生命能等多久？

How Long Can Life Wait?

Just because you haven’t moved from the sofa doesn’t mean you’re dead.

Maciej Cegłowski

Nov 07, 2026

How long can an organism remain inert and still be viable when conditions improve?

The question has big implications in three areas of space exploration. On time scales of years to decades, it dictates our approach to contamination risk when building space probes. On time scales of centuries to millennia, it has implications for how life may have spread around the early Solar System. And if individual organisms can survive for geological time periods—tens to hundreds of millions of years—then this greatly increases the likelihood of our finding life in asteroids and on Mars.

Survival in spaaaace

The first place the question of viability comes up is the problem of contamination. Rockets are highly septic structures that sit outside in the Florida sun and launch (from a microbial perspective) absolutely oozing life. When sending spacecraft to potentially habitable worlds, it’s important to know whether we can depend on harsh conditions in space to sterilize them, or have to try to do it ourselves. The question is particularly acute for Mars, where transit times are short, and where we already know conditions on the surface are not lethal enough to eliminate survivors.

The uncontested killer of organisms in space is ultraviolet light, which disrupts DNA after just a few seconds of exposure. Ozone and atomic oxygen in the upper atmosphere prevent short-wavelength ultraviolet light from reaching us on Earth, but organisms on the hull of a spacecraft are exposed to the full spectrum of these damaging rays. (The surface of Mars get a sterilizing dose too, though not as bad as free space.) Experiments in orbit have shown that it is UV light, far more than cold, vacuum, or cosmic radiation, that makes space lethal for microorganisms.

But those same experiments show that it takes very little to shield organisms from the worst of UV exposure. A thin film of dust, or a crevice in a spacecraft, or even just the bodies of fallen comrades, are all sufficient to protect surviving microbes a layer or two down. And once there’s shade to hide behind, the little ones do quite well for themselves.

In a three-year experiment begun in 2015, Japanese scientists hung plates covered with bacteria off the side of the International Space Station to see how well they would fare there. They found that after three years, colonies of deinococcus radiodurans were not just revivable, but in better shape than a control group that spent that time inside the space station. By their estimates, a pellet of D. radiodurans 1 mm in size should be able to survive direct exposure to interplanetary space for 2-8 years, and potentially stay viable there for decades if shielded from sunlight.

An interesting aspect of the Tanpopo research is the fact that D. radiodurans doesn’t have special defenses against DNA damage. If you just look at how badly it gets beat up by exposure to radiation and vacuum, it fares no better than bacteria like E. coli, which can’t survive in space at all. But D. radiodurans turns out to be a champion healer, outshining everyone in its ability to repair DNA, including double-strand breaks that cleave the molecule into fragments. Instead of protecting DNA directly, the bacterium employs chemical defenses (antioxidants) that protect the repair enzymes themselves.

The Tanpopo results and experiments like it are bad news for Mars exploration. They imply that live microbes from Earth have already reached Mars and are vegetating under the carapaces of landers. Whether these grim survivors pose a contamination hazard depends on how effective a barrier the Martian surface is to would-be colonizers, and whether transport pathways exist connecting the surface to any habitable Martian environment, which would presumably be deep underground.

The remarkable longevity of microbes in space also vindicates NASA’s policy of crashing expensive deep space probes into Jupiter and Saturn at the end of their missions instead of leaving them in uncontrolled orbits that, decades or centuries hence, could lead to an impact with a potentially habitable world like Enceladus or Europa.

Life inside a meteor

A second context where viability matters is in trying to evaluate lithopanspermia scenarios for the origin of life in the early Solar System. In these scenarios, a large impactor hits Planet A and ejects life-bearing rocks into deep space. These orbit the Sun for a while before eventually falling on Planet B. Because of the relative escape velocities and energies involved, Planet A in these liaisons is almost always Mars, and Planet B is usually Earth. If organisms can remain viable inside frozen rock for periods of thousands or hundreds of thousands of years, this dramatically increases the likelihood that life in the early Solar System could have moved between planets, and that life on Earth might have a Martian origin.

Lithopanspermia started off as a fringe idea, almost a provocation, put forth by Lord Kelvin in the 1870s to explain the origin of life on Earth. But time has been kind to the theory, and in recent decades it has found itself propelled to the edge of straight-up respectability by a series of important discoveries.

The first breakthrough came in the 1970s, when meteorite studies showed that heating during atmospheric entry was limited to the outer centimeter or two of rocky meteorites. This meant any meteorite larger than a golf ball could land on Earth without having its interior sterilized. If anything inside it was still alive, it had a chance of emerging intact, 1950s sci-fi movie style, to conquer the Earth.

The second big finding was an impact model developed in the 1980s to explain the enigmatic presence of small pieces of Martian rock on Earth. Scientists knew the rocks had to be from Mars because tiny inclusions of air trapped inside them had the same isotopic signature as the Martian atmosphere, as sampled by Viking. Clearly some cataclysm had been necessary to fling these rocks into space, but they had survived it without being vaporized or melting. In 1984, Jay Melosh, an expert on crater formation, found that there was a zone near large meteor impacts where direct and reflected shock waves could interfere to propel sheets of surface rock into space without substantially heating them1. This mechanism, called spallation, gave near-surface life forms in what Melosh called the ‘fecund zone’ a survivable pathway into space.

Important corroboration for this model came in 2000, when researchers studying remnant magnetism in a Martian meteorite called ALH84001 found that it had never been heated above 40˚C, about the temperature of a nice hot bath on Earth. The meteorite had started life sitting around on Mars, and was ending its life sitting around on Earth, without much happening in the interim that would distress a microbe. It was positive proof that rocks could make the full trip from Mars to Earth without getting cooked.

The next piece of the puzzle—whether microbes could even live inside of rocks—was settled decisively with the arrival of modern gene sequencing techniques in the 1990s. As I wrote about a few months back, this revolution established the existence of a deep biosphere on Earth, a layer of slow-living bacteria and archaea many kilometers deep, living contentedly in rock. These deep-dwelling organisms inhabit a world that is chemically and thermodynamically isolated from the surface, with generation times that can stretch into the thousands of years. Some of them even dine on radiolytic hydrogen gas, and are therefore completely decoupled from the biosphere as we know it. They have no reason to care what lies far overhead.

Obviously, such strong, independent microbes are excellent candidates for lithopanspermia theories. The one open question is how long they can survive in space. From a microbe’s perspective, t