Mark (left) and Scott (right) Kelly

And it's a long way from saying Scott Kelly's DNA "no longer matches that of his identical twin".

The findings that sparked this most recent wave of publicity were preliminary results that NASA announced in January from its studies that documented biological and physiological changes in Scott Kelly by comparing him to his Earth-bound, twin brother Mark, who is also an astronaut.

As part of the study, Scott spent a year in space while Mark, his identical twin, stayed on Earth as a control subject to look at the effects of space travel on the human body.

His gene expression - the transcribing and translation of genes, not the genes themselves - was what actually changed during his year on the Space Station. One of its editor's notes reads: "Editor's Note: This story was corrected to note that Scott Kelly's gene expression changed by 7 percent in space, rather than his genetic code". They witnessed a 7% change in gene expression. Gene expression refers to how active a particular piece of DNA is.

Colorado State University's Susan Bailey, who leads one of the research group, said Kelly's exercise routine and diet most likely caused the chromosomal bungee.

SHAPIRO: This information could give scientists a few clues about the long-term impact of space travel as NASA begins to plan missions to Mars.

"It's also not surprising that a lot of these expression levels are returning to normal as Scott recalibrates to Earth's environment", she said.

NASA's update came after some media outlets initially misreported that Scott Kelly's DNA itself had changed.

The twin brothers underwent medical testing before, during, and after lengthy mission. "However, the remaining seven percent point to possible longer-term changes in genes related to his immune system, DNA fix, bone formation networks, hypoxia, and hypercapnia".

That divergence in chimp versus human DNA comes from roughly 40 million mutations in the base-pairs, or letters, that make up the genetic code, Live Science previously reported.

Some changes included "cell-free" DNA in the blood, and changes in the telomeres, caps on the end of chromosomes which are involved in the ageing process. But it suggests that space flight does induce longer-term changes at the molecular level.

Kelly's one-year mission is a scientific stepping stone to a planned three-year mission to Mars, NASA said.

Nasa collected regular readings for metabolites, cytokines and proteins and discovered that spaceflight was linked to oxygen deprivation stress, increased inflammation and dramatic nutrient shifts which affect genes. However, it does give researchers further insights into how the body reacts to space.

  • Joey Payne