Update 11-4-2021: From AIAA’s email newsletter “The Daily Launch” for Thursday
11-4-2021:
Sun Produced Three
Coronal Mass Ejections Since Monday
SPACE (11/3) reports that
the sun has produced three coronal mass ejections (CMEs) since Monday. On
Monday and Tuesday, “a sunspot designated AR2887 unleashed two of these
outbursts. Then, later in the day on Tuesday (Nov. 2), a second sunspot called
AR2891 produced a CME as well.” The third ejection is moving more quickly than
the two others, “so it swept through all of one previous CME and part of the
other.” Scientists predicted that the resulting CME would arrive at Earth
Wednesday “and produce geomagnetic storms beginning on Thursday (Nov. 4).”
My take on this: This is exactly why any manned spacecraft or space station outside the Earth’s magnetic field absolutely must have a radiation shelter or shielding for its crew. All it takes is a big one of these to kill a crew. There was one between two Apollo moon landings in 1972.
Update 8-27-2021:
From AIAA’s “Daily Launch” email newsletter for 27 August
2021:
Scientists Suggest Safest
Time For Mars Travel
CNET
News (8/26) reports that a new study published this month
offers suggestions on how to avoid hazardous radiation on a trip to Mars. UCLA
said Wednesday that “The scientists’ calculations demonstrate that it would be
possible to shield a Mars-bound spacecraft from energetic particles from the
sun because, during solar maximum, the most dangerous and energetic particles
from distant galaxies are deflected by the enhanced solar activity.”
This news item makes sense only when you realize that there are two space radiation threats, not one. They are entirely different in characteristics, but linked. The galactic cosmic rays are a very thin drizzle of extremely energetic particles, which are very difficult to shield, without making the problem worse with localized floods of secondary radiation created by these particles hitting your shield.
The solar activity is a threat with large eruptions coming
from the sun. These are far less
energetic particles, but they come in
utterly enormous floods, such that the
radiation dose is quite lethal very quickly.
But, being lower-energy, they are much easier to shield.
The linkage between them is that when solar activity and the
threat of solar eruptions is higher, the
solar wind itself deflects more of the cosmic rays, reducing that threat to easily-tolerable
values. To travel during periods of high
solar activity, you must have radiation
shielding against solar eruption radiation,
but the radiation overdose threat is actually lower, because it is an easier type of radiation to
shield against.
On the other hand, if
you fail to shield against solar activity eruptions, you will kill your crew during the mission if
such an eruption strikes their vehicle in space. The slow,
drawn-out exposure to galactic cosmic rays raises the risk of cancer
later in life, long after the mission is
over.
The point of this update is that I am not alone in my understanding of this threat. That understanding is documented here in this article, and an earlier one posted in 2012.
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original article starts here:
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I got my data on risks and passive shielding from NASA’s own
site and documents. See
http://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm,
titled Spaceflight Radiation Health Program at JSC (no cited reference newer
than 1992).
or go to http://exrocketman.blogspot.com
and click on year 2012 then May 2, for
the article titled “Space Travel Radiation Risks”. That article, which I wrote and posted, abstracts the most relevant information from
the NASA source for the questions at hand.
Two Risks:
There is galactic cosmic radiation (GCR) and there are solar
flare events (SFE). GCR is a very sparse
trickle of extremely high-energy particles (mostly protons) that are very hard
to passively shield, and which can
induce secondary showers of other dangerous particles in some materials. The risk is modulated by the solar wind which
varies with the solar activity level.
SFE radiation is mostly proton and heavier particles emitted
from the sun sporadically, from
eruptions on its surface. These come in
very concentrated but brief events,
comprising much lower-energy particles that are far easier to passively
shield.
GCR: Roentgen
equivalent man (REM) per year = 42 + 18
sin(360o (t, year)/ (11 year)),
max 60 at solar min, min 24 at
solar max. Solar max is max sunspot
activity, with more frequent
eruptions. Although, eruptions can occur throughout the cycle.
SFE: from 1968 to 1970,
events every month or so ranging from 2 REM to 50 REM accumulated during
each event; from 1970 to 1972, events about every 6 months ranging from
about 50 REM to about 100 REM accumulated during each event; and in 1972 right between Apollo 16 and
Apollo 17, one event right at 5000
accumulated REM. There was one event of about
5 REM during Apollo 16. Figure 1 is a
plot of SFE events during the Apollo program,
direct from the NASA document’s Figure 10. The quoted numbers are for somebody outside a
spacecraft wearing only a spacesuit, per
NASA’s figure.
