Rain or shine, for more than 2.5 billion years living organisms could
always depend on one thing: gravity. People have now changed that by shooting
organisms into space. Weightlessness apparently does no great harm, so President
George Bush says that it is now time to send people to Mars. Biologists,
however, are not so sure.
At this week鈥檚 meeting of the American Association for the Advancement
of Science in Washington DC, scientists will report on efforts to prepare
for such a mission. Many argue that the biological effects of lengthy space
travel are the biggest imponderable. The fastest journey to Mars would take
eight months each way, according to NASA鈥檚 calculations.
Dennis Smith, vice-chancellor of the University of California at Irvine
and a long-stanging advisor to NASA, says the agency is nowhere near ready
to send people to Mars. 鈥榃e desperately need to do controlled experiments
in space,鈥� says Smith, but NASA has not bothered. That view is also shared
by Noel Hinners, chief scientist for the company Martin Marietta.
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鈥榃e鈥檝e known for 20 years what the potential problems were but done
little to solve them,鈥� says Hinners. The two scientists are preparing a
study for the National Academy of Sciences to help NASA to organise its
research.
NASA鈥檚 budget for life sciences is less than $138 million a year, out
of the agency鈥檚 total annual spending of $14 billion. Basic research takes
up a tiny percentage of that life sciences budget. 鈥楤ut this is not just
a plea for money,鈥� says Smith. NASA needs a different attitude. Most astronauts,
for example, object to being scientists鈥� guinea pigs.
Experiments in space also need a lot of ingenuity. Animals are often
poor models for humans; bones in rats are quite different from human bones.
Using humans would be unethical if, say, some astronauts did not exercise
in space as a comparison and suffered injury as a result. And biologists
at universities are wary of spending years designing an experiment that
may be postponed or lost in space.
The effects on astronauts of years in microgravity are the biggest mystery
to researchers. Joe Sharp, director of space research at NASA鈥檚 Ames Research
Laboratory explains: 鈥楴obody has the foggiest idea of the effect of even
40 per cent or 20 per cent gravity for extended periods.鈥� People will probably
go anyway, Sharp adds, 鈥榖ut we may kill a few on the way, and we may get
some wet noodles back.鈥�
Why should we worry when spacefarers have not suffered any serious harm
in so many years of space travel? There are several answers to this question.
First, only a handful of people have spent more than a few weeks at a time
in space. Secondly, it has been impractical or impossible to gather data
beyond vital signs and some blood and tissue samples.
What information has been gathered contains some subtle but ominous
signs. For example, cells in culture go awry aboard spacecraft. One European
experiment aboard a space shuttle showed that mammalian white blood cells
divided very slowly. These are cells that fight disease. Identical cells
were spun in a centrifuge on board to simulate gravity. The cells divided
normally. This kind of experiment, besides sounding a cautionary note, needs
repeating, says Smith.
Soviet and American tests with animals also found a decline in the production
of red blood cells that seemed to worsen the longer the animals were in
orbit. The phenomenon is called 鈥榮pace anaemia鈥� and its cause is unknown.
Moreover, the human body is not simply a bag of cells; no one knows how
years of microgravity would affect the complex interaction between cells
in an organ and between organs themselves.
It is well known that a type of osteoporosis, a demineralisation of
the bone, occurs when vertebrates spend time in microgravity. Why and how
it happens is unkown. The demineralisation is not uniform throughout the
skeleton, however. Studies of people kept in bed for long periods show a
redistribution of calcium in the skeleton.
Joan Vernikos, a life scientist at Ames, notes that the effects are
not quite like osteoporosis on Earth; microgravity appears to change the
levels of calcium in the plasma as well as the way minerals are deposited
in the matrix of bone. So simply making up the calcium that astronauts excrete
may not ensure that it is laid down properly in new bone.
In rats that have been sent into space, the breaking strength of long
bones decreased by between 30 and 50 per cent, says Richard Grindeland of
Ames. Another recent experiment in which rats with broken bones were sent
into orbit showed that they are slower to heal in microgravity than on Earth.
Muscles atrophy in microgravity. A host of countermeasures have been
devised and a few tried, such as exercise bikes and treadmills to which
crew members are strapped. This machinery is bulky and does little to provide
the 鈥榮hock鈥� to bones of earthbound exercise that they apparently require
for health. Moreover, muscles need different types of exercise; exercise
that contracts muscles but does not elongate them may not be adequate. Engineers
at NASA are working on several contraptions, such as inclined platforms
with a variety of weights and pulleys, that demand a wider range of effort.
