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What would happen if a large object hit the Earth?

Astrophysicist

So that we don’t take Real Estate, economics and investing too seriously and to keep our daily lives in perspective, here is our very own Property Chronicle NASA Astrophysicist and his column ‘Nothing Really Matters’.

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 100 meters (330 to 490 ft) 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 kilometer in diameter.The graph below shoes te cumulative number of PHAs detected since 1999 and you can see that the pace of finding new ones has decreased significantly. This means that we have discovered virtually all of these objects by the present time. Even so, if just one of these rare undiscovered birds were to strike earth it would be catastrophic, so we need 100% to be discovered. This figure, by the way, comes from the Wikipedia page for ‘Potentially Hazardous Asteroids’.

The largest known PHA is (53319) 1999 JM8 with a diameter of ~7 km, 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 3 meters across impact the Earth every year and deliver about 2 kilotons of TNT of energy. Objects 100 meters across collide with the Earth every few hundred years and deliver about 2 Megatons of TNT equivalent. A 1 kilometer-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 100 meters in size because they break-up and evaporate before reaching the ground. Still, the famous Tunguska Event in 1908 was a 50 meter stony meteor which evaporated about 20 kilometers above the Earth, and still flattened trees in a 30 kilometer 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!

Astrophysicist, zLead Article, zNewsletter

About Sten Odenwald

Sten Odenwald

Dr. Sten Odenwald is an award-winning astrophysicist and prolific science popularizer, who has been involved with science education for the COBE, IMAGE, Hinode and InSight spacecraft, as well as the Sun-Earth Connection Education Forum. He is currently the Director of Citizen at the NASA Goddard Space Flight Center in Maryland. His most recent book to be published by Arcturus Publications in London is 'A Degree in Cosmology'. Visit arcturuspublishing.com or his education website at the Astronomy Cafe (sten.astronomycafe.net) for details.

Articles by Sten Odenwald

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