Re: SPACE: Cassini Mission Consequences

Amara Graps (
Sat, 20 Sep 1997 12:19:18 -0800

>Although I have been in the periphery of the adversarial panoply of Cassini,
> I'd appreciate hearing your thoughts.


I've been following this Cassini debate very closely *.

My personal opinion is that the antagonists are using this as a banner for
their personal causes. I honestly don't know why it has carried so far.
My scientist friends are perplexed also. You may be interested
in a "back-of-the-envelope" calculation that Jeff Cuzzi performed
to demonstrate that the Plutonium risk is pretty small.
I attach that below.


* Lots of reasons. My PhD advisor at MPI-Heidelberg built the
Dust Detector instrument on Cassini, the Cassini Project at MPE-Garching
is funding my PhD project, and I have many friends and colleagues
who are project scientists on Cassini (including Jeff Cuzzi, the fellow
who wrote the essay below).


Subject: J. Cuzzi on Cassini plutonium hazard
From: (Bill Higgins)
Date: 1997/09/18
Message-Id: <>
Summary: Simple approximate calculation of worst Cassini accident effects
Keywords: Cassini plutonium 238 Saturn NASA
Distribution: world
Organization: Fermi National Accelerator Laboratory
Newsgroups: sci.astro,,sci.environment,talk.environment
[Fewer Headers]

I've just seen a message from Jeff Cuzzi, a prominent planetary
scientist working on the Cassini mission, and I thought I would pass
it along with his permission. Opinions are his, not mine and not my
employer's. He offers this additional disclaimer: "Remember it is
not an official NASA position but a calculation ANYONE could do!"

Bill Higgins Internet: HIGGINS@FNAL.FNAL.GOV
Fermi National Accelerator Laboratory

Date: Tue, 16 Sep 1997 22:14:46 -0700
From: (Jeff Cuzzi)
Subject: Plutonium primer

Cassini Plutonium for the technically minded
by Jeff Cuzzi

I'm sure we will all have friends and relatives asking us what's up
with the Cassini Plutonium issue as launch approaches in early
October. Allegations of risk have arisen due to Cassini's onboard
RTG's (Radioisotope Thermal Generators) which derive electricity from
decay of 72 lb (33kg) of Plutonium dioxide fuel.

In anticipation, I wanted to provide some "derived from basic
principles" satisfaction that the Cassini health threat is negligibly
small even in the extremely small chance that anything does go wrong
with the mission (either at launch or at flyby). The Cassini project
has devoted more than a million dollars to a thorough analysis of the
problem, but the back-of-an-envelope analysis below is a little easier
to grasp and serves as a calibration and sanity check.

I am a Cassini scientist, and neither a health expert, nor a nuclear
physicist. I do care about the health of the people of the world. I
had several discussions with a physicist at the Nuclear Regulatory
Commission (NRC) concerning decay rates and comparative relationships
to health effects. I also had this reviewed by the President of the
Health Physics Society, a 6500 member national organization (who has
publicly stated that NASA has done a very good job and has, if
anything, OVERestimated the health risks).

For my initial health effect data I relied on Web sites maintained by
the EPA and the Agency for Toxic Substances and Disease Registry
(ATSDR; part of the Center for Disease Control - see references
below); my NRC and Davis contacts confirmed these values and
identified their primary source (FGR-11, 1988). I suspect anyone can
reproduce the calculations below who can read a simple physics
textbook and the World Wide Web.

238-Pu decays by alpha-particle emission (like the longer-lived
weapons grade isotope 239-Pu, but 250x faster). The decay rate can be
calculated from the half life (88 yrs) and the number of nuclei per
gram, and is about 6E11 decays/sec/gm, defined as 17 Curie/gm. A Curie
(Ci) of 238-Pu and a Ci of 239-Pu have the same radiation damage
potential (they emit the same alpha particles). Because 238 decays
faster, it has a higher Ci/g rating by the ratio of half lives (about
250). The convenient unit is pico-Curies (10^-12 Ci = pCi).

Health standards are set by the International Commission on
Radiological Protection (ICRP), and found in FGR-11 and the ATSDR web
page. The conversion factors between radioactivity (Ci) and potential
tissue damage in rem (Roentgen Equivalent Measure, or more often
millirem (mrem = 10^-3 rem) are from the FGR-11 (note 1). They can be
derived from values on the ATSDR page as well. The ATSDR quoted Annual
Limit on Intake (ALI) is 20000pCi/yr for "workers", and the
corresponding dose limit is 5 rem/yr, giving a conversion factor of
0.25 mrem/pCi (note 1), in good agreement with the standard value of
0.29 mrem/pCi tabulated in FGR-11.

