Sunday, May 30, 2010

The Radiation Exposure Numbers Game

The effect of prolonged radiation exposure on the human body is one of the potential show stoppers of long term human spaceflight. Yesterday we heard Zubrin present his traditional "don't worry about it" speech and another speaker refer to his reasoning as "the numbers game". By this, he meant that no-one in the audience is going to check those numbers so Zubrin can say whatever he wants. I think this speaker was missing the point. Let's look at what Bob was actually saying.


Zubrin's Assumptions

  • Six month transit times to Mars are available every 2 years
  • Radiation exposure on the Mars surface is negligible due to the availability of dirt for shielding
  • Solar radiation in deep space can be shielded against, so galactic cosmic radiation is all that matters
  • The GCR dose in LEO is about half that of deep space. It's about 55%
  • Mars crews would consist of 5 people
  • The ISS will continue to be crewed permanently by 7 people

Under these assumptions, a Mars program for 10 years results in a total crew dose of 5 * 10 / 2 * Deep_Space_Dose. Whereas the total crew dose for the ISS will be 7 * 10 * 0.55 * Deep_Space_Dose. If we normalize we get 25 < 38.5, so clearly Mars is a safer mission than ISS in terms of radiation dose, right?

Well no, we can argue with almost all of his assumptions. Transit times are more like 8 months. Radiation exposure on the Mars surface isn't negligible because astronauts will be doing lots of EVAs. Solar radiation shielding makes the galactic comic radiation dose worse, etc, etc. But I think this is the wrong way to go about it. Arguing about the assumptions draws us into the trap of thinking about overall program doses which are irrelevant.

What matters is the individual astronaut's lifetime dose. In the ISS program today, astronauts are limited to 9 months non-consecutive stays on the station (either three 3 month stays or a 6 month and a 3 month stay). Under the assumptions above, an individual crew member's dose in the Mars program is 1 * 1 * 1 = 1 normalized year. In the ISS program the crew member's dose is 1 * 0.75 * 0.55 = 0.4125 normalized years.

As such, an individual astronaut is more than twice as likely to die of cancer when they return from the Mars mission than an individual astronaut in the ISS program, and that's under the bad assumptions. In reality it's much worse.

Now that doesn't mean they shouldn't go. Even if it was absolutely certain that 10 years of your life would be cut off by cancer from going to Mars there would be a line of volunteers snaking out the door. What it means is that NASA is legally not allowed to send astronauts under the existing radiation exposure laws (which ISS strictly follows). It means those laws need to be changed or we need technology to make that radiation exposure less.

4 comments:

  1. As best that I can tell, most estimates of deep space exposure start from the assumption that the habitat module will follow the same basic structural design of an Apollo capsule or ISS module. That is, aluminum outer walls followed by layers of insulation, micrometeorite shielding, etc. It would be interesting to see how low the exposures could be made if habitats were designed with exposure minimization as the top priority from the start (within roughly the same total mass).

    For example, one would arrange water and fuel tanks, food stuff storage, and all other hydrogen rich and low atomic mass materials to surround the living quarters or areas where the most time is spent (just doing this around the sleeping quarters could reduce the exposure considerably for 1/3 of a crew-members time.) Also, using Bigelow style inflatable modules would eliminate the metal in the walls and thus reduce secondary production. I suspect that even cumulative GRC doses could be lowered significantly with such measures.

    I'll also note that in the long term, a Mars cycler type of transport, like that championed by Buzz, could have its shielding mass increased to an arbitrary level over time since it is in a stable orbit and doesn't need fuel except for occasional orbital corrections. Each vehicle that makes a rendezvous with it can leave behind some mass for shielding.

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  2. As for radiation, the whole thing about the radiation exposure is way out of line. News reports in the past have misrepresented the risk, stating that it might prevent human missions to Mars. However, it could be easily managed with current technology and is within tolerable limits. An astronaut in a six-month journey to Mars, the time required with conventional propulsion, would be exposed to about 0.3 sieverts, or 0.6 on a round-trip. Eighteen months on the surface (if it takes so long to get there, you might as well stay awhile!) would bring another 0.4 sieverts, for a total exposure of 1 sievert. Limits set by NASA vary with age and gender but range from 1 to 3 sieverts.

    The danger lies in an unexpected intense solar flare but there are "good enuff" ways to add in the protection needed. One way to add more protection spacefarers aboard a Mars transport ship might be to surround them with the water they'd need for their journey or hydrogen used for fuel. The hydrogen in water, scientists have learned, is one of the best absorbers of particle radiation. And, of course, the "better" way to lower the dose received would with shielding technology such as a simple magnetic plasma bubble that NASA has been testing for years. This alone would protect the astronauts from most radiation on its trip to Mars. Add a radiation compartment completely surrounded by water or hydrogen as mention above to stop the fast and slow solar neutrons then you would have a very, very safe journey.

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  3. Clark, there's been a whole lot of studies of using propellant as shielding, including some that say aerobraking is human Mars missions make no sense because the mass you save from fuel has to go into shielding.

    LupusSolus, and your evidence for this wild claim is? The established medical evidence is that, although non-solar-flare radiation levels has only a minor effect on loss of mission, the statistical cancer risk to crew is beyond the acceptable levels for astronauts - who are already 5x above the acceptable levels for other government radiation workers. This is published in the medical literature, if you want to refute it you have to be able to point to overwhelming contradictory studies.

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  4. Quant,
    One thing that you missed on the return trip is that it is likely to take much longer than even 8 months, baring nuke engines. As such, this is one more reason why ppl need to be sent on one-way missions. It is safer to stay on mars for multiple decades than to do the return trip.

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