YOU can鈥檛 cheat fate for long鈥攊t鈥檚 the law. A mathematical law, to be
precise: the law of large numbers. This states that when an action is repeated
enough times, a particular consequence, however unlikely, will eventually
occur.
So if the chance of tossing a coin and getting heads is 50 per cent, then a
sufficiently large number of coin tosses will prove this. Russian roulette with
a six-chamber revolver offers players a 17 per cent chance of killing
themselves. Enough willing volunteers pulling the trigger enough times will
prove this. And when the chance of a rocket failing is 8 per cent, then expect
to lose eight rockets in every hundred. The law of large numbers is
incontrovertible, it is a law of nature and cannot be ignored. Flouting this law
can kill.
Which is why NASA should be worried. The American space agency and its
international partners are about to embark on the most complex construction
project in the history of humankind. Next week, barring any last-minute delays,
the first component of the International Space Station (ISS) will lift off from
Baikanur Cosmodrome in Kazakhstan aboard a Russian Proton rocket. The ISS is an
orbiting laboratory made up of more than 30 separate modules that will be
connected in space. When complete, the station will cover an area larger than
two American football fields and will be powered by more than 4000 square metres
of solar panels. Assembling this giant will require nearly 100 rocket launches
and more than 1000 hours of spacewalks.
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The statistician looks at such an endeavour and sees danger. Every hour in
the high radiation levels of space increases the chances of contracting a fatal
cancer. Every strut on the station has a chance of failure, and every square
centimetre of its area has a chance of being punctured by space debris or
micrometeors. And every rocket launch raises the prospect of an explosion.
But NASA appears remarkably nonchalant about these risks. While it has
analysed some of the individual risks to the space station, it has not yet
calculated the overall probability of a catastrophe. Critics ask how this can be
when safety is supposed to be NASA鈥檚 top priority. A catastrophe during assembly
could delay the station for years and possibly render it a piece of orbiting
junk. It is even possible that astronauts will die because of this project.
Without proper risk analysis, say the critics, NASA is facing its most difficult
challenge unprepared.
Next week鈥檚 launch will be the beginning of a massive series of chance
events. Whether NASA has calculated the risks or not, it is destined to find out
what they are. The law of large numbers should see to that. Take radiation. When
a high-energy particle strikes a molecule of DNA, there鈥檚 a chance that it will
cause an error in the genetic code and lead to a fatal cancer.
Living in a field of radiation is like playing Russian roulette with
high-energy particles as bullets. And the conditions that astronauts have to
face are far more severe than those on Earth. 鈥淭here are electrons, cosmic rays,
particles from the Sun,鈥� says Gautam Badhwar, a scientist with NASA鈥檚 Johnson
Space Center in Houston. 鈥淭hey鈥檙e very high energy and they鈥檙e very hard to
stop.鈥� Since these particles cannot be stopped, astronauts on the space station
will have to learn to live with them.
Tracy Yang, also at the Johnson Space Center, has studied the effect of
radiation on astronauts. He counted the mutations and aberrations in chromosomes
from astronauts who had spent several months aboard the Russian space station
Mir. 鈥淚n general, after a complete mission, you look at the increased frequency
of chromosomal aberrations,鈥� says Yang. By comparing the damage done during the
Mir flight with damage done to tissue by gamma rays in the lab, Yang concluded
that the astronauts had been exposed to about 0.15 sieverts鈥攖he equivalent
of a few thousand chest X-rays. 鈥淭hat鈥檚 not something to sneeze at,鈥� says
Badhwar. In the US, civilians are allowed no more than 0.05 sieverts per
year.
But just what risk this dose represents is a matter of debate. Humans have a
high chance of contracting cancer in ordinary life and the effects of small
doses of radiation are difficult to assess against this background rate.
However, in 1990, the US National Research Council, based in Washington DC,
estimated that if 1000 people were exposed to 0.1 sieverts, all in one gulp, an
extra eight fatal cancers would occur in that group above the number normally
expected.
Risk of cancer
Calculating the effect on astronauts is even more difficult. Much of the
assembly works will be carried out during the solar maximum when solar flares
can boost radiation levels in space by a factor of ten or more. But while the
astronauts鈥� dosages could be considerably higher than 0.1 sieverts, they would
receive this dose over a relatively long period of time鈥攁 factor which
researchers believe mitigates damage.
Badhwar has estimated the increased risk of cancer that a 180-day mission
would produce. 鈥淚t depends on age and sex, but it amounts to somewhat less than
1 per cent,鈥� he says. A chance of less than 1 per cent is not a huge risk, but
not something to be ignored. Especially when the law of large numbers comes into
play.
The space station will be in orbit for at least 10 years and perhaps 20 or
more, housing a maximum of seven astronauts at a time. If 100 astronauts stay on
the space station during this time, the odds of one of them dying from their
radiation exposure are even. 鈥淎stronauts will be told how much risk there is in
terms of radiation dose for the mission,鈥� says Yang. 鈥淪pace exploration brings a
lot of benefit to society, so it seems reasonable.鈥�
Space debris also represents a considerable danger. Forty years of human
activity in space has left its mark. The Earth is surrounded by a vast cloud of
orbiting junk ranging from spent rocket stages and defunct satellites to flecks
of paint and even human excrement. The US Space Command based in Colorado
Springs tracks orbiting junk but can only see pieces that are larger than 10
centimetres across. The ISS can be moved out of the way of these pieces. But
impacts from smaller, unseen pieces are inevitable.
Eric Christiansen is chief analyst at the Hypervelocity Impact Test Facility
at the Johnson Space Center and has studied how debris damages the surfaces of
spacecraft. While a piece of junk only a few centimetres across may sound
insignificant, it can cause considerable damage when travelling at several
kilometres a second. Christiansen and his colleagues have developed a shield for
the ISS that provides layer after layer of protection from space debris. 鈥淸Five
centimetres] above the pressure shell, there鈥檚 an aluminium bumper, then there
are layers of ceramic cloth, and layers of Kevlar cloth,鈥� explains Christiansen.
