One of the most popular end of the world scenarios depicted in science fiction and Hollywood blockbusters is the impact event, when a large celestial object such as a comet, asteroid, planet or meteorite on a collision course with Earth threatens to devastate the planet. This is not pure fantasy, we are constantly bombarded from space, but fortunately there appears to be an inverse relationship between the size and frequency of impact events. Small celestial objects such as meteorites frequently collide with Earth but are either vapourised by the atmosphere or are too small to have an effect when they hit with the Earth's surface. However, when larger objects strike the planet the impact can cause abrupt, global changes that effect climate, biodiversity and even the Earth's magnetic field.
The last major impact collision occurred some 65 million years ago. It is called the K-T boundary event and marked the end of the Cretaceous and start of the Palaeogene period, but it is popularly known as the event which most probably caused the extinction of the dinosaurs. An asteroid estimated to be approximately fifteen kilometres wide slammed into the Earth at Chicxulub in Mexico at a speed twenty times faster than a speeding bullet and hit the Earth with unimaginable force estimated at one billion times more powerful than an atomic bomb. There was an immediate effect as huge fireballs, earthquakes, continental landslides and tsunamis caused widespread devastation. The blasted material ejected at high velocity into the atmosphere shrouded the planet in darkness producing a global winter which wiped out about seventy percent of known species.
The largest impact event to have occurred in modern recorded history was the Tunguska blast, an explosion that took place over remote Siberia in 1908. Eight hundred square miles of forest was destroyed and an estimated eighty million trees were flattened lying in a radial pattern which pointed to the blast's epicentre. The seismic shock waves reached England and the night skies glowed in Asia. It is believed that the blast was caused by an asteroid about 35 metres wide which entered Earth's atmosphere travelling at a speed of about 33,500 miles per hour and heated the air surrounding it to 44,500 degrees Fahrenheit. At a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs. Because most of the asteroid was consumed in the explosion it did not cause an impact crater.
Impact events can have unimagined consequences; the K-T boundary proved disastrous for dinosaurs but indirectly produced the opportunity for the rise of mammals. The theory of exogenesis hypothesises that the complex organic matter that is the foundation of life and is contained in comets, asteroids and meteorites is spread around the universe by impact events.