Was Einstein Wrong about Space Travel?

Consider a pair of twins. One gets a job as an astronaut and rockets
into deep space. The other stays on Earth. When the traveling twin
returns home, he discovers he's younger than his brother.

This is Einstein's Twin Paradox, and although it sounds strange, it is
absolutely true. The theory of relativity tells us that the faster you
travel through space, the slower you travel through time. Rocketing to
alpha Centauri---warp 9, please---is a good way to stay young.

Or is it?

Some researchers are beginning to believe that space travel could have
the opposite effect. It could make you prematurely old.

"The problem with Einstein's paradox is that he forgot to fold in
biology---specifically, space radiation and the biology of aging," says
Frank Cucinotta, NASA's chief radiation health officer at the Johnson
Space Center.

While the astronaut twin is hurtling through space, Cucinotta explains,
his chromosomes are exposed to penetrating cosmic rays. This can damage
his telomeres---little molecular "caps" on the ends of his DNA. Here on
Earth, the loss of telomeres has been linked to aging.

So far, the risk hasn\'t been a major concern: The effect on shuttle and
space station astronauts, if any, would be very small.  These astronauts
orbit inside of Earth\'s protective magnetic field, which deflects most
cosmic rays.

But by 2018, NASA plans to send humans outside of that protective bubble
to return to the moon and eventually travel to Mars.  Astronauts on
those missions could be exposed to cosmic rays for weeks or months at a
time. Naturally, NASA is keen to find out whether or not the danger of
"radiation aging" really exists, and if so, how to handle it. 

Science is only now beginning to look at the question. \"The reality is,
we have very little information.  Not many people have looked at whether
radiation increases the rate of telomere loss,\" says Jerry Shay, a cell
biologist at the University of Texas Southwestern Medical Center at
Dallas. With support from NASA, Shay and others are studying the
problem.

**A Lit Fuse**

Like the fuse of a time bomb, telomeres are long strands of
repeating DNA that shorten each time a cell divides.  When the telomeres
become too short, the cell essentially self-destructs: It can no
longer divide and will eventually die.

Without this built-in fuse, human cells would be able to
continue growing and dividing indefinitely.  In fact, scientists believe
that cells evolved telomeres as a way of preventing the
out-of-control cell growth of cancerous tumors.  Because of telomeres,
most human cells can only divide about 50 times before the time bomb
goes off.

One current theory of aging holds that, as the cells of a person\'s body
start to hit this telomere-imposed limit, the lack of fresh, new cells
causes the typical signs of aging: wrinkled skin, failing organs, weaker
immune system, etc.

Whether or not telomere loss actually causes aging remains a matter of
debate, Shay notes.  The fact that shortened telomeres go hand in hand
with aging is well documented.  People with shorter telomeres, for
example, are known to not live as long on average as people with longer
telomeres.  But mere correlation doesn\'t prove whether telomeres are in
fact the cause.

\"It\'s hard to prove cause and effect in these things.  But I think
there\'s a sufficient number of these correlative studies from a variety
of different investigators that one has to start believing that short
telomeres are a marker of aging,\" Shay says.

Recent research, performed by Frank Cucinotta and colleagues, showed
that iron-nuclei radiation (a chief component of cosmic rays) does
indeed damage the telomeres of human cells.  See references below.

To prove this, they exposed laboratory dishes containing a kind of human
blood cell called lymphocytes to beams of both iron nuclei and gamma
rays.  Until recently, such a thorough analysis of telomere damage
would have been prohibitively time consuming \[WHY? Are telomeres so
numerous?\].  But a new cell-staining technique called RxFISH (Rainbow
cross-species Fluorescence In Situ Hybridization) allowed Cucinotta and
his colleagues to look at the damage to all of the telomeres
simultaneously.

\"We had this surprising result that iron particles are much
more damaging to telomeres than gamma rays,\" Cucinotta says.  He
suggests that this difference might be due to the wider path of damage
caused by the iron nuclei.  Telomere strands wrap into elongated loops,
like little knots on the ends of chromosomes.  Gamma rays can only
strike one side of these loops or the other, but iron nuclei can affect
both sides at the same time, inflicting lasting damage on the telomere. 
This explanation is still speculative, however.

The task now is to quantify the risk telomere damage might pose
to astronauts, so that mission managers and the astronauts
themselves can make informed decisions about the risks they face.  In
all likelihood, the effects will be modest, Shay says.

\"We\'re talking about subtle things.  These people are probably not
going to wind up in wheelchairs or something like that from being
in space,\" Shay says.

For example, astronauts who have had the greatest exposure to
space radiation, such as the Apollo astronauts who traveled to the
Moon, tend to get cataracts about 7 years earlier than other astronauts,
on average.  Cataracts are a common symptom of aging.

Of greatest concern is possible aging of the brain and spinal cord.  
Experiments with rats show that brain tissue is vulnerable to "aging" by
iron-nuclei radiation, according to research by Jim Joseph of Tufts
University and Bernie Rabin at the University of  Maryland. 

\"It looking more and more likely that this could be a problem for
long-term space travel,\" Cucinotta says.

However, if scientists can tease apart the exact ways that iron-
particle radiation affects telomeres, they may be able find a way
to avoid or correct it.  The solution could be as simple as a
pill containing DNA-repair molecules---or something else. \"There are
many ways that we can intervene,\" Shay says.

One way or another, NASA hopes to ensure that the men and women
who someday walk on Mars won't get gray hairs---before their time.

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references:

Durante, M., et al. 2006. Chromosomes Lacking Telomeres are Present  in
the Progeny of Human Lymphocytes Exposed to Heavy Ions.
Radiation Research, Jan;165(1):51-8

Joseph, J.A., et al. 1992. Possible \"accelerated striatal aging\" 
induced by 56Fe heavy-particle irradiation: implications for manned 
space flights. Radiation Research, Apr;130(1):88-93

Rabin, B.M., et al. 2005. A longitudinal study of operant responding  in
rats irradiated when 2 months old. Radiation Research, Oct;164(4 Pt
2):552-5