In 2007, a group of microscopic animals left Earth aboard a Russian satellite, were exposed to the vacuum of space for ten days, and returned alive. Some even laid eggs that hatched successfully. The animals weren't genetically engineered superorganisms or the product of decades of selective breeding. They were tardigrades—eight-legged creatures smaller than a poppy seed that have been plodding across moss and lichen for 600 million years.

The Paradox of Fragility and Invincibility

Tardigrades look absurdly vulnerable under a microscope. Their pudgy bodies and stubby legs give them the appearance of a child's drawing of a bear. They move slowly (hence their name, which means "slow steppers") and feed on simple organisms in water films around moss and soil. Nothing about their lifestyle suggests they should survive conditions that would instantly kill almost anything else on Earth.

Yet these animals can withstand radiation doses of 5,000 to 6,000 Gray—a thousand times what would kill a human. They've survived temperatures near absolute zero and as high as 150°C. They've endured pressures that would crush most life forms and even survived being shot from a gun at nearly 3,000 feet per second. When Israeli lunar lander Beresheet crashed on the Moon in 2019, thousands of dehydrated tardigrades were aboard. They might still be there, in suspended animation, waiting for conditions that will never come.

The Tun State: Shutting Down to Survive

The key to tardigrade survival lies in a trick called anhydrobiosis. When their environment dries out, terrestrial tardigrades lose almost all their body water and curl into a compact form called a tun. In this state, their metabolism drops to 0.01% of normal levels—essentially pausing life itself.

This isn't simply dehydration. As water leaves their cells, tardigrades produce special proteins that protect their cellular machinery. CAHS proteins (cytoplasmic abundant heat soluble) form gel-like structures that stabilize proteins and membranes. Other protective molecules replace water molecules, preventing cellular collapse. The process takes hours, during which the tardigrade carefully orchestrates its own shutdown.

When water returns, tardigrades rehydrate within hours. Their metabolism restarts, their legs unfurl, and they resume their slow march across their microscopic world. Individuals have been revived after 30 years in a freezer.

Why Radiation Doesn't Kill Them

The space experiments revealed something unexpected: tardigrades don't actually prevent radiation damage. When researchers examined their DNA after radiation exposure, they found the same breaks and lesions that occur in human cells. Tardigrades accumulate damage just like any other organism.

What makes them different is their repair system. In 2016, researchers identified a protein called Dsup (Damage suppressor) unique to tardigrades. Dsup binds to DNA through electrostatic attraction—its positively charged regions latch onto the negatively charged DNA backbone. This binding serves two purposes: it physically shields DNA from some radiation damage, and at high concentrations, it helps compact fragmented DNA pieces, maintaining genome organization even when the double helix breaks.

When scientists inserted the Dsup gene into human cells, those cells showed reduced DNA damage from radiation-mimicking drugs. The protein worked across species, suggesting it's a straightforward physical mechanism rather than something requiring tardigrade-specific cellular machinery.

The Repair Crew

Dsup isn't the whole story. A 2024 study identified another protein, TDR1 (Tardigrade DNA Repair protein 1), that tardigrades produce specifically in response to radiation. Unlike Dsup, which appears to work preventatively, TDR1 is part of the cleanup crew. It enters the cell nucleus, binds to damaged DNA, and somehow facilitates repair.

What surprised researchers was that tardigrades also use the same DNA repair genes found across all life—from bacteria to humans. When exposed to radiation, tardigrades upregulate these ancient, conserved repair pathways. They're not using alien technology; they're using the same tools other organisms have, just more effectively and in combination with their unique proteins.

The DNA damage gradually disappears after exposure, sometimes within hours. This efficiency distinguishes tardigrades from organisms like bacteria, which can tolerate radiation but take much longer to repair damage, or from organisms that prevent damage through different mechanisms, like thick protective pigments.

Space Changes the Equation

The 2007 FOTON-M3 mission exposed tardigrades to conditions no amount of evolutionary pressure on Earth could have prepared them for. In low Earth orbit, they faced not just cosmic radiation but also solar UV radiation and the vacuum of space simultaneously.

Some tardigrades were exposed to space vacuum alone. Others got vacuum plus full solar UV radiation. The survival rates varied dramatically depending on exposure type, but some individuals in every group survived. More impressive: females laid eggs during the mission, several hatched, and the newborns showed normal development.

This suggests tardigrades' survival mechanisms are robust enough to handle combinations of stressors they've never encountered. The tun state protects against vacuum and temperature extremes. The DNA repair systems handle radiation. Together, these independent adaptations—evolved for surviving terrestrial extremes like desiccation and background radiation—accidentally made tardigrades space-capable.

What Tardigrades Tell Us About Life's Limits

Tardigrades didn't evolve to survive space. They evolved to survive Tuesday afternoon when the moss dried out. The fact that these adaptations work in space tells us something about the nature of extreme environments: they might not be as different from each other as we think.

The proteins that protect tardigrade DNA could inform radiation protection for human cells, particularly for cancer patients undergoing radiotherapy or astronauts on long-duration missions. Researchers have already demonstrated that Dsup reduces DNA damage in human cells. Whether this could translate to practical applications remains unclear, but the principle works.

Perhaps more significantly, tardigrades expand our sense of what's possible for life. If a microscopic animal that evolved in moss can survive the Moon's surface, what else might be out there, equally unprepared but accidentally capable? The tardigrades on Beresheet's crash site won't grow or reproduce—the Moon has no water to revive them. But they're probably not dead either. They're waiting, in their tuns, in a place where nothing was supposed to survive at all.