The uncertainty of science: For reasons that are not yet understood, a flatworm fragment flown to ISS in a microgravity experiment regenerated with two heads.

But the most dramatic difference was a type of regeneration observed in one of the 15 worm fragments sent to the ISS. That worm returned to the scientists with two heads (one on each end of its body), a type of regeneration so rare as to be practically unheard of — “normal flatworms in water never do this,” Levin told Live Science. When the researchers snipped both heads off back on Earth, the middle portion regenerated into a two-headed worm again.

“And these differences persist well over a year after return to Earth!” Levin said. “Those could have been caused by loss of the geomagnetic field, loss of gravity, and the stress of takeoff and landing — all components of any space-travel experience for living systems going to space in the future,” he said.

The flatworms that flew in space showed other significant differences from the control group that stayed on Earth, further suggesting that for flatworms at least the environment of weightlessness causes more problems that were expected.