Commentary | Curiosity’s Radiation Results
Last month, the investigators on the Radiation Assessment Detector (RAD) instrument aboard NASA’s Curiosity rover announced some of their findings. According to the scientists, the RAD measurements indicate that the crew of a human Mars expedition using present-day propulsion technology (six-month transits each way, 18 months on the surface) would receive a round-trip radiation dose of about 0.6 Sievert (Sv), or 60 rem (1 Sv = 100 rem.)
This result was not surprising. In fact it was entirely consistent with the data reported by many of the same scientists using the Mars Radiation Environment Experiment, or Marie, onboard the Mars Odyssey spacecraft during its outbound cruise to Mars in 2001, or estimates published by me in “The Case for Mars” in 1996, based on models developed still earlier by Viking scientist Ben Clark.
What was new, however, was that NASA headquarters chose to represent these predictable results as dramatic findings presenting a show-stopper for human Mars exploration. “The findings, which are published in the May 31 edition of the journal Science, indicate radiation exposure for human explorers could exceed NASA’s career limit for astronauts if current propulsion systems are used,” the official release stated. “Exposure to a dose of 1 Sv, accumulated over time, is associated with a 5 percent increase in risk for developing fatal cancer. NASA has established a 3 percent increased risk of fatal cancer as an acceptable career limit for its astronauts currently operating in low-Earth orbit.”
Thus, with an estimated mission dose of 0.6 Sieverts, the risk would be 3 percent, right up against the limit that NASA has devised for itself. Therefore, in compliance with its own regulations, the agency’s leadership is allegedly justified in avoiding the challenge of human Mars exploration, at least until such time as radical advanced propulsion systems capable of much faster interplanetary transits become available. They will let us know just as soon as that happens. In the meantime, we should all be content with a human spaceflight program that continues to spend billions of dollars every year for the foreseeable future in order to go nowhere.
This argument is irrational for several reasons.
In the first place, the 3 percent risk estimated for the 60 rem dose is overstated. The most comprehensive and authoritative study of radiation risk to humans is “The Effects on Populations of Exposure to Low Levels of Ionizing Radiation,” published by the Advisory Committee on the Biological Effects of Ionizing Radiation (and therefore known as the BEIR report) of the National Academy of Sciences in 1972. According to its Summary and Conclusions, “Cancer induction is considered to be the only source of somatic risk that needs to be taken into account in setting radiation protection standards for the general population. … In these populations, the excess mortality from all forms of cancer corresponds to roughly 50-165 deaths per million persons per rem during the first 25-27 years after irradiation.”
That is, if we take the extreme high end of the BEIR vulnerability estimate, 165 deaths per million person-rem, a dose of 60 rem would represent a 1 percent probability of contracting a fatal cancer sometime in the following quarter-century.
Furthermore, it must also be noted (as the BEIR report itself does) that the BEIR risk estimates themselves are extremely conservative and undoubtedly overstated, because they are based on the “linear hypothesis” that posits equal risk to small doses accumulated over time to large doses taken all at once. This is clearly false. For example, a person can drink a glass of wine every night for a year without harmful effect, but drinking 100 glasses (let alone 365) in one night would be fatal. Yet, based on the linear hypothesis, one would falsely predict a 1 percent chance of death from a single glass. The linear hypothesis makes the same wild error in purposely overestimating radiation risk. Yet even so, using that method, the upper end of the BEIR report places Mars mission radiation induced cancer risk at 1 percent.
In the second place, even if one were to stipulate to a much greater degree of risk associated with a given radiation dose, the NASA leaders’ argument for remaining parked in low Earth orbit makes no sense because they are already exposing astronauts to cosmic radiation there. That is, the interplanetary radiation dose rates measured by RAD and Marie are a factor of two higher than those astronauts currently receive on the international space station (ISS). But the ISS is manned constantly, while a Mars mission would spend only about 40 percent of its time in transit, with the other 60 percent being spent on Mars where plenty of shielding material is available. So assuming crews of equal size, the crews of the ISS over the next 10 years would collectively receive the same cumulative total radiation dose, and thus incur the same risk of a cancer fatality, as would the crews of five human Mars missions launched at every biennial opportunity over the same period.
As a third point, we note that the strategy of delaying human Mars exploration until faster space transportation becomes available is disingenuous, because none is on the way. The claims made by the Ad Astra Rocket Co. that its Variable Specific Impulse Magnetoplasma Rocket (VASIMR) could enable 39-day Mars transits are nonsensical, because they are based on the hypothetical availability of impossible space nuclear electric power systems simultaneously 10,000 times as large and 100 times lighter than any that have ever been built, and NASA has no program to build space nuclear electric power reactors of any type, regardless. Nuclear thermal rockets could enable four-month Mars transits, but NASA is not working on them either. Four-month transits could also be achieved with chemical propulsion simply by adding on more stages, but in that case overall mission safety would be much better served by using the increased propulsive capability to add more payload while keeping the transit time to six months. In fact, regardless of the propulsion system the six-month outbound transit is best for safety because it is the trajectory that uniquely provides a two-year free return to Earth without any need to loop into the hot inner solar system. If you try to fly to Mars faster, that critical mission safety feature is lost.
Finally, it must be said that the notion that stalling the human spaceflight program in low Earth orbit to avoid crew risk on meaningful exploration missions is a responsible humanitarian position is fundamentally flawed. This is so not only because it immorally degrades astronauts from explorers to guinea pigs, imposing risks on them as research subjects for others while preventing them from achieving purposes worthy of their courage. It is also so because the human spaceflight program costs a lot of money. If the goal is not space exploration but lifesaving, that money could be much better spent elsewhere.
For example, the U.S. Federal Highway Administration saves a life for every $3 million it spends on its repair operations. At funding levels on the order of $6 billion per year, the human spaceflight program is consuming funds that could otherwise save 2,000 lives per year. That is the cost of the enterprise we are engaged in. Conducted at such staggering cost, the human spaceflight program really needs to deliver. Only by opening of the solar system to humanity can the program stand worthy of such a price. For the space agency to continue to demand such sacrifice on the part of the public while refusing to do what is necessary to achieve its mission is morally indefensible.
If we are to have a human spaceflight program, it needs to go somewhere. NASA’s leaders should not be seeking excuses not to.
Robert Zubrin is president of Pioneer Astronautics and the Mars Society and the author of “The Case for Mars.” His latest work, “Mars Direct: Space Exploration, The Red Planet, and the Human Future,” was recently published by Penguin.