24 ASTRONOMY • JANUARY 2020From March 2015 to March 2016,
now-retired astronaut Scott Kelly lived
and worked on the International Space
Station. During that time, Scott’s identical
twin brother, Mark — also a retired astro-
naut — went about his daily life on Earth.
Because identical twins share everything,
right down to their DNA, the brothers are
the basis of NASA’s Twins Study, which
compares their bodies pre- and post-
f light. The goal is to learn more about the
changes humans undergo as a result of
living and working long term in space.
On April 12, 2019, the 10 research
teams associated with the study published
a summary paper in Science. Each of their
projects focused on a different aspect of
human health, such as metabolism, DNA
replication, gene expression, cognition,
immune response, and microbiome.
Some of the body’s functions, such as
immune response to vaccination, did not
change during spacef light. But the teams
did note changes in Scott’s body weight,
gene expression, gut microbiome, carotid
artery thickness, and even the length of
his telomeres — the protective caps at theend of DNA strands — among others.
Most changes persisted while Scott
was aboard the space station and
returned to baseline after time spent back
on Earth. For example, thousands of
Scott’s genes changed the way they turned
on and off during spacef light. In particu-
lar, his body activated several genes that
remained dormant in Mark — a response,
the researchers believe, that shows his
body adapting to his new environment.
Within six months of
returning to Earth,
more than 90 percent
of Scott’s gene
expression levels had
returned to normal.
Some changes
were unexpected. The telomeres that
protect our DNA shorten as we age or in
response to certain stressors. The team
expected Scott’s telomeres would shorten
in comparison with Mark’s while Scott
was in space, a more stressful environ-
ment. But Scott’s telomeres instead grew
longer during spacef light, while Mark’s
stayed the same. Upon returning toEarth, Scott’s telomeres shortened back
to normal within 48 hours. And several
months later, Scott had more short telo-
meres than before embarking, indicating
at least some effects of spacef light may
be long-lived.
The results are still good news for
future astronauts on missions such as
extended stays on the Moon or journeys
to Mars. “Overall, the findings were that,
‘Hey, there are no showstoppers in going
to Mars,’ ” Scott says.
But “there are areas
we need to investi-
gate further,” he
adds, such as the
genetic changes he
underwent. It’s also
vital, he says, to determine how to better
protect astronauts from radiation. “From
a physiological perspective I think that’s
the longest pole in the tent, so to speak, to
going to Mars,” he says.
But the most notable result, according
to Scott? “After spending a year in space,
I am now not only smarter, but also
better-looking than my brother Mark.”7
Twin astronauts show how
spaceflight affects human health
NASA’s Twins Study
took advantage of
twin astronauts Scott
(left) and Mark Kelly
to examine how
spaceflight affects the
human body. Scott
spent a year in space,
while Mark continued
life on Earth to serve
as a control. NASAAt least some effects
of spaceflight may be
long-lived.