Tardigrades are one of nature’s most indestructible lifeforms. These microscopic animals can survive both freezing and boiling temperatures, pressures equivalent to those six miles under the ocean, and even the vacuum of outer space.
But for one pair of scientists, a lingering question remained: can tardigrades survive being shot out of a gun headlong into an impact target?
It’s a worthy hypothetical in any context, but there also happens to be a legitimate scientific reason to conduct such an experiment. For decades, scientists have speculated about the possibility that hardy organisms might be able to survive trips between planets by hitchhiking on meteorites. This theory of interplanetary cross-pollination, known as panspermia, has implications for understanding how life might have emerged on Earth and whether it is common elsewhere in the universe.
With this in mind, Alejandra Traspas and Mark Burchell, a PhD student and professor of space science at the University of Kent, respectively, sought to determine whether spacefaring tardigrades would be able to withstand the sudden impact of arrival at an alien world.
In a study published this month in the journal Astrobiology, the researchers point out that “there is no knowledge of how [tardigrades] survive impact shocks” and so “accordingly, we have fired tardigrades at high speed in a gun onto sand targets, subjecting them to impact shocks and evaluating their survival.”
“We had no real info, only guesswork,” Burchell said in an email. He noted that past studies of tardigrade-scale seeds break apart when they impact at speeds over 2,200 miles per hour, and at shock pressures of 1 gigapascal (GPa), which suggested that it “might be an interesting regime to test” actual tardigrades in the same conditions.
“The results were however a surprise in that the tardigrades seemed to recover from impacts, right up to speeds which started to physically tear them into pieces,” Burchell added.
Traspas and Burchell used a special type of scientific equipment called a two-stage light gas gun to fire out their tardigrade-laced projectiles. Light gas guns are used by scientists, for instance at NASA, to test the effects of high velocity impacts (hypervelocity). Their use of gunpowder and compressed hydrogen gas allows them to reach speeds high enough to test, for example, the effects of space debris slamming into satellite shielding. The team selected the tardigrade species Hypsibius dujardini for the study, and induced the animals to enter their “tun” state, a special form of suspended animation, by freezing them for two days before the experiment.
The researchers then fired at a total of six shots, each containing a few tardigrades, at speeds ranging from about 1,240 to 2,230 miles per hour, which is faster than a bullet exiting a traditional firearm. The animals impacted the sand target at shock pressures ranging from 0.61 to 1.31 GPa.
Amazingly, tardigrades in the four out of the six shots survived the high-speed and shock pressure of the impact. However, these individuals recovered much more slowly from the experiment compared to a control group of tardigrades that had been frozen for the same period, but were spared from being fired out of a gun. That lag time in recovery “suggests that the impact shock had a more significant effect than freezing alone,” according to the study.
Alas, the tardigrades shot at the two highest speeds—around 2,000 and 2,230 miles per hour—did not survive. In fact, their bodies were fragmented when they were recovered, revealing that even the sturdy tun state could not protect them from the shock of impact. The results indicate that the upper limit of impact speeds that tardigrades can survive hovers somewhere around 1,845 miles per hour.
Most meteorites exchanged between planets impact at higher shock pressures than the maximums probed by the experiment, suggesting that the “arrival of a tardigrade on Earth, for example by way of a meteorite impact, is not likely to be a viable means of a successful transfer even for such hardy organisms,” according to the study.
In other words, “any transfer process between bodies that involves a shock of above 1 GPa is not a viable process,” Burchell explained. “And it is the shock that is important, not just the speed.”
“If you hit a porous material as the target such as aerogel”—a spongy material used to capture comet dust in NASA’s Stardust mission—”then you can keep the pressure below 1 GPa up to about 6 km/s [13,400 mph] impact speed,” he continued. “But this is not a natural scenario, so in nature you need to find niche transfer routes which minimize the shock.”
Traspas and Burchell speculate that extremophiles traveling between a planet and its natural satellites, such as Earth and the Moon, or Mars and its moon Phobos, might have a better chance of survival, though these organisms would still face long odds. Indeed, even if an organism did make it to a new world alive, the long recovery time of the experiment survivors hints at bodily damage that could hinder reproduction, potentially curbing interplanetary travelers to a single generation.
The new research sheds light on these hypothetical instances of panspermia in the wider universe, but it’s also relevant to the prevention of planetary contamination in our own solar system as a result of spaceflight. For instance, the Israeli spacecraft Beresheet may have spilled tardigrades on the Moon when it crashed during an attempted landing in 2019.
Burchell said that it’s difficult to predict whether tardigrades could have survived that impact because there are so many unknown factors. Regardless, the new study provides important context for future missions that hope to prevent contaminating other worlds with biological specimens.
Traspas and Burchell also apply their findings to concept missions to Saturn’s moon Enceladus or Jupiter’s moon Europa. Scientists think these worlds likely bear subsurface oceans under their ice crusts, making them tantalizing candidates for life within our solar system.
These moons spew out watery plumes into space, enabling spacecraft to fly through these cosmic sprinklers to collect samples that might contain any alien life. The new study constrains ideal collection techniques that would ensure that organisms picked up in this process don’t end up smashed to bits, like some of their tardigrades.
“If appropriate attention is given to the mission design (orbit or flyby) and collection method (solid collectors or underdense collectors such as aerogel), it may be possible to successfully sample the plumes of Europa and Enceladus for such life-forms,” Traspas and Burchell noted in the study. “Indeed, the idea that these plumes may be responsible for icy satellite panspermia in their respective planetary systems could be investigated.”
There is arguably no bigger mystery in science than the origin of life on Earth and its possible existence beyond our planet. Now, we are one step closer to understanding these open questions, thanks to a gun that shoots tardigrade bullets.