The Case for Mars (21 page)

Now, in addition to causing prompt radiation sickness and death when dealt out in overwhelming amounts, smaller chronic doses of radiation can increase the sta
tistical probability of cancer in humans and other animals. This is because the radiation-induced toxin introduced into a cell via a radiation hit can be a carcinogen. The exact relationship between such chronic doses and delayed cancer effects is not universally agreed upon, but it has been studied in far greater detail than the effects of any of the chemical carcinogens present in the human environment. For example, prior to 1960, large-scale radiation treatment to spinal bone marrow was used in Britain to deal with ankylosing spondylitis. Numerous follow-up studies were done on those so treated to look for radiation-induced leukemia. In the largest of these studies 14,554 adult patients were tracked for twenty-five years following their treatments, which ranged from 375 to 2,750 rem each.
¹¹
Of the group studied, sixty individuals died from leu
kemia, which compares unfavorably with an expected death rate from leukemia of six for a random group in the contemporary British population. Nevertheless, despite the huge doses, the fatality rate of the irradiated group was less than 0.5 percent. On the basis of this and hundreds of other such studies, the authoritative National Academy of Sciences-National Research Council study known as the Biological Effects of Ionizing Radiation (BEIR) report
¹¹
estimated the statistical probability of fatal cancer within thirty years induced by chronic doses of radiation totaling 100 rem in individuals over the age of ten (see
Table 5.1
).

TABLE 5.1
Estimates of Cancer Risk Due to Chronic Radiation Doses Totaling 100 rem

So, according to BEIR estimates, the likelihood of fatal cancer is 1.8 percent within thirty ye
ars for every 100 rem received. So if a female astronaut gets a dose of 50 rem over the course of a two-and-one-half-year Mars mission, and after her return lives thirty years until she dies of old age, the chance of her getting a fatal cancer due to that exposure would be 50/100 × 1.81% = 0.905%. (The chance of her getting a fatal cancer within one year would be 1/30th this amount, or 0.03 percent. The risk of radiation-induced cancer occurring dur occe mission itself is almost negligible.) If the astronaut were a male, the chance would be slightly less, 0.68 percent, because of the elimination of breast cancer risk. Assuming these astronauts don’t smoke, the chance of them dying of cancer if they did not go to Mars would be about 20 percent. Therefore, by taking the dose associated with the trip, they increased their cancer risk from 20 percent to somewhat less than 21 percent..

Now, in the above example I cited a chronic (not prompt) dose of 50 rem delivered over the course of a two-and-one-half-year Mars mission. The question then is, how do the mission profiles available to a present-day piloted Mars mission affect the radiation dose the crew may be expected to receive?

There are two kinds of radiation that can affect astronauts on a Mars mission: solar flares and cosmic rays.

Solar flares are composed of floods of protons that burst forth from the Sun at irregular and unpredictable intervals on the order of once per year. The amount of radiation dose a solar flare would deliver to a completely unshielded astronaut can be hundreds of rem in the course of several hours, which as we have seen would be enough to cause radiation sickness or even death. However, the particles composing solar flares individually each have energies of about one million volts, and can be stopped relatively easily by a modest amount of shielding. For example if we look at the three largest solar flares recorded in history, those of February 1956, November 1960, and August 1972, we find that the dose they would have delivered to an astronaut protected only by the hull of an interplanetary spacecraft like our hab (which with its hull, furniture, miscellaneous engineering systems, fittings, and other objects has about 5 grams per square centimeter of mass spread around its periphery to shield its occupants) would have averaged about 38 rem, while if the astronaut had gone into an onboard pantry storm shelter (where the Mars Direct hab has about 35 grams per square centimeter of shielding see
Figure 5.1
) he could have been shielded by stacked provisions reducing the dose to about 8 rem.
¹²
,
¹³
,
¹⁴
If he had
been sitting in the hab
on Mars
during an
event representing the average of these flares, he would have taken about 10 rem if outside the shelter, or 3 rem within the shelter. (The Mars surface doses are much lower because the planet’s atmosphere and surface shields out most of the flare.)

FIGURE 5.1
A schematic of the Mars Direct hah. In the event of a solar flare, the airlock could double as a storm shelter for the crew
.

Cosmic-ray doses are different. Because they are composed of particles with energies of billions of volts, it takes meters of shielding to stop them, which basically makes cosmic-ray shielding impossible during interplanetary flight. On Mars, however, cosmic-ray shielding can readily be provided by the planet itself, which masks out all cosmic rays coming from below, and by the use of sandbags to block out at least part of the cosmic-ray dose impinging upon the hab from above.

Also, unlike solar flares, cosmic rays don’t occur in huge occasional floods. Rather they occur as a fairly constant but thin rain of radiation. An astronaut in the hab during flight through interplanetary space will take a cosmic-ray dose varying between 20 rem and 50 rem per year, depending upon where the Sun i
s in its eleven-year sunspot actiity cycle. The biggest cosmic-ray doses will occur during the period of minimum solar activity, because during the period known as “solar max” the Sun’s magnetic field expands and actually manages to shield the whole solar system to some extent against cosmic rays coming in from interstellar space. So as an average, however, 35 rem per year of cosmic-ray doses can be expected during interplanetary flight. If the crew was unsheltered on the surface of Mars, the cosmic-ray dose would be about 9 rem per year, while under shelter (a sandbag-roofed hab) it would be about 6 rem per year. Since the crew will be spending most, but not all of its time on Mars in the hab, 7 rem per year of cosmic rays is probably a reasonable average for that phase of the mission.

If we put all this data together with the flight profiles of the conjunction and opposition missions, and assume that a solar flare equal to the average of the three worst in history occurs at a rate of once per year during the mission, we obtain the predicted radiation doses shown in
Table 5.2
.

