The “tun” is a survival state, not a spacesuit
When conditions become dangerously dry, some tardigrades curl their legs inward and contract into a compact, shrunken form called a tun. This is part of cryptobiosis: a state in which normal activity falls so low that growth and reproduction effectively stop. The animal is not actively functioning in the way it does when hydrated; it is waiting out conditions it could not withstand while active.
Drying changes what the body has to protect
The key is losing most of the water in the body. Water is essential to ordinary cell chemistry, but it can also form damaging ice crystals as temperatures fall. In a tun, the tardigrade’s cells are dried and stabilized by protective molecules, including specialized proteins. These help keep cell structures from collapsing as the animal loses water and faces other stresses.
That preparation matters in space. Vacuum can pull water from an exposed animal, while the absence of air changes how heat moves. A hydrated tardigrade would not simply carry on living in those conditions. A tun, by contrast, can withstand some periods of exposure and resume activity if it later encounters water and conditions are suitable.
Tough does not mean invulnerable
Cryptobiosis is a pause with limits, not immunity to anything space can do. Survival depends on the species, the exposure, and the conditions; damage can still accumulate. The remarkable trick is more specific: by becoming nearly dry and dormant before the worst conditions arrive, a tardigrade can endure stresses that would quickly overwhelm its active, water-filled body.
