NASA has a mystery to solve:
Can people go to Mars, or not?
“It’s a question of radiation,”
says Frank Cucinotta of NASA’s Space Radiation Health Project at the
Johnson Space Center. “We know how much radiation is out there,
waiting for us between Earth and Mars, but we’re not sure how the
human body is going to react to it.”
NASA astronauts have been in
space, off and on, for 45 years. Except for a few quick trips
to the moon, though, they’ve never spent much time far from Earth.
Deep space is filled with x-rays and protons from solar flares, gamma
rays from newborn black holes, and cosmic rays from exploding stars.
A six-month trip to Mars, with no big planet nearby to block or deflect
that radiation, is going to be a new adventure.
NASA weighs radiation danger
in units of cancer risk. A healthy 40-year-old non-smoking American
male stands a (whopping) 20% chance of eventually dying from cancer—if
he stays on Earth. If he travels to Mars, the risk goes up.
The question is, how much?
“We’re not sure,” says
Cucinotta. According to a 2001 study of people exposed to large
doses of radiation—e.g., Hiroshima atomic bomb survivors and,
ironically, cancer patients who have undergone radiation therapy--the
added risk of a 1000-day trip to Mars lies somewhere between 1% and
19%. “The most likely answer is 3.4%,” says Cucinotta, “but
the error bars are wide.”
The odds are even worse for
women, adds Cucinotta. “Because of breasts and ovaries, the risk to
female astronauts is nearly double the risk to males.”
Researchers who did the study
assumed the Mars-ship would be built “mostly of aluminum, like an
old Apollo command module,” says Cucinotta. The spaceship’s
skin would absorb about half the radiation hitting it.
“If the extra risk is only
a few percent… we’re OK. We can build a spaceship using aluminum
and head for Mars.” (Aluminum is a favorite material for spaceship
construction, because it’s lightweight, strong, and familiar to engineers
from long decades of use in the aerospace industry.)
“But if it’s 19%… our
40something astronaut would face a 20% + 19% = 39% chance of developing
life-ending cancer after he returns to Earth. That’s not good.”
New materials for spacecraft construction would need to be developed
to block radiation.
The error bars are large,
says Cucinotta, for good reason. Space radiation is a unique mix
of x-rays, gamma-rays, high-energy protons and cosmic rays. Atomic
bomb blasts and cancer treatments, the basis of many studies, are no
substitute for the “real thing.”
The greatest threat to astronauts
en route to Mars is galactic cosmic rays—or GCRs for short. These
are particles accelerated to light speed by distant supernova explosions.
The most dangerous GCRs are the heavy nuclei of iron atoms. “They’re
1000 times more energetic (1 GeV) than protons accelerated by solar
flares (1 MeV),” notes Cucinotta. They barrel through the skin
of spaceships and people like tiny cannon balls, breaking the strands
of DNA molecules, damaging genes and killing cells.
Astronauts have rarely experienced
a full dose of deep space GCRs. Consider the International Space
Station: it orbits Earth only 400 km above the surface. The body of
our planet, looming large, intercepts about one-third of GCRs before
they reach the ISS. Another third is deflected by Earth’s magnetic
field. The skin of the ISS absorbs about half of what’s left
before it reaches the crew. Space shuttle astronauts enjoy similar reductions.
Apollo astronauts traveling
to the Moon absorbed higher doses—about 3 times the ISS level--but
only for a few days during the Earth-Moon cruise. GCRs may have
damaged their eyes, notes Cucinotta. On the way to the moon, Apollo
crews reported seeing cosmic ray flashes in their retinas, and now,
many years later, some of them have developed cataracts. Otherwise the
crews don’t seem to have suffered from their travels. “A few
days ‘out there’ is safe,” concludes Cucinotta.
But astronauts traveling to
Mars will be “out there” for a year or more. “We simply don’t
know what cosmic rays will do to us when we’re exposed for so long,”
he says.