TABLE 5.2
Radiation Dose Experienced on Mars Missions

As discussed in the previous chapter, the Mars Direct mission would use the conjunction trajectory, whose estimated round-trip mission radiation dose varies between 41 and 62 rem, depending upon whether the Sun is at solar min or solar max phase of its eleven-year cycle. So, the 50-rem estimate for a round-trip Mars mission is realistic, reflecting an average value between solar min and solar max conditions. We can also see that the worst expected solar flare dose on the Mars Direct mission is
about 5 rem, far below the 75 rem threshold for any prompt radiation sickness effects.

Looking at
Table 5.2
, note also how silly the arguments for the opposition-class mission are from the standpoint of radiation dose reduction. Despite its much greater mass and cost, and much lower mission value (due to its limited stay on Mars), the total radiation dose received on the opposition-class mission is more than the conjunction mission, and its expected prompt dose from solar flares is 75 percent higher. But basically, the chronic doses experienced on either of these trajectories are predictable, and are negligible when compared to all the other risks that must be accepted in manned space flight. The only real risk from radiation is the possibility of a freak solar flare delivering a prompt dose far in excess of anything that has been measured over the past fifty years. The chance of this occurring is much higher on the opposition-class mission due to its close-in pass to the Sun. Thus, there is no radiation-dosage rationale for choosing the opposition mission plan over a Mars Direct-style conjunction or even minimum energy trajectories. Quite the contrary, from the perspective of radiation hazards, the opposition trajectory is the worst choice possible.

By the way, contrary to the scare-mongering of certain people who would like to obtain large research budgets in this area, there is nothing extraordinary about cosmic-ray radiation doses compared to other types of radiation doses. Cosmic rays deliver about half the radiation dose experienced throughout life by people on the surface of the Earth, with those living or working at high altitude receiving doses that are quite significant. For example, a trans-Atlantic airline pilot making one trip per day five days a week would receive about a rem per year in cosmic-ray doses. Over a twenty-five-year flying career, he or she would get more than half the total cosmic-ray dose experienced by a crew member of a two-and-one-half-year Mars mission.

So, once again, using only chemical propulsion, not warp drive, we can fly a crew to Mars and return them home with radiation doses limited to 50 rem or so. While such doses are not to be recommended to the general public, they represent a small fraction of the total risk of not only space travel, but such common recreations as mountain climbing or sailboarding. Radiation
hazards are not a show stopper for a piloted Mars mission.

ZERO GRAVITY

Another dragon barring the path to Mars is the menace of zero gravity. Long-duration exposure to zero gravity carries the risk of serious deterioration to human muscles and bone tissue, we are told, and, therefore, before astronauts go to Mars we must undertake a long-term program of experimentation with human subjects exposed to extended periods of zero gravity on board the Space Station. This program will require several decades, many billions of dollars in “microgravity life science research,” and a few dozen human beings willing to sacrifice their health to “scientific research.”

I find this argument bizarre. Now, it is certainly true that spending long periods in zero gravity will cause cardiovascular deterioration, decalcification and demineralization of the bones, and a general deterioration of muscular fitness due to lack of exercise. Zero gravity also depresses some aspects of the body’s immune system. These effects are well documented from the experiences not only of the U.S. Skylab astronauts, who spent up to three months at a time on-orbit, but of Soviet cosmonauts, many of whom have spent stints in zero gravity on their
Mir
space station of over six months, and some for almost eighteen months—nearly three times the duration of the trans-Mars or trans-Earth cruises required to perform the Mars Direct mission. In all cases, near total recovery of the musculature and immune system occurs after reentry and reconditioning to a one-gravity environment on Earth. The demineralization of the bones ceases upon return to Earth, but actual restoration of the bones to preflight condition appears to be a very extended process. The Soviets have experimented with various countermeasures to zero gravity, including intensive exercise, drugs, and elastic “penguin suits” that force the body to exert significant physical effort in the course of routine movement. As might be expected, programs of intensive (three hours a day) exercise have proven effective in reducing general muscular deconditioning, and to some extent cardiovascular deterioration, but countermeasures taken to date have shown little benefit i
n slowing bone demineralization. It should be understood that while these effects are all quite tangible and definitely not desirable, they are not too extreme—in no case have such zero-gravity “adaptations” prevented astronauts or cosmonauts from satisfactorily performing their duties while they are in the zero-gravity environment, and after even the longest flights, crew members recovered enough to become basically functional again within forty-eight hours after landing. Indeed, within a week of landing, the members of the eighty-four-day duration Skylab 3 crew were able to play strong games of tennis. The recovery time to functionality upon Mars arrival after a six-month zero-gravity exposure should be swifter, because the crew will only have to deal with reacclimation to Mars’ 0.38g environment after landing, instead of the lg shock experienced after reentry on Earth. The point, however, is that an awful lot of research has already been done in this area, and we know what the effects are. Given that is the case, we can rightly ask whether it is necessary, or even ethical, to subject further astronaut crews to such experimentation solely for the purpose of more exhaustive research on zero-gravity health deterioration effects. I don’t think it is. In fact, given what we know today, I’d have to classify the proposed program of continued experimentation on humans with long duration zero-gravity health effects as unethical and worthless—and I know a lot of astronauts who agree with me on that point. It just doesn’t make sense to expose dozens of astronauts to a larger zero-gravity dose than a Mars mission might provide in order to “insure the safety” of a much smaller crew who actually fly there. Doing so is like training bomber pilots by having them fly their planes through real flak. If you are willing to accept the health consequences of long-duration exposure to zero gravity, you might as well take your licks in the process of actually getting to Mars.