The tardigrade is fondly nicknamed the water bear. It's absurdly ursine. Underneath the lens of a microscope, a see-through, segmented microbe plods into view. Its head turns left and right as if snuffling for a scent. It's less than a millimetre long with eight legs that are plump and clawed. In suspended animation, it curls up and hardens its shell; waiting for water like a seed. It's the first animal known to survive the vacuum of space.

A water bear is very tolerant. It can survive temperatures from -272 °C to 150 °C, pressure six times greater than at the bottom of the ocean, doses of radiation lethal to humans, and 30 years of no food or water. Since it withstands rather than thrives in the conditions of space, it isn't classified as an extremophile. It's a microscopic astronaut. Proof that life can survive off-world. Long enough, perhaps, to hop between Earth and Mars.

To find one, simply soak a piece of moss in water.

(Image credit: University of North Carolina at Chapel Hill)