According to a new theory, there were two giant impacts in a row, separated by 100,000 to 1 million years, involving a Mars-sized body called Theia and proto-Earth. In this image, the proposed hit
To date, nearly 28,000 near-Earth asteroids (NEAs) have been found by survey telescopes that continually scan the night sky, adding new discoveries at a rate of about 3,000 per year. But as larger
They can be natural, like supereruptions or a major asteroid impact of the scale that helped kill off the dinosaurs, or they can be human-made, like nuclear war or an engineered virus.
The risk of humanity extinction by giant asteroid impact is addressed. A 100 km sized asteroid impact may transform the Earth into an inhospitable planet, thus causing the extinction of many life forms including the human species. The exact reason for such a result remains nevertheless uncertain.
A very large asteroid impact 65 million years ago is thought to have contributed to the extinction of about 75% of marine and land animals on Earth at the time, including the dinosaurs. It created the 180-mile-wide (300-kilometer-wide) Chicxulub Crater on the Yucatan Peninsula.

Certainly something dramatic happened to life on Earth, because geologists have defined the K-T boundary and the end of the Mesozoic Era on the basis of a large extinction of creatures on land and in the sea. An asteroid impact, or a series of gigantic eruptions, or both, would have had major global effects on atmosphere and weather.

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major asteroid impacts on earth