In 1984, NASA left Bacillus spores in space for nearly 6 years; after LDEF returned to Earth, 1–2% of the exposed spores were still alive

Imagine a life form so tough that it can survive nearly six years in space without food or oxygen and through temperatures swinging from freezing cold to scorching heat. This is pretty much what happened to a group of ordinary soil bacteria known as Bacillus subtilis. In April 1984, NASA sent trays containing these microscopic spores into orbit aboard a satellite known as the Long Duration Exposure Facility, or LDEF. The landmark study, ‘Long-term survival of bacterial spores in space,’ by Gerda Horneck and colleagues, published in Advances in Space Research, reports that the satellite ended up floating around in orbit for a much longer period of time than expected, and when it finally returned to Earth, researchers found that some of the spores had survived the entire journey.

A satellite stuck in orbit for years

LDEF was designed to study the effects of long-term exposure to the space environment on materials and biological samples. It was originally scheduled for retrieval by March 1985, under a year after launch, by a Space Shuttle crew. Different complications delayed LDEF recovery many times, including the famous delay following the Challenger tragedy of 1986. Columbia's crew finally retrieved LDEF in January 1990, by which point it had spent 69 months, just under six years, in low Earth orbit. During that time, the samples were exposed to vacuum, thermal cycles, and cosmic radiation. Before launch, scientists loaded millions of spores onto small metal trays, some spread thin in single layers of about a million spores each, and others stacked in dense multilayers containing about a hundred million spores each.


Why many spores survived

When researchers finally opened the trays, the results were surprising. Spores protected from direct sunlight performed much better because ultraviolet radiation was one of the most lethal hazards in the space environment. Spores that were placed densely in layers and protected could survive at a rate of up to 80%. Not all spores that were exposed to sunlight were killed either. Out of the roughly one million spores in each unprotected monolayer sample, the study reports that as many as ten thousand viable spores were still recovered, a small fraction next to the shielded batches, but still a striking sign of resilience given what those cells went through.

The secret is a tough outer shell

Bacillus subtilis spores do not behave as typical bacteria do. As soon as conditions become hostile, the cell stops most biological processes and wraps its DNA in a very compact, dry protective layer, allowing it to remain dormant until conditions improve. As Wayne Nicholson and his colleagues explain in a comprehensive review titled "Resistance of Bacillus Endospores to Extreme Terrestrial and Extraterrestrial Environments," it is this dormancy state that allows a spore to survive harsh conditions that would kill an actively functioning bacterium, from boiling heat to the vacuum of space.

What sunlight does that vacuum doesn't

It might seem strange that space itself was not the biggest threat. Vacuum alone had little effect on spore survival. The damage was done by solar ultraviolet radiation, which normally does not get through because of our planet's atmosphere and ozone layer. In its absence, UV radiation destroyed DNA in the spores roughly four orders of magnitude faster than any other factor in space. As a broader review of decades of space microbiology research puts it, extraterrestrial solar UV radiation was found to be the single most damaging factor microorganisms encounter in space.

Why this matters beyond curiosity

This experiment is often discussed in relation to the older concept of panspermia, the idea that life can travel through space from planet to planet, or even between star systems, inside rocks or dust ejected by asteroid collisions. If even simple bacterial spores could survive nearly six years of exposure to space and revive afterward, it suggests that some life forms may survive an interplanetary journey with limited protection. This study is also relevant to today's space agencies, which work to sterilize spacecraft headed to Mars and other planets because spores like these are difficult to eliminate completely.

More than three decades after LDEF returned home, its spore specimens remain a widely cited example of how some Earth-based life-forms can survive harsh conditions much better than anyone could ever have imagined possible. A little shielding made a major difference to how many spores lived, and even without it, life still found a way to hang on.