Finding out is the mission
of NASA’s new Space Radiation Health Institute, located at Brookhaven
National Labs in New York. It opened in October 2003. “At the
institute we have particle accelerators that can simulate cosmic rays,”
explains Cucinotta. Researchers expose mammalian cells and tissues to
the particle beams, and then scrutinize the damage. “The goal is to
reduce the uncertainty in our risk estimates to only a few percent by
the year 2015.”
Once the risks are known, NASA
can decide what kind of spaceship to build. It’s possible that
ordinary building materials like aluminum are good enough. But
if they’re not, “we’re going to have to consider new designs.”
How about a spaceship made
of plastic? “Plastics are rich in hydrogen—an element that
does a good job absorbing cosmic rays,” explains Cucinotta.
For instance, polyethylene, the same material garbage bags are made
of, absorbs 20% more cosmic rays than aluminum. “We couldn’t
build a whole spaceship from plastic, but we could use it to shield
key areas like crew quarters.” (Indeed, this is already done
onboard the ISS.)
If plastic isn’t good enough
then pure hydrogen might be required. Some advanced spacecraft
designs call for big tanks of liquid hydrogen fuel. “We could
protect the crew from radiation by wrapping the fuel tank around their
living space,” speculates Cucinotta. Of course, psychologically
speaking, astronauts might rather face cosmic rays than spend a year
living inside their spaceship’s gas tank.
“I’m confident we’ll
eventually solve these problems,” says Cucinotta, “and be able to
send people to Mars. We have a lot to learn between now and then.”
TWO FORMS OF RADIATION: SOLAR
FLARES AND GCRS.
On August 6, 1972, a solar
flare erupted. Billions of tons of protons raced toward Earth.
The particles, moving nearly as fast as light, swarmed around and past
our planet, enveloping Earth in one of the five biggest space radiation
storms ever recorded. On the planet below the event passed mostly unnoticed.
Earth’s atmosphere and magnetic field warded off the radiation, and
no one was harmed.
But what if an astronaut had
been “out there?”
Only four months later the
astronauts of Apollo 17 were. In Dec. 1972 they spent 12 days cruising
to the moon and back, protected only by the aluminum shell of their
spacecraft. If they had left on August 6th, Gene Cernan, Ron Evans
and Harrison Schmitt, would have absorbed, in a single hour, about 100
times more radiation than a person would living on Earth at the top
of a high mountain … in a lifetime.
This is a story often told
to illustrate the radiation dangers of space travel. A solar flare
can erupt at any moment, peppering astronauts’ bodies with light-speed
particles they rarely encounter on Earth. It sounds bad.
“In fact, those Apollo astronauts
would have been OK,” says Frank Cocinotta, the chief scientist for
NASA’s Space Radiation Health project at the Johnson Space Center.
“The command module was shielded well enough to ward off a strong
solar flare.”
Radiation doses to humans are
usually described in rem—short for R. E. M. A rem has units
of Joules/gm and measures the amount of damage to human tissue from
a dose of ionizing radiation.
“The August ’72 radiation
storm would have delivered a sudden dose of 30 to 40 rem inside an Apollo
command module,” says Cocinotta. For comparison, the threshold
for radiation sickness—nausea, loss of appetite and fatigue--is about
75 rem. “Fatalities don’t begin until 300 rem,” says Cocinotta.
“Without medical care, the death rate is 50%.”
So the Apollo astronauts wouldn’t
have gotten sick. And they wouldn’t have died.
For comparison, astronauts exposed to the August ’72 storm would have received a dose of about 40 rem. EXTRA CANCER RISK.
“That’s if they were
To get sick from a sudden dose of radiation, you would have to absorb about 75 rem. You would feel nauseous, etc. But after a while, your body would repair the damage, and you would recover. A sudden dose of 300 rem is more serious. Without medical care, there’s a 50% chance that you qould quickly die.