Several expressions can be found for EPA-allowable levels of
radioactivity. The ATSDR web page gives a mixture of recommended
limits for the public and for "occuptional exposure" in rem, Annual
Limits on Intake (ALI) in pCi/yr, and in Derived Air Concentration
(DAC; pCi/m3) levels. These are generally consistent with a 10 times
lower limit for the general public than for workers, but my NRC
contact says the DAC's for the general public are maybe another 10
times smaller than can be inferred from this web page (probably
factors for time off-job as fraction of 24 hr, etc).

Also, it appears that the 500 mrem annual limit for the public cited
by ATSDR probably includes the unavoidable background level of 360
mrem/yr from Radon gas, cosmic rays, the dentist, etc. My NRC contact
thinks this would be consistent with his knowledge of an ICRP
recommendation for the public of no more than 100 mrem annually above
the annual background.

Presume a worst case scenario involving vaporization of ALL the Pu-238
that is in the RTG's. This 'astrophysical accuracy' calculation makes
no allowance for removal of Pu into the ocean, by rainout, deposition
onto uninhabited terrain, etc. The 72lb of Cassini fuel is actually
nearly 30% oxygen and less active Pu isotopes, so is only 50 lb Pu-238
= 23 kg = 400,000 Ci (about 17 Ci/g). The volume of air in the
Northern troposphere and stratosphere (which receive 99% of the Pu) =
2 pi X (10 + 40) X 6000^2 km3 = 10^19 m3.

Dispersion of all this vaporized Pu in the northern atmosphere gives a
radiation density of about 0.04 pCi/m3, comparable to the allowable
DAC. The ATSDR numbers imply that you breathe air at about 0.1
liter/sec (plausible) so get 3000 m3/yr, or about 120pCi/yr. the
conversion factor above (0.25) gives a 50 year dose of 30 mrem from
each year of breathing this Plutonium - less than 10% of the annual
background. You'd need to breathe it for 10 years just to get the
equivalent of one year of natural radiation. Meanwhile, of course, it
is being lost from the system so the real numbers are far smaller. And
this is using ALL the Plutonium.

Looked at another way, all the Pu settles out eventually, providing
2000 pCi/m2, probably over a few years. If a person has a cross
section of 1 m2 and inhales ALL the fallout in this area, he gets a
500 mrem 50 year dose. This is still considerably smaller than the
18000 mrem we naturally receive over the same 50 year period.

For comparison, 500 mrem total dose is about the same as one
mammogram. Of course, most of this settling Pu misses people's noses
and mouths, and if this amount of Plutonium were mixed into the top 1
mm of soil, it could be shipped as non-radioactive material. And this
is using ALL the Plutonium.

No credible indication has ever been found of increased health risk
even to the many people who worked milling Pu in the Hot and Cold War
days. The only documented health effects I have been able to find are
on the ATSDR web site (see references). Dogs (apparently beagles)
inhaled Plutonium at a rate of 1400 - 100,000 pCi per kg body mass in
a day, and suffered lung damage, even cancer, depending on dose, after
several months to years.

Allowing for 20 kg body mass, these dog martyrs consumed, in one DAY,
amounts which would be 14 to 1000 times the average person's share of
the entire Cassini Pu load as overestimated above. The president of
the Health Physics Society has himself done extensive research on mice
that confirms these dog results.

Vaporization of all the Plutonium is, of course, a gross overestimate.
Forget (for a moment) the one-in-a-million probability that ANY kind
of flyby mishap will even occur which leads to reentry and
vaporization. Even if a mishap does occur, only a tiny fraction of
the Pu is able to end up in people (this is the analogue of the fact
that there are enough germs in one sneeze to give a billion people a
cold - it's the distribution problem that stops this from happening).

The Cassini project and its consultants have done exhaustive analyses
of this problem. Atmospheric incineration and ground impact have both
been considered. The RTG housing itself probably does come apart under
entry heating, but the triple-protected modules (2 layers of carbon
composite, and an iridium cladding on each Plutonium golf ball) are
extremely durable, and designed to withstand atmospheric deceleration
and heating. They hit the ground at terminal velocity - only 100-300
feet/second, or one-tenth the speed of a rifle bullet. Rifle bullets
don't vaporize on impact. Neither do meteorites; they dig a little
hole. So the units might dent the hood of your car pretty badly, or
make a hole in your yard, but won't spray pulverized plutonium all
over your house. All this has been tested.

