What would happen if a large object hit the Earth?

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!

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. This photo taken by a dash camera shows the event. 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.

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 or Paris)

160 m         500          3.0           Destroys large urban areas

350 m        5000          6.0           Destroys area the size of a
                                         small state. Ocean impacts produce

700 m        15,000         12           Land impact destroys Virginia, 
                                         Tiawan and ocean impact causes 
                                         major tsunami.

1.7 km      200,000         30           Land impacts affect climate, 
                                         global destruction of ozone, 
                                         tsunamis destroy coastal communities.

3.0 km      1 million       60           Large nations destroyed, widespread
                                         fires from ejecta, climate change.

7.0 km      50 million      125          Mass extinction, global conflagration, 
                                         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 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 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 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!

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.