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Even the most difficult-to-kill tardigrades cannot always withstand being shot out of a gun.

That means even the most durable water bears might not be able to survive a crash landing on a new world.
After all, tardigrades aren't fully invincible.

Water bears are microscopic organisms that are nearly impossible to destroy. They can go without food or water for years, tolerate freezing and boiling temperatures, and withstand blistering radiation and the vacuum of space. However, in a recent experiment, these death-defying organisms were put to the test in a novel way: by discharging tardigrades from a rifle.

Researchers report online May 11 in Astrobiology that tardigrades survived slamming into objects at speeds of up to 825 metres per second. Faster water bears, on the other hand, blasted apart on collision. Even intrepid small animals like tardigrades would struggle to survive a crash-landing on a new planet, according to the research.

Panspermia — creatures travelling between worlds on planetary debris churned up by meteorite collisions — now has new boundaries. Knowing whether life is capable of hopping between planets could help scientists figure out how life began on Earth and assess the risk of Earthly lifeforms on spacecraft polluting other parts of the solar system.

The tardigrade-carrying Israeli spacecraft Beresheet, which crashed into the moon in 2019, inspired researchers to test water bears' ability to resist high-speed collisions. Is it true that these tardigrades are still alive?

Scientists inserted dormant Hypsibius dujardini tardigrades into nylon bullets after freezing them to put them in a state of suspended animation. The rounds were shot from a five-meter-long instrument known as a two-stage light gas gun, which resembles a cannon more than a rifle.

The machine hurled tardigrades into sacks of sand designed to resemble the lunar surface at speeds ranging from 550 to 1,000 metres per second.

The tardigrades were killed by slamming into sand at speeds greater than 825 metres per second, or around one gigapascal of shock pressure. Even the water bears that managed to survive were not spared. They could take up to 36 hours to recover after being submerged in water. Water bears that had been frozen and thawed without being pummelling with sand recovered in nine hours.

These findings indicate that the Beresheet tardigrades did not survive their lunar landing. Similarly, space objects slam into planets and moons in the solar system at thousands of metres per second, much too fast for tardigrades to survive. Splashing down in water, on the other hand, may provide a softer landing. Panspermia in the solar system is difficult for [animals like water bears], but it is possible.

Microbes, which are smaller and more resilient than water bears, may have a greater chance of migrating across worlds. Other studies have found that these organisms can resist hits of thousands of metres per second, with survival rates of one in 10,000 or less. There are also tardigrades that are far tougher than the ones used in this experiment. He wonders if other types of water bears would be more splatter-resistant.