The Invisible Killers
We have the technology to send astronauts to Mars. But can we return them safely to Earth?
- By John F. Ross
- Air & Space magazine, January 2006
(Page 3 of 5)
Once the door opens, the white-painted cinderblock hallway cuts left, then right, then left again, a precaution against errant particles escaping. The hallway ends at the chamber, which contains a 30-foot track of parallel stainless steel bars; the bars follow the path of the particles and disappear into a hatch in the wall. As the particles travel down the track toward this room, a series of powerful magnets attached to the bars accelerates them and focuses their path.
Marcelo Vasquez, an energetic Argentinian-born biologist and physician, is chief of medical research at NSRL. Presently, he is using mice to look at the effect of ionizing radiation on cognitive function. Vasquez and his colleagues built a three- by three-foot plexiglass pool with a small platform within. They trained mice to swim to the platform and climb on it. After the mice grew proficient at the task, the scientists recorded their times.
Vasquez then strapped three trained mice at a time to a small block of Lucite and irradiated them in the accelerator chamber. Next, he put them back into the water and found that it took the mice longer to find the platform than it had before. The radiation exposure, says Vasquez, caused the animals to lose brain cells quickly. He does acknowledge, however, that in his experiments he administers high doses of radiation, and one can’t necessarily extrapolate directly from the mouse results to what humans will experience.
Vasquez brings up on a computer screen a series of slides showing mouse brain cells exposed to increasing levels of ionizing radiation. The network of axons and dendrites, the structures that enable cells to communicate with one another, first appears as a field densely packed with shapes and fibers. By slide four, the picture is sparsely filled: Cell nuclei look like they have exploded, their contents spread randomly. The image is reminiscent of a block in World War II-era Dresden after a bombing run. The radiation, explains Vasquez, doesn’t kill all the cells, but it severely disrupts the flow of signals. Vasquez’s colleague, Derek Lowenstein, chairman of Brookhaven’s collider accelerator program, has given voice to deep fears among scientists by asking: “Will astronauts come back blithering idiots or not?”
Vasquez is also concerned about other factors that may exacerbate radiation damage. “We’re testing mice here on Earth in a comfortable 1 G environment,” he says. Put people in space, and “their physiology will be stressed and that can’t help their response to radiation damage.”
Another NSRL researcher, Betsy Sutherland, is studying the cellular destruction wrought by ionizing radiation. If an ionizing particle hits DNA in a cell’s nucleus, it can cut one or both strands of the double helix like a chainsaw ripping through a tree branch. Evolution has ensured that organisms have mechanisms to repair insults to genes, which occur regularly from such sources as the sun’s ultraviolet rays and natural toxins contained in food. Proteins move quickly to reattach broken strands and splice in new sections of DNA if necessary. Cells too badly damaged to be fixed get tagged by the p53 gene, which orders the cell’s death. From the organism’s perspective, it’s better that a cell die than become fixed incorrectly: Cells with mutations could lead to cancer or defects that can be passed on to the next generation. Sutherland says that ionizing radiation appears to impede the p53 gene from doing its job.
Sutherland and other biologists have noted other disturbing effects of radiation on cells, such as “the bystander effect,” in which damage to one part of DNA causes damage to other DNA segments far away.
The researchers at the NSRL are well aware of the limitations of the work here. For example, they can shoot only one type of heavy-ion radiation at a time; in space, astronauts will be exposed to a barrage of many kinds. It is also difficult for researchers to design a lab simulation that shows how space radiation is distributed among various parts of the human body. The European Space Agency built a simulated human torso, Phantom, which was attached to the outside of the International Space Station in 2001. The dummy contained actual human bone, plastic material simulating soft tissue, lighter material representing lung tissue, and a covering of Nomex to simulate skin. Phantom was also enclosed in material simulating a spacesuit. About 350 radiation meters were placed throughout the torso, including the sites of critical and susceptible organs, such as the brain, heart, thyroid glands, and kidneys. The results from Phantom’s exposure turned out to be similar to those predicted by NASA models. The experiment also showed that more than 80 percent of the radiation that hit the dummy came from cosmic rays; protons, on the other hand, were weakened by passing through the spacecraft and Phantom’s skin.