What a year in space does to your genes

The same DNA, different cellular activity

A gene is a stretch of DNA that a cell can use to make a working product, often a protein. Gene expression is the process of turning those instructions into action. Cells adjust which genes they use as conditions change—much as a workshop changes which tools it brings out without rewriting its manual.

That distinction matters in space. The Twins Study did not find that Scott Kelly became genetically a different person from his identical twin. It tracked shifts in how cells used genetic instructions, alongside other biological measurements. Many shifts eased back toward earlier levels after his return; some lingered.

A body responding to an unfamiliar environment

Orbit changes several demands on the body at once. Microgravity alters how tissues and fluids behave, while radiation can damage cells. The stress of a long mission, disrupted sleep, and changes in routine also affect the signals cells receive. Those signals can prompt cells to increase or reduce the use of particular genes, including genes involved in immune activity and other repair or stress responses.

A change in expression is not automatically harmful, nor does it mean a gene has been permanently switched on or off. It may be a temporary adjustment. Researchers therefore looked not only for changes during flight, but also at whether measurements moved back toward Kelly’s preflight levels after landing.

What twins can—and cannot—tell us

Mark Kelly provided a rare comparison: an identical twin who shared much of Scott’s genetic background but remained on Earth. That makes differences between them useful clues about what might be associated with spaceflight. But the study had only one astronaut, so it cannot cleanly separate the effects of space from every other difference between the twins or establish what will happen to every traveler.

The lasting lesson is that the body’s response is not a simple genetic rewrite. Space can alter the cellular instructions being used, and recovery does not happen on one timetable: some activity shifts back quickly, while other changes take longer or remain detectable after the astronaut is home.