EXPLORATION used to be a fairly straight-forward affair. The intrepid picked
a destination, lined up a few sponsors, got crew and gear together and off they
went. Maybe they made it, maybe they didn鈥檛.
Space exploration is a little different. With current rocket systems, it
would take us more than 100 000 years to get to Alpha Centauri, one of the
nearest stars, and much longer to reach a habitable planet. While new
technologies may shave a few years off the timetable, it is still going to take
thousands of generations of galactic explorers to reach new solar systems鈥�
in other words, we are going to need to reproduce in microgravity.
Sex in space is a crowd-pleaser so not surprisingly reproductive biology is
already a staple of modern space research. But for now, most experiments amount
to a few days of tinkering, not the months or years needed to gauge the impact
space will have on generations of people. 鈥淲e have a huge way to go to
understand what would happen to a human embryo in space,鈥� says physiologist Jim
Pewelczyk of Pennsylvania State University at University Park, who worked on the
Neurolab shuttle mission in 1998.
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First, there are the logistics of coupling two moving, weightless
bodies鈥攁rguably one for NASA鈥檚 engineers. But solve that puzzle and
another presents itself. Our reproductive systems don鈥檛 work as they should in
space. Altered light-dark cycles knock out male testosterone and sex drive,
while the female menstrual cycle misfires or stops altogether. Reproductive
technologies such as chemically induced ovulation and artificial insemination
could solve these problems. But in space exploration, difficulties lurk behind
every airlock.
Earthbound explorers may lose the odd digit to frost or snake bites, but
space explorers could end up losing whole organ systems. Without gravity, bone
density falls rapidly and muscles deteriorate. 鈥淚f you look at cosmonauts who
were up for more than a year, they had to be carried out of the spacecraft when
they returned to Earth,鈥� says biochemist Marc Tischler of the University of
Arizona at Tucson.
Space biologists are developing drugs and exercise programmes to combat
physical decay in adults (seeNew 杏吧原创, 20 September 1997, page
28), but that won鈥檛 help children born in space. 鈥淸They] would be even more
debilitated because they wouldn鈥檛 be starting with a normal structure,鈥� says
Tischler. No one knows how many of the body鈥檚 organs would be adversely affected
if forced to develop in microgravity, although one of the most critical clearly
is. Mice fetuses flown aboard the space shuttle have brains with smaller and
fewer neurons than normal.
鈥淭here are significant differences between the functioning of cells on Earth
and cells in space,鈥� says neuroscientist Richard Nowakowski of the Robert Wood
Johnson Medical School in Piscataway, New Jersey. For now, the full extent of
the differences, or even exactly how mammalian cells sense gravity, is a
mystery.
Slightly better understood is how some parts of the body鈥檚 inner ear
balancing system develop in space. Rat fetuses that spent part of their
gestation aboard the space shuttle lost their sense of balance, as demonstrated
by the slowness with which they righted themselves when placed on their backs in
a bath of water.
No surprise there. Study after study shows that every sensory system, from
sight to touch to smell, needs to be stimulated for normal development, and
there鈥檚 no reason why our sense of balance should be exempt. What is a surprise
is that some balancing skills actually improve in animals born in microgravity.
The part of the balance system that helps the body reorientate and stops you
falling over after turning a somersault, is enhanced in baby rats that spend
most of their life before birth in space. The rats may have fine-tuned this
aspect of balance as their mothers walked along the sides and ceiling of their
cages, something they couldn鈥檛 do on Earth, says developmental psychobiologist
Jeffrey Alberts of the University of Indiana at Bloomington, who ran the
study.
Some creatures even seem to manage to compensate for microgravity. Take
frogs. They start out as unicellular organisms that soon turn into creatures
that we would recognise. Gravity may help the developing embryo tell what鈥檚 up,
what鈥檚 down and where to put the head and limbs, but it鈥檚 not a major
requirement, says Richard Harland of the University of California at Berkeley.
Other mechanisms seem to be at work, including the point at which the sperm
enters the egg. Eventually, the head pops out of the side of the embryo opposite
the site of sperm entry.
But before the embryo reaches that stage, it becomes a 10 000-cell blob
called an early gastrula. Cells jostle into new positions, and different types
of tissue start to come into focus. 鈥淚t鈥檚 a transitional stage,鈥� explains Steven
Black of Reed College in Portland, Oregon. 鈥淭hey still look round, but they
ain鈥檛 no eggs.鈥� When Black examined the early gastrulae of African clawed frogs
that had been fertilised in space, he found that the 鈥渞oof鈥� was thicker than
normal. That鈥檚 a potential problem since the brain develops in this area. But in
time, the embryos appeared to compensate for the space environment. 鈥淟iving
tadpoles returned from space were virtually normal,鈥� Black says. Still, frogs
are frogs and no one knows what effect microgravity would have on the early
gastrulae of human interplanetary explorers.
Astral selection
But just for the sake of argument, imagine that biology solves all the
problems that are yet to be discovered about reproducing in microgravity. Our
space pioneers would still face at least one more perilous confrontation with
nature. Space flight would offer the ideal environment for evolutionary
change鈥攁 small, confined population subject to extreme inbreeding and
exposed to high doses of mutation-inducing radiation.
鈥淚f you were in space for many generations, would you start to lose your
feet?鈥� asks Jerry Brown, a space science specialist at the US Space Foundation
in Colorado Springs, Colorado, which promotes space exploration. 鈥淚nstead of
arms and legs you might have four arms. That would be far more useful in
microgravity than a pair of legs. But when you got to a planet, you鈥檇 be back to
crawling about on all fours.鈥�
Or would space explorers lose their distance vision or much of their hearing
because there鈥檚 nothing to see out the window and little to hear? 鈥淭here are no
crickets in space, no distant thunder, no cry of the loon,鈥� says Brown. Unlike
explorers past, space explorers would probably bring along a semblance of home
in the form of computerised virtual reality displays. Still, says Brown, 鈥渨hat
we would have is a person developed as a space thing rather than an Earthbound
迟丑颈苍驳鈥�.
This could be a major problem once our interplanetary explorers reached their
destination. Instead of the physical agility and stamina of a Peary, Scott or
Shackleton, they would be more likely to be atrophied, misshapen beings ready to
hit the new planet with a splat, unable to move in the stronger pull of a
planet鈥檚 gravity.
Of course, we may have faster-than-light spacecraft and artificial gravity by
the time we鈥檙e ready to go. We may even discover a wormhole鈥攁 cosmic
shortcut through space鈥攖o speed us to our destination.
No one knows. But improving rocket technology could be the least of the
battle. Unless the biological hurdles are cleared as well, the best spaceship
ever built won鈥檛 get us to new worlds in any sort of shape to survive on them.