Manned Mars mission: The big picture
2. Is it wise?
3. Blasted by a ray gun
4. Can't we get along?
Galactic cosmic rays contain a variety of
elements. Although lighter nuclei are more common, hefty, high-speed
iron nuclei can pack quite a punch. Could your brain handle it?
Graph: National Space Biomedical Research Institute
Clouds of interstellar gases collapse into
a star, which much later forms a supernova. The explosion ejects
vast quantities of particles, including cosmic rays. Could these
babies bang your bod? Copyright ©Drs.
R. Mewaldt, E. Stone, and M. Wiedenbeck, California Institute of
Technology, courtesy NASA
Space travel, classic style. In 1994, astronaut
Mark Lee tested a jetpack powered by small nitrogen thrusters, in
the bay of Space Shuttle Discovery, more than 200 kilometers above
Earth. Because Lee was in low orbit, he was largely protected from
cosmic rays. Would speeding heavy ions buffet the brains and bodies
of Mars-bound explorers? Photo: Shuttle
Crew STS-64, NASA
Here's a fun fact: Space is filled with nasty
radiation, including electromagnetic radiation (X-rays and gamma
rays) and cosmic rays (protons and heavy ions). At Earth's surface,
the atmosphere and Earth's magnetic field repel or temper the vast
majority of these deadly rays.
In orbit, above the atmosphere, astronauts face higher doses of electromagnetic radiation and protons, but the magnetic field, like a protective cocoon, still repels dangerous cosmic rays. Further from Earth, however, levels of cosmic-rays - including heavy ions like iron, silicon and carbon -- increase dramatically.
Due to their high mass and extreme energy, cosmic rays
can be even more damaging than X-rays or gamma rays, says Marcelo
Vazquez, an associate scientist at Brookhaven National Laboratory
in New York who is also a liaison between the lab and NASA.
Heavy ions can kill a cell on impact, or cause the cell to die via programmed cell death, or apoptosis. But a heavy ion can also kill other cells, Vazquez says. "You would think it would need to hit a cell to produce damage," he says, but the initial impact liberates electrons that can damage cells beyond the path of the original particle.
That's called the "bystander effect," but the picture gets even worse, Vazquez says: "Cells that have been damaged can send chemical messengers to other cells, producing secondary damage."
This is not just a theoretical problem for interplanetary travel, says Vazquez, who studies heavy ions for NASA's Space Radiation Laboratory.
While X-rays and gamma rays can cause cancer, Vazquez focuses instead on cell death, especially in the central nervous system. The prospect of damage to the brain arose after an Apollo 11 astronaut saw flashes of light. Scientists began to suspect that the cause was cosmic-ray impacts on the brain, eye or optic nerve - all tissues of the central nervous system.
But aside from moonshots, the danger was theoretical,
since Earth's magnetic field protects astronauts in orbit near the
That comfortable equation would change on a trip to Mars: An experiment by Vazquez and colleagues found signs of serious brain damage in mice they zapped with heavy ions. Before the ray blasts, the mice learned to locate a shallow spot in a pool, where they could easily keep their heads above water. After exposure to heavy-ion radiation, the mice had some difficulty remembering the location of the shallows, a sign of possible damage to brain areas involved in spatial memory and learning, says Vazquez.
The rays came from Brookhaven's Alternating Gradient Synchrotron. Although the radiation resembled
galactic cosmic rays, the dose was 10 times what a Mars mission
The results were disturbing. But an estimate of radiation exposure on a three-year Mars exposure produced a separate cause for concern, Vazquez adds: 46 percent of cells in the hippocampus [a center of memory and learning] would be struck by at least one heavy ion (with an atomic mass above 16). "If this is true, and the heavy ions produce such damage, it's really scary, because 46 percent of the cells can be potentially be destroyed," he says.
While Vazquez stresses that these results come from a computer model, not actual data, he points out that the hippocampus is not an optional hunk of brain hardware: It's necessary for learning and memory.
Although heavy ions are emerging as a major hazard of interplanetary space travel, Vazquez says, "we don't have a sufficient amount of hard data on heavy-ion induced brain damage. We have hypotheses and some data that suggest a threat; potentially it could be a show-stopper."
The obvious solution to these dangers would be to mount a heavy (and therefore expensive-to-launch) shield on your Mars-bound spacecraft. But that could backfire. Just as cosmic rays make secondary electrons after hitting a cell, they can also release secondary particles (other heavy elements, neutrons and X-rays) after hitting a shield. And these aren't too healthy for astronauts and other living things.
If shields are ungodly expensive and of questionable value, what about shielding cells themselves? Such "countermeasures" (argot, much?) could include drugs or food supplements that reduce or reverse radiation damage.
One major candidate is antioxidants, which, as any supplement-stuffed, Earth-bound health freak can tell you, play a starring, but still murky, role in the quest to prevent cancer on Earth. Antioxidants are chemicals that alter the chemical balance away from oxidation, which plays a fundamental role in cancer and other aging processes.
Previous studies of astronauts returning from orbit found reductions in "total antioxidant status," a measure of the total ability of cells to counteract oxidation. That indicates that a long space voyage would increase vulnerability to cancer and degenerative diseases.
Could antioxidants reduce this vulnerability? Perhaps, according to a recent study for the National Space Biomedical Research Institute. Ann Kennedy, a professor of radiation oncology at the University of Pennsylvania, and her colleagues found that low doses of an antioxidant called selenomethionine could absolutely block the oxidative effect of heavy ions in rats.
with iron ions, like those found in cosmic rays, reduced the antioxidant
status of rat cells, raising the chance of cancer. But when the
rats ate the food supplement selenomethionine, their antioxidant
status improved, even when radiation was present. Could selenomethionine
protect astronauts on a voyage to Mars? Diagram
courtesy Ann Kennedy (see "Selenomethionine Protects..." in the
Indeed, selenomethionine was also somewhat effective at combating the oxidative effects of other types of space radiation. Another positive: The compound is already considered safe enough to be the center of a large prostate-cancer prevention trial.
How does selenomethionine reverse the oxidative effects of radiation? Possibly by increasing the activity of two genes linked to cancer prevention: ATR, which helps repair damaged DNA, and CHK2, which suppresses tumors.
One final question: Why focus on antioxidant status rather than count cancers? Because cancer prevention trials take so long that many researchers prefer to focus on "surrogate biomarkers." These are things you can measure (like antioxidant status) that are probably related to the thing that you care about (cancer).
"We want to identify something that appears to be correlated with the induction of cancer, so when we see a change in the endpoint, you can assume that you will affect the yield of malignancies," Kennedy says. Now, having proven that selenomethionine restores antioxidant status, she wants to see if it also blocks cancer formation.
How can we be sure that astronauts
can work together?