Oddly enough, according to cancer specialist Dr. John Dicello of the John Hopkins University School of Medicine, radiation "is relatively poor at inducing cancer."

Chemicals, he says, can do far more damage, as shown by the strong tie between environmental contaminants and increased levels of cancer.

But for space travelers, radiation is a tremendous worry. That's because astronauts are exposed to far more radiation than we typically encounter on Earth. And it's a different kind of radiation.

Cosmic rays from deep space, for instance, are composed of heavier particles than our bodies are used to, and they have little trouble breaking strands of DNA.

Broken DNA, by itself, is not necessarily cause for alarm. DNA strands break all the time. Even a physical blow will do it.

"If you hit yourself with a hammer," notes Dicello, "that can do a lot more damage than most radiation exposures." Because this kind of damage occurs so frequently, the body has evolved mechanisms to handle it.

Sometimes, he explains, cells with damaged DNA simply destroy themselves. Other times, they try to repair the damage.

Problems start when they do this incorrectly. They might, for example, insert a chunk of DNA in the wrong place. Or they might attach it to the wrong chromosome.

When that happens, it's possible that the mistake will let that cell ignore constraints designed to make cells behave. A cancer can start when altered genes allow the cell and its descendants to multiply too freely.

So far the story sounds simple: Radiation damages DNA. Repairs are bungled. Cancer ensues.

But it's not so simple, says Dicello, not at all. Radiation can affect human tissue in unpredictable ways, and the chain of events leading from radiation to cancer is vexingly complex. "If I really understood it, I'd probably win the Nobel Prize."

Consider the following: Some astronauts, veterans of long space missions, have "significant chromosome aberrations" in their blood cells.

These aberrations may be "associated with the development of cancer," says Dicello, but they do not, by themselves, cause cancer.

For that to happen, cells with aberrations must undergo a series of further mutations.

According to the National Cancer Institute, "the number of cell divisions that occur during this process can be astronomically large - human tumors often become apparent only after they have grown to a size of 10 to 100 billion cells."

Years, even decades, might pass between the onset of the problem, the exposure to radiation, and the appearance of a tumor.

Because of the delay, it's very difficult to determine exactly when or why a cancer starts. That's the bad news.

The good news - for astronauts and for the rest of us - is that there are many places along this slow developmental path at which an incipient cancer can be stopped.

Indeed, researchers have pinpointed some of the genes involved3 and they're working on treatments targeted directly at those genes.

Understanding how to stop the cancer, however, doesn't necessarily tell us how it starts.

Cells often react in unexpected ways to radiation, notes Dicello. For example, there's a puzzling phenomenon known as adaptive response. Sometimes, when tissue is exposed to damaging radiation, it not only repairs itself, but also learns to repair itself better next time.

How that works is still being investigated.

Furthermore, radiation damage is not always proportional to the amount of radiation experienced. "Our research shows some unusual things," says Dicello. Some types of chromosome aberrations are very sensitive to radiation.

"Deliver a low dose, and they take off." Other types of aberrations require much higher doses. Researchers are still trying to sort out which is which � and why?

These uncertainties make it hard to predict how human tissue will react to space radiation.

Astronauts, points out Dicello, will encounter at least two kinds of radiation: (1) high-energy cosmic rays from distant exploding stars and (2) less-energetic protons and photons from flares on our own sun - and they can be exposed to both at the same time.

While researchers know something about how cells respond to each kind of radiation separately, some of Dicello's work suggests that exposure to these two types of radiation mixed together could produce as-yet unpredictable results.

The damage could be less than the two kinds added together - or it could be more! There could, perhaps, be an adaptive response in which lightweight solar protons stimulate repair processes to help reduce the effects of the heavy cosmic ray ions.

Or something totally unexpected could happen.

There's still a lot to learn. Dicello lists some of the questions: "How important is adaptive response? How important is the effect of cells on each other? How important are antioxidants? We don't yet know."

"The answers are important to everyone," he adds. Understanding how the body deals with damaged DNA could help doctors prevent complications from the radiation treatments given to cancer patients.

It might help them deal with the fallout from, say, a terrorist's dirty bomb or DNA-problems caused by environmental or chemical pollution.

Eventually, Dicello believes, researchers will figure it out. And when that happens, people on Earth will benefit at least as much as people in space...

...who can then turn to lesser worries, like double-jawed aliens.

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