It would be the end of the world as we know it. A relatively small lump of rock – a small asteroid, perhaps only a few hundred metres across – plunging to Earth would devastate a continent or trigger tsunamis. Civilisation would be set back several centuries.

It is a real risk, say a group of astronauts and astronomers who are to highlight the threat facing humanity by marking 30 June as Asteroid Day. Supporters include Martin Rees, the astronomer royal; guitarist Brian May; biologist Richard Dawkins; Apollo 9 astronaut Rusty Schweickart; Nobel laureate Sir Harold Kroto; British astronaut Helen Sharman; and cosmonaut Alexey Leonov. Their aim is to highlight the dangers facing Earth and to help raise funds to build satellites to track deadly asteroids that may be lurking in near-Earth space.

The chosen date marks the anniversary of the “Tunguska event”, when a small asteroid or comet exploded above Siberia with the force of 1,000 atomic bombs. The largest impact event on Earth in recorded history, it occurred on 30 June 1908.

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