The second biggest impact that Earth has experienced was probably the K-T event, which happened about 65 million years ago. An asteroid approximately 10 kilometres across hit what is now Chicxulub, in the Yucatan Peninsula in the Gulf of Mexico, with an explosive force of about 100 thousand billion tonnes of TNT - about 500 million times greater than that of the Hiroshima atomic bomb. The Chicxulub crater is now buried, but geophysical surveys estimate its diameter to be around 200 kilometres. The environmental changes that followed wiped out 60% of all species then living, including dinosaurs.

First of all, there was a rise in temperature, as energy from the impact heated the atmosphere. The energy was sufficient to fuse nitrogen and oxygen in the atmosphere to nitrogen oxides. Heat radiated in all directions, setting fire to vegetation and starting global firestorms. The impact also generated enormous tidal waves.

Because the meteorite hit sulphur-rich rocks, sulphur oxides were also added to the atmosphere. Then, as water vapour plus smoke and soot from the fires led to a drop in temperature, the nitrogen and sulphur oxides started to rain down as nitric and sulphuric acids.

All in all, conditions were hostile, to say the least. Only species that could escape the devastation, for example by hibernating or burrowing, or because they lived deep in the ocean, survived.

Although the K-T event had dramatic consequences, it was still not the largest impact that Earth has experienced. That happened right at the start of Earth's history, when Earth was hit by an object about the size of Mars.

	This is an extract from 'Collision course', chapter 3 of the Voyage in Space
	and Time booklet