By way of comparison,
the Earthly natural background in the US is near 300 milli-REM (0.30
REM) per year. Worldwide is not significantly different.
A rough rule of thumb:
500 REM accumulated in a short time (hours or days) is considered lethal
to 100% of those so exposed.
NASA’s Astronaut
Exposure Rules:
These vary with the affected organ (eyes, skin,
and 5 cm inside the body as representative of blood-forming organs or “BFO”), but the lowest values are for 5 cm
inside. NASA’s exposure rules limit
exposure to 50 REM accumulated in any one year,
25 REM in any single month, and a
career limit that varies with age and gender,
but peaks at 400 REM accumulated over an entire career.
These are illustrated with Tables 1 and 2 lifted from the
NASA document, and presented here as
Figures 2 and 3, respectively. Use the equations in Figure 3 to calculate
career limits. These exposure
limitations represent approximately twice the exposures nuclear workers are
allowed to face, with a single-digit
percentage increase in cancer risk expected.
Effectiveness of
Shielding Materials:
These divide into the effects of aluminum, water,
and hydrogen, which fairly well
bounds a lot of possible materials. The
true risk here is the SFE event, of a very large magnitude, such as the 5000 REM 1972 event. That’s outside in nothing but a space
suit. Inside the command module, the effect of the spacecraft structure
reduces the exposure to 500 REM. That’s
a 10:1 reduction for the spacecraft hull (remember, SFE particles are lower-energy and far easier
to shield passively).
Based on the NASA document’s Figure 9, given here as Figure 4, the plot for the 1972 event reduces 500 REM inside
the command module to 20 REM at 20 g/cm2 of aluminum shielding
added to the effects of the spacecraft hull.
Use the density of aluminum (2.7 g/cm3) to find the actual physical
thickness of this aluminum to be 7.4 cm. You would want an aluminum shield that thick
or thicker to survive a 1972-magnitude SFE event, and stay barely within the month exposure
limit.
The effect of the various materials as passive shielding for
GCR is given in Figure 5, which is
lifted from the NASA document’s Figure 6.
In terms of shield mass per unit area of hull, hydrogen is the most effective, and aluminum the least, with water in between. Note how the curves flatten at larger masses
per unit area, leading to stronger
differences in the amount of shield material required.
Here we ignore the shielding effect of the spacecraft hull structure
as negligible against the more energetic radiation. For 20 g/cm2 (7.4 cm thick)
aluminum, 60 REM/year reduces to 40
REM/year, which is well within the
annual limit. You get the same exposure
at only 10 g/cm2 water at 1 g/cm3, which is 10 cm thick, and 3 g/cm2 hydrogen, which at .07 g/cm3 is some 43 cm
thick.
The problem with “over-killing” the shielding is the
secondary shower of dangerous particles created by the high-energy GCR particles. This gets to be a problem if you make the
shield too thick, and it simply doesn’t
happen with the lower-energy SFE particles.
That makes passive shielding a real trade-off for design purposes. This is less a problem with spacecraft
design, and more of a problem with
surface habitation design and construction.
The temptation would be to pile too much regolith atop the roof, causing the secondary scatter problem.
Figure 5 – Effectiveness of 3 Materials Against GCR, From NASA Document Figure 6
My
own recommendation would be to use 15-20 g/cm2 water, some 15-20 cm thick, which would reduce 60 REM/year GCR to about
30 REM/year. Against the
GCR, the same protection obtains at some
50 g/cm2 aluminum, which is
about 18.5 cm thick. These are similar
thicknesses (15-20 cm water vs 18-19 cm aluminum), but very different masses per unit area: 15-20 g/cm2 water vs 50 g/cm2
aluminum. Water is simply the lighter
shield for the same effect, by about a factor of 2.5 to 3.
We don’t have anything but aluminum to look at in Figure 4 the
for the SFE event. At the same 50 g/cm2
aluminum that looks good against GCR,
the 500 REM SFE event, as
measured inside the command module, gets
reduced to a 2 REM/event exposure. Assuming
the same attenuation ratio and mass per unit area ratio between aluminum and
water that we saw with GCR in Figure 5, then
we should see the same 500 REM to 2 REM reduction of SFE radiation that 50 g/cm2
of aluminum provides, with only 15-20
g/cm2 water, which is 15-20
cm thick. That’s an assumption
requiring verification.
