Impact of Asteroids

Among the various theories which try to trace the cause of mass extinctions on the planet, one widely accepted theory hints at the impact of asteroids. It is backed by the fact that there do exist some examples of hard-hitting impact of these celestial bodies on the planet.
Asteroids are small celestial bodies composed of rock and metal, which revolve around the Sun. Their size can vary; some asteroids are large and roughly spherical in shape, due to which they appear like miniature planets, while others are small and irregularly shaped. The matter of these celestial bodies also differs. While asteroid 'Vesta' is believed to have a nickel-iron core, olivine mantle, and basaltic crust, asteroid '10 Hygiea' is composed of carbonaceous chondrite.
Asteroid Impact Facts
Most of the small asteroids are piles of rubble held together by gravity. Some asteroids supposedly have traces of amino-acids and other organic compounds, and it is assumed that the impact of asteroids on the Earth could have initiated life on the planet by seeding it with the necessary chemical substances.
Although, it is believed that the asteroids may have planted life on the Earth, they are also capable of having catastrophic effect on the planet. Asteroids with a diameter of 5 to 10 meters and energy equal to 15 kilotons of TNT enter the Earth's atmosphere approximately once every year, while those with a diameter of over 50 meters enter the Earth's atmosphere approximately once in a thousand years. Their impact can be devastating if they hit the Earth, but they disintegrate in the upper atmosphere and most of the particles are vaporized, thus rendering them harmless.
Extinction Events
In the distant past, our planet has experienced a massive climate change and large-scale extinction of animal and plant species ascribed to the impact of some large asteroids on the planet. The exogenesis theory suggests that the origin of life on the Earth must have been triggered by the impact of some asteroid carrying organic chemicals needed to initiate life. The Cretaceous-Tertiary extinction event, which led to the end of dinosaurs, and the Permian-Triassic extinction event, that led to the end of the Permian period finishing off 90% of the species on the planet, are believed to be triggered by asteroid impact.
Impact Craters
The craters on the solid bodies in the solar system, which were attributed to volcanic activities till 1960s, were actually formed due to the impact of celestial bodies. Researchers have determined that in the last 600 million years, at least 60 objects with a diameter of five km or more, have struck the Earth. Even the smallest of these possessed energy of ten million megatons of TNT, and would have formed a crater, approximately 95 km in diameter, on the surface of our planet.
The Rio Cuarto craters in Argentina are believed to have formed due to the impact of asteroids that struck this area around 10,000 years ago. In 1490, 10,000 people reportedly lost their lives due to a hail of stones from the sky. This is believed to have been caused as a result of disintegration of a large asteroid after it entered the Earth's atmosphere. In 1908, a mid-air explosion of an asteroid destroyed 80 million trees near the Podkamennaya Tunguska River in Siberia. In many cases, the sighting of foreign bodies like asteroid or meteoroids is reported as a fireball in the sky.
Millions of asteroids will pass by the Earth in the years to come. The Lincoln Near-Earth Asteroid Research (LINEAR) and the Near Earth Asteroid Tracking (NEAT) are projects initiated by NASA to detect asteroids or other such foreign bodies coming close to the Earth. The need of the hour is to develop an intercept system in space to destroy or deflect foreign objects heading towards our planet, before they prove catastrophic.