Factoring in these issues, the projects finds that the average
expected dose (per person) is only 1 mrem over the entire 50 year
lifespan of the at-risk population. Comparing this to the above upper
limit of about 500 mrem/50 yr, one gets a distribution efficiency
factor of about .002. If a sneeze had the same efficiency then each
sneeze would give 2 million people a cold. So the project's
distribution efficiency factor, which includes the difficulty of
burning through the carbon-composite and Iridium cladding of the fuel,
is hardly unreasonable and actually seems quite conservative.

Given the low distribution efficiency, the "average" person receives
practically no Pu at all. So what's all the fuss about? There is a
very narrow range of "hot" particle sizes (about 6-10 micron radius)
that is both large enough to have a significant radiation damage
potential (in the range that damaged dogs' lungs) AND small enough to
have any conceivable chance of being inhaled (but only a very, very
small chance - see note 2).

Because of the high density of the Pu (11 g/cm3), the aerodynamic
radius is 11 times the actual radius. That is, cigarette smoke
particles as large as 6-10 microns are inhalable with small
probability (a percent or less), but Pu particles of the same size
behave like 60-100 micron carbon grit. If ALL the Cassini Pu were in
this 6-10 micron size range, there would be 5 E11 particles to
distribute - "100 for each person" is what the critics might say. But
in reality there are enormous reduction factors that must be

For instance, the fraction of Pu fuel that is actually vaporized is
probably less than 10%. The fraction of all released particles that
lie in the narrow hazardous size range is perhaps 1%. The fraction of
Pu that ends up landing where people live (say, the 20 largest cities)
is roughly their area fraction or say 0.0001. The fraction of these
grit particles that are actually inhaled, because of their large
aerodynamic size of about 100 microns, is also small - surely less
than 0.01 (note 2). There is slop in these estimates, but they are
plausible "delivery inefficiences" and lead to 500 inhaled "hazardous"
particles worldwide, consistent with the Cassini project's far more
careful estimate of 100 additional fatalities over a 50 year period.

Recall that the probability of this happening in the first place is
one in a million; another type of celestial mishap with the same
probability, impact of a mile-wide asteroid, would kill over a billion
people. Also recall that a billion people will die from cancer
unrelated to Cassini during this same 50 years.

The health hazard numbers are even smaller for a launch-related
accident (even while it is perhaps 1000 times more "probable" at
1/1500 chance of Pu-release related to launch accident), because a far
smaller amount of Pu is vaporized and fewer people are exposed. The
RTG's have been exhaustively tested under conditions comparable to
such accidents; their Carbon-Iridium protection scheme is incredibly

Overall, I think the above simple arguments make the more exhaustive
analysis done by the Cassini project very easy to understand and
accept. The health hazard due to Cassini Plutonium really is
negligible. Statistics in the World Almanac verify that a person's
risk of dying from Cassini is a million times smaller than his or her
risk of a fatal auto accident while driving one mile.


1) For the cognoscenti, all doses given here are effective (whole
body), equivalent (radiation type independent), committed (50-year)
doses (unless specified as annual). This is necessary to compare
different sources of radioactivity. There are factor-of-2 or 3
differences depending on how soluble the Plutonium is; the values on
the web page are appropriate for "insoluble" Plutonium such as the
Cassini ceramic form. The basic constants are thus the 50-year
integrated effective (whole body) damage-causing dose in mrem from a
certain quantity of radioactivity in pCi.

2) The human nose is 100% effective at filtering particles that are 10
microns or greater and 95% effective at filtering particles over 5
microns. These particles can then be excreted easily. The critical
size for deposition in lung cells is 1-2 microns. Once inhaled, the
material is subject to removal processes involving incoproration in
mucous suspension and being swept out by the action of the cillia
which line the portions of the lung which are exposed to air
(Glasstone and Dolan 1977).


FGR-11 (1988), or Federal Guidance Report-11: "Limiting values of
radionuclide intake and air concentration and dose conversion factors
for inhalation, submersion, and ingestion"; K. F. Eckerman et al, EPA
Report EPA-520/1-88-020. This is based on standards developed by the
International Commission on Radiological Protection, and is endorsed
by the President of the United States.

Glasstone and Dolan (1977), Department of Defense Publication, "The
Effect of Nuclear Weapons"

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JPL Cassini Home Page:

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