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Imminent arrival of tardigrade aliens greatly exaggerated
The half a millimetre-long creatures that look like a tiny eight-legged caterpillar are resiliant beyond imagination and fascinate scientist as much as science-fiction writers
(L to R) SEM (scanning electron microscope) image of Milnesium tardigradum in active state and a drawing of a tardigrade on a grain of sand from 1840 [(L to R) doi:10.1371/journal.pone.0045682.g001 and E de Laplante - Doyere/both Creative Commons]

Lithopanspermia is a proposed method of life being distributed around the universe, whereby extremely hardy organisms might travel on rocks ejected from one planet that crash land on another. It means alien life forms travelling from one planet to another to start a new life.

The organisms that could potentially complete this journey would need to be very tough indeed. In the first place, they would need to survive within a rock travelling through space, which would subject them to very high doses of cosmic radiation.

The precise radioactive dose would depend on how long the rock is travelling between the two planets. Rocks that travel between Earth and Mars would be travelling so long that the dose received even deep in the centre of the rock would be enough to kill any known living organism.

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