鈥淚t can stop a marble-sized aluminium particle moving at 7 kilometres a
蝉别肠辞苍诲.鈥�
Nevertheless, any junk larger than a marble but too small to be avoided
represents a considerable danger. 鈥淭here鈥檚 a residual risk that a particle can
penetrate,鈥� explains Christiansen. 鈥淚t varies from module to module, but there鈥檚
about a 1 to 2 per cent chance per module of a penetration within a 10-year
period.鈥� Node 1, which will be launched in December, has an 0.75 per cent chance
of getting a hole in 10 years. The main xenon tank that neutralises electric
charge on the station by emitting xenon ions has a chance of puncture of 0.45
per cent, while the Hab module where astronauts will live has a 1.8 per cent
chance.
The Russian components fare slightly worse, since they were designed before
engineers started thinking seriously about the threat from orbital debris. And
although NASA is deploying extra shields for some of the Russian components,
they鈥檙e still the most vulnerable.
Per module, 1 or 2 per cent isn鈥檛 much to worry about, unless there are a
large number of modules. And then the law of large numbers steps in. 鈥淏ecause
there are a lot of modules, 30 or so, there鈥檚 a 24 per cent chance of a
penetration,鈥� explains Christiansen. And given that the space station will
probably be in orbit longer than 10 years鈥攎aybe 15, 20, or more鈥攖he
chances go up even further. Over 20 years, the chance of penetration is about 42
per cent.
Most puncture wounds from a piece of debris this size would be about the size
of a coin. The atmosphere would leak out slowly, giving the astronauts time to
fix the hole. 鈥淵ou might be able to slap a piece of Band-Aid on and stabilise
it,鈥� explains Christiansen. And lessons learned from the Mir mishap would make
such a penetration less likely to be a catastrophe.
But there are some events that can be catastrophic. A puncture in an
inaccessible area of the station might be impossible to patch, rendering an
entire segment unusable. A hit to a critical component, such as the xenon tank,
will cause an explosion of the pressurised gas inside, perhaps destroying part
of the station. Worst of all, a larger hole could cause the module to burst like
a balloon, a phenomenon known as catastrophic unzipping. To reduce this risk,
NASA increased the thickness of the pressure hulls by 50 per cent. But the risk
is still there. 鈥淭he probability鈥檚 very small, less than 1 per cent [over 10
years],鈥� says Christiansen. 鈥淚t鈥檚 down in the mud.鈥�
Hypervelocity impact
There鈥檚 one risk, however, that makes the others pale in comparison. It鈥檚 the
very act of launching objects into space鈥攁 dangerous proposition by any
standards. Rockets fail, it鈥檚 a fact of life when you鈥檙e trying to do business
in space. In mid-August, a Lockheed Martin Titan-IV rocket exploded, destroying
a reconnaissance satellite and costing a billion dollars. Two weeks later, a
Boeing Delta-III rocket blew up on its maiden flight. And in September, a
Ukrainian Zenit rocket failed, destroying 12 satellites owned by the Globalstar
telecoms consortium鈥攁nd that was the third rocket failure within a
month.
NASA, however, has unreal expectations. 鈥淭he assumption is that there will be
no failures. That will probably not be the case,鈥� says Thomas Young, a member of
NASA鈥檚 Advisory Council, a board of engineers, scientists and industry
representatives set up to advise the space agency.
The space station components will be lifted into orbit using mainly Russian
Soyuz and Proton rockets and the space shuttle, all of which have a risk of
failure. The most reliable uncrewed rockets fail roughly 8 per cent of the time
though the shuttle is considerably more reliable. But nearly 100 rocket launches
will be needed just to assemble the station and keep it supplied. Once again,
the law of large numbers steps in. The chance of a rocket failure at some point
during the assembly is roughly 99 per cent (This Week, 9 May 1998, page 4), in
other words, almost a certainty. In fact, NASA should expect to lose five
launchers during the course of the ISS assembly.
Most serious of all is the prospect of a shuttle blowing up and killing
everybody on board. Such a disaster would set the assembly back years and could
even lead to it being abandoned entirely. In September, the NASA Advisory
Council, estimated that the probability of such an explosion at some point
during the ISS assembly could be as high as 60 per cent.
Given these figures, the NASA Advisory Council has been shocked to find that
NASA has not formally analysed the risks involved. According to Frederick
Gregory of NASA鈥檚 Office of Safety and Mission Assurance, NASA has not done a
formal probabilistic risk analysis 鈥�(PRA)鈥� to determine the probability of a
disaster during the assembly of the space station. In September, however, he
promised that such an analysis would be done. But as 快猫短视频 went
to press with only days until the first launch, the study was not complete.
But to NASA鈥檚 Advisory Council, this simply isn鈥檛 good enough. 鈥淎n in-depth
understanding on where the risks are is critical,鈥� says Brad Parkinson ,
Professor of Aeronautics and Astronautics at Stanford University and Chair of
the NASA Advisory Council. 鈥淎 state-of-the-art PRA should be done.鈥�
As Julie Swain, a cardiologist at the University of Kentucky and a member of
the Advisory Council, says: 鈥淚t鈥檚 the biggest construction project in the world,
and the opportunity for something to go wrong is phenomenal.鈥� Launch after
launch, spacewalk after spacewalk, year after year in an orbit filled with
debris and radiation will eventually take their toll. 鈥淚t鈥檚 dangerous. It will
always be dangerous,鈥� says Swain. 鈥淚t ought to be expected that people are going
to die.鈥