1. Why Scientists Compared an Astronaut with His Twin
Space changes the body in many ways, but it is hard to know which changes come from space and which would have happened anyway. Two people who started life with almost the same DNA give researchers a better "before and after" comparison than two strangers. NASA's Twins Study compared Scott Kelly, who lived on the International Space Station (ISS), with his identical twin brother Mark, a retired astronaut who stayed on the ground. Ten research teams took part [2]. They looked at the body, at molecules inside cells, and at thinking skills [2].
Here is the twin idea in a sentence. Identical twins come from a single fertilized egg and have the same DNA. Fraternal twins come from two eggs and share about half their DNA, like any brother or sister [4]. So Mark was the "same DNA, but staying on Earth" comparison for Scott.
2. What Happened to Scott's Telomeres in Space
The result that got the most attention involved telomeres, the protective caps at the ends of chromosomes. In Scott's blood cells, telomeres became about 14.5% longer during the flight. Within roughly 48 hours of landing they shortened again, and they were back near their preflight average within months [1]. NASA said the lengthening was confirmed with several different tests [3].
It is tempting to say that space made Scott younger. The study does not support that. Scott also had more very short telomeres than before the flight, and that was still true about six months after he returned [1]. The paper reports the change; it does not settle why it happened.
3. Which Changes Went Away After Landing, and Which Stayed
Genes make proteins when a cell "switches them on," and spaceflight changed how active many of Scott's genes were. NASA's summary says about 93% of the changes in his gene activity returned to normal after landing. The remaining 7% hint at longer-lasting changes in genes linked to the immune system, DNA repair, bone formation, and low oxygen and high carbon dioxide in the body [3].
That number depends on how you count. In the paper, one group of blood cells had 481 genes that changed during flight, and 112 of them (23.2%) were still changed after he returned [1]. A single percentage hides these differences between cell types, so "most went back, some did not" is the safer way to say it.
Thinking skills showed a similar pattern. NASA reported no significant drop in Scott's mental performance during the flight. Speed and accuracy fell more after landing, which may have come from readjusting to gravity and a busy schedule after the mission [3]. The paper also lists reduced cognitive function after return among the changes that persisted [1].
4. Why One Pair of Twins Cannot Speak for All Astronauts
The study had one astronaut in space. With a single person, the authors write, it is impossible to tell whether a change was caused by spaceflight or happened by coincidence. They call the work "hypothesis-generating," meaning it suggests what to test next, and say it needs to be combined with studies of more astronauts [1].
The twins were also not perfect copies of each other's lives. Before this mission, Scott had already spent about 180 days in space, while Mark had spent about 54 days [1]. And the authors note that predictions for missions longer than one year are still estimates to be tested [1].
5. Why Identical Twins Are Not Exactly Alike Even Without Space
If identical twins have the same DNA, why can they differ at all? A well-known 2005 study compared "epigenetic" marks, chemical tags on DNA and its packaging that help switch genes on and off. Twins were similar when young, but older twins showed clear differences in these marks, and these differences were linked to differences in gene activity [5]. Genetics teaching materials also point out that identical twins can differ in personality, talents and some diseases, and attribute this to outside influences [4].
This is one more reason a twin pair is a starting point rather than a proof. Two brothers with matching DNA can still be two different people with different lives.
6. How to Read the Original Studies Yourself
If this made you curious, the Science paper is free to read through Europe PMC and PubMed Central. Try finding the figure that shows telomere length over time and see how the line rises in flight and drops after landing [1]. NASA's own pages are shorter and written for general readers [2][3]. A good question to carry with you: if you were designing the next study, how many astronauts would you want, and what would you measure first?