About 50,000 years ago, an object about 150-meters across traveling at 45,000 mph created the Barringer Crater in Arizona that is 1,500-meters across. (Credit:Wikipedia-NASA). It was a 10-megaton explosion.
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 December 2024, astronomers at the IAU Minor Planets Center had cataloged about 2,437 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 shows the 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 is provided by the Vera Rubin Observatory (LSST) website.
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, yet 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!
On February 15, 2013 a once-a-century, 20-meter asteroid literally ‘came out of nowhere’ and exploded over the town of Chelyabinsk in Russia with 500 kilotons of energy, approximately 30 times the yield of the nuclear bomb over Hiroshima. There were many photos of this event taken but nearly all are copyright-protected and licensed. Here is a set of links to some representative images. Just Google ‘Chelyabinsk Meteor’ and you will find many more.
SciTechDaily.com….[Link Here].
RIA NOVOSTI/Science Photo Library – [Link Here]
Over 1,500 people were injured or hospitalized for cuts from flying glass. Had this event happened over New York City, over 100,000 people might have been hospitalized and most of the window glass from sky scrapers would be laying in the streets. Needless to say, the consequences of asteroidal impacts depend on the size of the asteroid. Here is a table of some possible consequences:
Size Yield Crater Effect
(Megatons) (km)
..........................................................
75 m 100 1.5 Land impacts destroy area
the size of Washington
160 m 500 3.0 Destroys large urban areas
350 m 5000 6.0 Destroys area the size of a
small state.
700 m 15,000 12 Land impact destroys
a small country
1.7 km 200,000 30 Severe climate effects.
Tsunamis destroy coastal
communities.
3.0 km 1 million 60 Large nations destroyed,
Widespread fires.
7.0 km 50 million 125 Mass extinction,
long term climate change.
16 km 200 million 250 Large mass extinction.
For the ocean impacts of objects about 300 meters 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 atmospheric concussion wave, which would flatten poorly designed buildings and cause fires just below the impact. Fortunately, humans occupy so little of the surface of 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 700 meter range would produce major tidal waves that would just reach the shores of many continents. In the 1 -2 kilometer range, these tsunamis would be 300 feet high and travel 20 or more kilometers inland putting at risk about 100 million people or 10 percent 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 dwellings 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!