In 2007, researchers exposed dehydrated tardigrades—eight-legged animals less than a millimetre long—to the vacuum of space for ten days. Those shielded from the Sun’s ultraviolet radiation survived at rates similar to Earthbound controls, then laid eggs that hatched normally after they were rehydrated.

In September 2007, a Russian Foton-M3 capsule carried trays of dehydrated tardigrades into low Earth orbit. Once the spacecraft reached altitude, a European Space Agency exposure platform opened those trays directly to space.

For ten days, the animals experienced a near-perfect vacuum and the natural radiation environment 258 to 281 kilometres above Earth. Some samples were protected from sunlight. Others received ultraviolet wavelengths that the atmosphere normally prevents from reaching the ground.

When the capsule returned, researchers added water. Tardigrades exposed to vacuum without solar ultraviolet radiation revived at rates broadly comparable to dehydrated controls that had remained on Earth. The recovered adults then laid eggs, and those eggs hatched at statistically similar rates to the controls.

The result is often compressed into the claim that tardigrades can live in space. The actual finding is narrower and more useful. These were inactive, desiccated animals, not tiny astronauts walking around in a vacuum. Their survival depended strongly on whether the Sun could reach them.

They went into orbit as dried tuns

Tardigrades are a phylum of microscopic, eight-legged animals commonly called water bears. Most are well under a millimetre long. They require a film of water when active and can be found in marine and freshwater habitats as well as mosses, lichens and soils.

Many terrestrial species can survive the disappearance of that water by entering anhydrobiosis. The animal retracts its legs, contracts into a compact shape called a tun and loses almost all free water. Metabolism falls towards levels that instruments struggle to distinguish from zero.

This is a form of cryptobiosis, not ordinary sleep. ScienceBlog’s recent account of a 46,000-year-old nematode revived from permafrost described a related strategy: life processes pause until water and suitable conditions return.

The dehydrated state was essential to the 2007 result. The experiment did not establish that an active, hydrated tardigrade could be placed in open space and survive. It tested animals biologically prepared for water loss before launch.

TARDIS separated vacuum from sunlight

The project was called TARDIS, for Tardigrades in Space. It flew within the Biopan-6 facility provided by ESA on Foton-M3, launched from Baikonur Cosmodrome on September 14, 2007.

The team used adults of two species, Richtersius coronifer and Milnesium tardigradum. According to the peer-reviewed report in Current Biology, the experiment divided flight samples among three exposures.

One group experienced space vacuum while being shielded from solar ultraviolet light. A second received vacuum plus ultraviolet A and B, wavelengths from 280 to 400 nanometres. The third received vacuum and the full solar ultraviolet range from 116.5 to 400 nanometres, including short wavelengths blocked by Earth’s atmosphere.

All flight samples also encountered ionising solar and galactic cosmic radiation. Ground controls remained dehydrated under ordinary laboratory conditions. This structure allowed the researchers to distinguish the effects of vacuum and background space radiation from the added damage caused by direct sunlight.

The capsule returned on September 26 after twelve days away from Earth, but the Biopan trays had been open to space for ten of those days. The results appeared in 2008. The distinction between mission duration and exposure duration explains why summaries sometimes give different numbers for the same flight.

Microbial spores, seeds and lichens had already shown forms of survival after exposure beyond a spacecraft. The TARDIS team described its result as adding the first animal to that short list, an important distinction because an animal body coordinates many specialised cells and tissues rather than persisting as one cell or a dormant seed.

Vacuum alone did not reduce overall survival

After recovery, the researchers rehydrated the animals and followed them for more than three weeks. A tardigrade was recorded as vital only when it displayed coordinated leg movement, not merely a twitch or an intact-looking body.

Across both species, the vacuum-only samples and controls followed broadly similar survival curves. The paper found no significant difference in their comparisons except at one late observation for R. coronifer. That isolated result did not change the overall conclusion that exposure to vacuum produced no general loss of survival.

This does not mean vacuum is harmless. In an ordinary active animal, low pressure causes dissolved gases to expand and exposed water to evaporate or boil. A recent ScienceBlog examination of documented human vacuum exposures shows how quickly an oxygen-dependent body becomes incapacitated.

