The collision, except for those near the impact, would unlikely be fatal to the world’s population. But it would throw up so much dust that billions of people could expect a slow death.
Huge quantities of dust would bring an "eternal winter" in which the sun would be obscured and crops would fail, leading to mass famine.
Dr Dave Rothery, a vulcanologist, foretells a similar end, but thinks the dust would be spewed into the atmosphere by a super-volcano.
If one erupted, around 400cu km of molten rock and debris would be blasted into the sky, much of it remaining in the atmosphere as volcanic dust.
"It would put so much ash and sulphur dioxide into the atmosphere that photosynthesis may break down," he warned. A similar, though less devastating, eruption took place in 1816 when a volcano in Indonesia put so much dust into the atmosphere it became known as "the year of no summer".
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