Facing drying, a tardigrade retracts its legs, expels most of its body water and contracts into a tun. It synthesises protective molecules, including trehalose in some species and a group of intrinsically disordered proteins that vitrify the cytoplasm into a glass-like solid, immobilising cell structures rather than letting them collapse.
In that state tardigrades have survived temperatures near absolute zero and above 100 degrees Celsius, pressures far exceeding the deep ocean, and thousands of grays of ionising radiation. In 2007 tuns of two species were exposed to the vacuum and radiation of low Earth orbit on the FOTON-M3 mission and some survived, the first animals shown to endure open space.
The qualifiers matter. Exposure durations in these experiments are often short, survival rates are frequently low, and hydrated tardigrades are far more vulnerable. They are not, as often claimed, unkillable; they are exceptional at pausing. How the disordered proteins work in molecular detail is still being worked out.