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Like all of the terrestrial planets (and the icy objects in the outer Solar System), Earth has been subjected to innumerable impacts – large and small – by asteroids and comets. Studies of the ages of lunar craters indicate that Earth and the Moon underwent a very heavy bombardment prior to about 3.8 billion years ago. Indeed, the formation of the Moon is generally thought to be associated with a collision between Earth and a Mars‐sized intruder (see Chapter 4). Such a violent episode would have been characterized by global melting of Earth's crust, perhaps delaying the establishment of life.

A glance at the heavily cratered Moon shows that there must have been many millions of impacts during Earth's 4.6 billion year history. Yet, at the present time, there are only 190 confirmed impact structures on Earth's surface. They range in size from the 2,000‐km wide Vredefort feature in South Africa to a 15‐m diameter hollow in Kansas.

Since about 70% of Earth is covered in water, it is logical to assume that most impact craters are hidden beneath the sea. However, this alone can not account for the extremely modest number of visible impact structures compared with the Moon.

The main factor is Earth's much younger crust. Plate tectonics, combined with active weathering, erosion, and deposition, mean that the ancient crust, which recorded impacts prior to 200 million years ago, has largely been destroyed or covered by sediments (see Surface Features). Only a few of the biggest impact structures, some dating back more than 2 billion years, can still be found on the continents.

Since the end of the heavy bombardment, the number and size of the impacts has decreased dramatically. Even so, Earth is still exposed to occasional collisions, with an estimated population of 1,000 near‐Earth objects (NEOs) that are more than 1 km in diameter. However, a more likely threat comes from smaller objects, which are much more numerous. By February 2019, 1,947 NEOs had been classified as potentially hazardous (see Chapter 13).

An example of the potential threat was provided in 1908 when a 60‐m wide object – probably a meteorite – exploded near Tunguska, Siberia. Although it disintegrated 6–10 km above the ground, the blast leveled 80 million trees over an area of 2,000 sq. km.

A similar, but smaller, event occurred on February 15, 2013, when a 10–20‐m wide object exploded in the air above the Russian city of Chelyabinsk. The blast injured about 1,500 people and damaged more than 7,000 buildings, collapsing roofs and breaking thousands of windows.