A tun starts from a different physiological condition. The tardigrade has already removed most of the water that vacuum would pull from its tissues and suspended the chemistry that normally depends on that water.

Ultraviolet light was the harder test

Solar ultraviolet radiation changed the result sharply. Full-spectrum exposure killed nearly all the animals. No R. coronifer in that group revived, while only a small fraction of M. tardigradum recovered.

The ultraviolet A and B treatment was also damaging, although outcomes differed between the species. ESA’s summary of the experiment identified ultraviolet radiation as the most difficult part of the space environment.

The contrast explains why “survived space” needs a qualifier. Vacuum, cosmic radiation and dehydration were tolerable for many of the shielded tuns over ten days. Direct solar ultraviolet exposure was not. A tardigrade’s reputation for indestructibility is an exaggeration; the animal has a remarkable but measurable survival envelope.

The eggs were laid after the flight

Revival was only the first test. Adults that recovered from the vacuum-only exposure were maintained after rehydration and allowed to reproduce.

For both species, eggs laid by the flight animals hatched at rates that did not differ statistically from eggs laid by the Earthbound controls. This is the basis for saying the survivors reproduced normally. It refers specifically to hatching success, not proof that every later measure of health or lifespan was unchanged.

The timing also matters. The adults did not lay these eggs while sitting outside the spacecraft. They were dehydrated during exposure, returned to Earth, received water and only then resumed activity and reproduction.

The study included a separate test in which eggs themselves went into orbit. Eggs protected from ultraviolet light hatched at control-like rates after direct exposure to vacuum and cosmic radiation. None of the eggs exposed to solar ultraviolet light hatched. Once again, sunlight rather than vacuum set the sharper boundary.

A cell can survive only if its structure survives drying

Removing water from a cell is normally destructive. Membranes deform, proteins lose their working shapes, molecules aggregate and reactive chemistry damages DNA. Tardigrades do not simply tolerate that chaos after it happens. They change the physical state of their cells as they dry.

Research since the Foton mission has identified tardigrade-specific proteins that can form protective networks or glass-like matrices. These materials immobilise sensitive molecules and help preserve membranes and proteins until water returns. Antioxidant systems and DNA repair contribute as well, with different tardigrade lineages using different combinations.

NASA molecular biologist Thomas Boothby described the dry interior as becoming increasingly viscous and then glass-like in a NASA discussion of water bears in space. Rehydration dissolves that matrix and releases stabilised cellular components to work again.

The promise is not limited to spaceflight. Researchers have attempted to use tardigrade-inspired chemistry to stabilise medicines such as insulin. Preserving a biological molecule without continuous refrigeration is an engineering version of the problem a tun solves, although translating a whole-animal survival strategy into a reliable medical material requires separate testing.

Later missions asked a different question

TARDIS demonstrated survival after direct exposure. It did not explain every gene or protein responsible, and it did not test a population living actively for generations in space.

NASA’s Cell Science-04 investigation later carried active Hypsibius exemplaris tardigrades aboard the International Space Station. Rather than opening them to vacuum, the study kept them inside a culture system and examined short-term and multigenerational changes in gene expression under spaceflight conditions.

The NASA investigation overview describes the goal as identifying genes required for adaptation and survival under extreme stress. That is a different experiment from asking whether a dried animal can persist outside a spacecraft, but it follows directly from the mystery TARDIS exposed.

Ten days in orbit is not immortality

The 2007 result does not show that tardigrades could cross interplanetary space unprotected. Ten days in low Earth orbit is short compared with a trip between planets, and the radiation environment beyond Earth’s magnetic protection is harsher. The animals were shielded during launch, re-entry and recovery.

It does not establish that a tardigrade could live, feed and reproduce in a vacuum. It does not test years of accumulated molecular damage, impact into another world or whether a viable population could establish itself there.

What it does establish remains extraordinary. A multicellular animal can suspend its active biology, endure ten days of direct space vacuum and resume coordinated movement when water returns. Survivors can then produce eggs whose hatching success is indistinguishable from that of carefully matched controls.

The shield was not incidental and the dehydration was not fine print. They were the experiment’s central lesson. Tardigrades survived space not by ignoring its hazards, but by entering a state in which vacuum had far less left to take away.