When conditions dry, many tardigrade species contract into a barrel shape called a tun, retract their legs, and lose the great majority of their water. Intrinsically disordered proteins unique to the group form a glass-like matrix inside the cells, stabilising membranes and proteins that would otherwise collapse. Metabolism becomes undetectable.
In this state tardigrades have survived exposure to vacuum and solar ultraviolet in low Earth orbit, temperatures near absolute zero and above the boiling point of water in laboratory tests, and very high doses of ionising radiation. Rehydration can restore activity, though survival rates in these experiments are often well below one hundred per cent.
The popular framing of tardigrades as indestructible is wrong in two ways. Active, hydrated animals die readily from heat, drying and crushing. And survival in orbit is not the same as living in space: the animals were dormant and passive throughout. How the disordered proteins protect cellular structures in detail is still being worked out.