It is believed that organisms soon adapt to microgravity and establish
a new biological steady state. But rats sent into space by Wesley Hymer
of Pennsylvania State University showed that readapting, whether on mars
or back on Earth, may not be straightforward. Hymer鈥檚 rats were unable to
produce more than half the normal amount of pituitary growth hormone. This
hormone helps to control growth of muscle, bone and the cells of the immune
system. Hymer speculates that the pituitary cells package hormone molecules
differently in weightless conditions than they do under gravity.
鈥楴ASA鈥檚 approach to this has been an engineer鈥檚 solution,鈥� complains
Smith. 鈥滺ave them take calcium and run on the treadmill for four hours鈥�,
they say. Our approach is to do basic research first.鈥� One such engineering
solution to weightlessness that NASA is now contemplating is spinning the
spacecreaft to create at least some gravitational effect on the astronauts.
But a rotating craft exerts Coriolis forces, which work in a different direction
from a centripetal force. The physiological effect of these forces over
months is unknown.
Microgravity is not the only threat to humans. 鈥楻adiation is one of
the most pressing questions,鈥� says Steve Fogelman, deputy director of life
sciences at NASA. With the exception of the astronauts who visited the Moon,
space travellers do not go beyond low-Earth orbit, where even the attunuated
magnetic field and atmosphere provide some protection from radiation. Deep
space, however, is saturated with cosmic rays.
There is also a significant risk of a solar flare during a mission to
Mars, which would increase the flux of protons in deep space to dangerous
levels. Large doses of protons will kill human cells. Unshielded, an astronaut
could be exposed to hundreds of rads during a solar flare.
Cosmic rays include heavy particles, those with high energies, such
as iron, would penetrate the craft and bore through human cells. Even if
they do not damage people immediately, they could alter DNA and increase
the risk of cancer. One analysis by Stuart Nachtwey of NASA estimates the
extra risk of cancer over an 18-month voyage, including 30 days on Mars,
is only 1 per cent. But he notes that three-quarters of the dose would be
from heavy ions, whose biological effects are 鈥榩oorly understood鈥�.
No doubt an advertisement in the Los Angeles Times for volunteer space
travellers would attract plenty of candidates. But would any be psychologically
or physically fit enough to spend two or three years in a weightless box?
Smith says the message from Soviet data is that one year is the most people
can take.
For all his staying power, Yuri Romanenko, who endured 326 days in the
Mir space station, spent almost half his waking hours on exercise machinery
near the end of his trip. On other flights, Soviet cosmonauts could work
only about four hours a day. The Soviets ended a mission early because of
the crew鈥檚 cardiovascular problems and, says Smith, a 鈥榣ot of fighting.鈥�
快猫短视频s are beginning to argue that psychological fitness could be the
biggest hurdle to a Mars mission.
Criticisms like those levelled during the meeting of the American Association
for the Advancement of Science are having some effect. 鈥楾here is a lot of
room for improvement,鈥� acknowledges Fogelman. The agency has recently outlined
some ambitious plans, including a bigger budget for the Ames laboratory
to carry out more experiments.
A powerful voice has been the influential panel of experts, led by Norman
Augustine of Martin Marietta. Its report to NASA and the White House on
the future of the US space programme put life sciences at the top of the
list of experiments for the US space station to be built by the end of the
century. That could reinstate plans to include a centrifuge. The device
is essential to determining the effects of varying degrees of gravity on
everything from cells to people.
NASA has also begun to design an environment that can sustain a crew
during a Mars mission. Mike Shafto, one of NASA鈥檚 human performance engineers
at the Ames laboratory, is studying 鈥榓nalogue environments鈥� where people
are isolated in a strange environment for long periods 鈥� such as the Antarctic,
or inside Aquarius, a submersible in the Caribbean Sea near the Virgin Islands.
Shafto is also eliciting what he calls 鈥榳ar stories鈥� from former astronauts.
He will build mock-ups of a capsule for a Mars mission in the hope of
identifying those errors 鈥榙esigned into鈥� the automation. 鈥榃e can鈥檛 afford
any Chernobyls in space,鈥� says Shafto. At the same time, he must avoid putting
too much automation on board and turning the astronauts into bored passengers.
There are also various schemes in progress which aim to safeguard space
travellers. Water is a relatively good shield against cosmic rays. For solar
flares, a system of early warning satellites around the Sun could detect
the first sign of trouble. Sensors on the craft bound for Mars would detect
light from a flare and the crew could take shelter in a shielded room before
the slower protons arrive.
The report from the National Academy of Sciences will highlight microgravity,
radiation and psychological fitness as the three main thrusts of an invigorated
biology programme. Whether it comes about depends as much on politicians
as on scientists. Only half in jest, Sharp and Vernikos at Ames suggest
creating a research centre dedicated to space biology in the home district
of an important politician. 鈥業t could be in Indiania,鈥� says Vernikos. Indiana
is home of Vice-President Dan Quayle, head of Bush鈥檚 National Space Council.