 700m range would produce major tidal waves that would just reach the shores of many continents. In the 1km to 2km range, these tsunamis would be 300ft high and travel 20km or more inland putting at risk about 100 million people or 10% of the world population. Such an impact would be known several days in advance by direct detection by NORAD so the question is whether enough people could make it to safety. Of course when they return to their coastal homes and cities, most of these would be severely damaged or washed away by the tremendous return tide!
I am not even going to mention collisions with larger asteroids which could occur every few million years or so. There would be enough devastation caused by the more frequent ‘super Tunguska’ events to keep us busy!
Article originally published by The Astronomy Cafe.
