AW: [ RadSafe ] Two unrelated questions for the group

[Rainer.Facius at dlr.de]

Douglas, I address your second question:


 


Since I don not know this article I can comment only with some general remarks.


 


For long term missions outside the shielding provided by the geomagnetic field planning manned space missions has to make allowance for two sources of ionising radiation. One is the rather regularly varying irradiation by galactic cosmic radiation (GCR). During maximum solar activity - as we had it about 3 years ago and will have it about every other 11 years, their intensity is minimal and vice versa. 


 


Without dramatic progress in propulsion technology the most feasible (also financially) mission to Mars will take some 422 days in space (round-trip) and 525 days on Mars' surface.


 


For such a mission, depending on the mass of shielding material available, the equivalent dose from GCRs to the blood forming organs (BFO - a proxy for the effective dose) according to present prediction capabilities will range between 280 to 918 mSv depending further on the solar activity during the mission and it s atomic composition. Hydrogen rich material like polyethylene (PE) performs better as shielding material due to its smaller contribution to secondary fragmented ions in comparison to Aluminium (AL). In the table below, Min refers to a mission during solar minimum, Max to solar maximum and the numbers give BFO equivalent dose in mSv.


 


                             AL.....918
             Min
                             PE.....846
5 g/cm**2
                             AL.....383
             Max
                             PE....353
-------------------------------------
                             AL.....852
             Min
                             PE.....748
10 g/cm**2
                             AL.....364
             Max
                             PE....317
-------------------------------------
                             AL.....769
             Min
                             PE.....649
20 g/cm**2
                             AL.....339
             Max
                             PE....280
-------------------------------------




Obviously, planning the mission with respect to its position in the solar cycle would argue to have it during solar maximum. As regards shielding by matter, the energies of the GCR heavy ions make them very difficult to shield against. A factor of 4 in mass (that has to be propelled too) at most yields a reduction in dose by a factor of 1.3!


 


The physics behind these predictions is sufficiently well understood by now. The great uncertainty is whether the radiation quality factors used in the conversion of absorbed dose to equivalent dose truly reflect the radiobiological effectiveness of the GCR ions. I would not be surprised if finally it turns out that they do not. Assuming that they do, the above doses accumulated during nearly 3 years may appear gigantic for terrestrial radiation workers. Nevertheless, the safe return of the crew will not be compromised by health effects ensuing from these exposures. A second unresolved longstanding uncertainty in this statement of course is that the cellular and tissue reactions which under terrestrial 1 g conditions counteract and control the development of radiogenic cancers retain their normal efficiency under the numerous and persistent physiological changes brought about by long duration weightlessness. Even if not, a health detriment which might jeopardize a safe return is unlikely, yet, increased late mortality from cancer will be a likely risk.


 


The second radiation component in interplanetary space is energetic charged particle radiation, mainly protons, from solar particle events (SPE) commonly called flares. Unfortunately, the probability that large events will occur is maximum during solar maximum and essentially nil during minimum solar activity! 


 


In contrast to GCR heavy ions, however, their lower energies make them easier to shield against. Nevertheless, the intensities in extreme SPEs may reach levels where in interplanetary space behind a shielding of even 10 g/cm**2 (often called a "radiation  storm shelter"; whether AL or PE does no more matter here) still acute doses of about 1.3 Gy to the bone marrow, 2.5 to the lens and 2.6 to the skin are to be expected for what we may call a worst case event according to our present experience. Only if the sensitivity to induction of early deterministic radiation effects had been significantly enhanced by weightlessness (again the same question mark!) would we expect that such exposures could trigger a fatal mission abort. Some kind of transient performance decrement might be expected and then it could depend on the criticality of the mission phase where such an SPE occurs. The frequency of such extreme events (one in 50 years is our current sample size) is too far low to have us expect more than one such extreme SPE during a mission. One most essential prerequisite of course is that mission planning and control can ensure that indeed under all conceivable circumstances the crew will have time to find shelter. 


 


We have some indications that our sun might enter into an era of more violent activity. When and to what amount that might invalidate our 'worst case' predictions can only be answered by prophets.


 


I hope that helps and reinforces your son's resolve to go ahead.


 


Best regards, Rainer


 


Dr. Rainer Facius


DLR, German Aerospace Center


Institute of Aerospace Medicine 


Dept. Radiation Biology


51147 Cologne


GERAMNY


+49 2203 601 3147


________________________________

Von: radsafe-bounces at radlab.nl im Auftrag von Minnema, Douglas
Gesendet: Fr 02.12.2005 16:42
An: Radsafe (E-mail)
Betreff: [ RadSafe ] Two unrelated questions for the group




All,

I have two unrelated questions that I'm combining into one e-mail for
convenience.

The first one has to do with Am-241 sources in smoke detectors.  What
happens when the smoke detector is burned in a building fire?  Is the source
expected to survive intact or is there a potential for a release of some
type?  I would assume this has been considered multiple times, but I haven't
found the answer yet. If somebody can point me in the right direction I'd
appreciate it.


Second question:  My 15-year old son has a subscription to "Air & Space
Smithonian" magazine, and in the December 2005/January 2006 issue there is
an article entitled "The Invisible Killers: Can Astronauts Survive the
Radiation on a Journey to Mars?".  Has anybody seen this yet?  I'm not
familiar with current efforts in protecting astronauts from space radiation,
but some of the statements made in this article are clearly wrong, which
leads me to suspect that either the article was designed to be inflammatory
or the author just didn't understand the information he was given.  A couple
examples:

"From World War II atomic bomb detonations in Japan and the 1986 accident at
the Chernobyl nuclear reactor near Kiev, Russia, we know the effects of
brief but intense pulses of radiation: nausea, immune system shutdown,
central nervous system damage, and death within minutes to hours."

"Derek Lowenstein, chairman of Brookhaven's collider accelerator program,
has given voice to deep fears among scientists by asking: "Will astronauts
come back as blithering idiots or not?""

"The U.S. Occupational Safety and Health Administration treats astronauts as
radiation workers."  "Today, the law limits the amount of radiation that
nuclear workers, including astronauts, receive to 5,000 millirem over the
course of their careers."

Since my son's long-held goal is to pilot the first spacecraft to Mars (and
yes, he is working hard towards that goal), he was obviously curious about
the article.  He does understand that ionizing radiation is often
misunderstood or mis-stated in the media (he is the son of an HP), but in
this case I can't answer all of his questions.

If somebody knows about this article or would be willing to look at it for
me, I'd much appreciate it.  Please contact me directly and I'll get a copy
of the article to you if necessary.

Thanks,

Doug Minnema PhD, CHP
<Douglas.Minnema at nnsa.doe.gov>


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