Lessons for
Spacecraft Design:
Water is the best shielding material, because it is the lightest, while providing practical thicknesses, unlike hydrogen. My best guess is that storable
propellants should resemble water in their shielding properties. They are light molecules made of light
atoms, like water, and have densities far more comparable to
water than to liquid hydrogen.
The recommended water shield is 15-20 g/cm2
(15-20 cm thickness), which could be
water, wastewater, or even frozen food. It could also be storable propellants like
the hydrazines and NTO oxidizer. Increase
the thicknesses for wastewater,
ice, or frozen food: a good guess is a factor of 1.5 to 2 increase
in thickness over straight water.
Against GCR and ignoring any effects of the spacecraft hull, 15-20 cm of water should reduce 60 REM/year
of GCR to something near 30 REM/year. This
might, or might not, be practical for the entire spacecraft, but putting water in one form or another
around the sleeping quarters might be,
in addition to a designated radiation shelter space.
There is some benefit of the spacecraft hull reducing SFE
from a 5000 REM/event outside the hull to a 500 REM/event inside the hull. Since that is still a lethal dose, further shielding is absolutely required!
That same 15-20 cm of water
should reduce an inside-the-hull 500 REM/event to something nearer 2
REM/event, which is well within the
monthly limit, even if multiple such events
occur spaced rapidly together. This
gives us a lot of margin in the case of an event far larger than the 1972 SFE
event.
Any spacecraft design should incorporate its flight control station
within the designated-shelter radiation shielding so that critical maneuvers
may be flown regardless of the solar weather. Shielding about the sleeping quarters is also
recommended for purposes of reducing GCR exposure.
Exposures Calculated
for a Mars Mission at 60 REM/Year GCR with Three 1972-Class SFE Events:
The mission is 9-month transit/13-month near (or on)
Mars/9-month transit. One SFE event
occurs during each transit, and the
other occurs while the crew is on or near Mars.
Calculations are made with and without the sleeping quarters
shielded, for 1/3 of clock time during
each day. Shielding about the sleeping
quarters and the designated shelter is spacecraft hull plus 15-20 cm water-equivalent
for SFE, just 15-20 cm water-equivalent
for GCR. How this might actually be done
was shown conceptually in Figure 6 above.
The first radiation exposure year is 3 months pre-mission on
Earth, then 9 months in transit to Mars. The second radiation-exposure year is 12
months on Mars. The third radiation-exposure
year is one month on Mars, 9 months in
transit to Earth, and 2 months
post-mission on Earth. Earthly exposure
is at the 0.3 REM/year rate.
The 9 month transit is 0.75 year. Without any shielding effects at all, 45 REM are accumulated during transit for the
year in which transit occurs.
If there is sleeping quarters shielding, its presence cuts the GCR to a rate of 30
REM/year while sleeping. Then based on clock times, a 2/3-1/3 split occurs between the 60 and 30
REM rates: that is a rate of 50 REM/year
applied to a 9 month transit. Thus the
crew accumulates 37.5 REM during the transit,
which goes toward the total accumulated exposure during the year in
which the transit takes place.
While on or near Mars,
the planet blocks half the spherical “sky”, for a net in-space unshielded GCR rate, assumed unattenuated by the planet’s
atmosphere, of 30 REM/year, accumulated during each year spent on
Mars. The stay is 13 months, so without sleeping quarters shielding, 30 REM counts toward the first full year on
Mars, and one month’s worth (2.5 REM)
counts toward the second year on Mars and in-transit home.
If there is shielding about the sleeping quarters on
Mars, the same 2/3-1/3 split applies to
rates of 30 and 15 REM/year, reducing
the effective exposure rate to 25 REM/year.
In that case, the year on Mars
accumulates 25 REM, and the 13th
month accumulates 2.1 REM.
Add 2 REM to the accumulation in any one month for SFE
events during the transits and during the stay on Mars. There must be a designated radiation shelter
for SFE events, even while on Mars, or the exposures could easily be lethal. That assumes no attenuation of the radiation
by Mars’s thin atmosphere.
The result is depicted graphically in Figure 7. Again,
shielding is 15-20 cm of water-equivalent.
Figure 7 – Radiation Profiles for 15-20 cm Water
Shield, with or without Sleeping
Quarters Shield
These results show a marginal yearly exposure during year
3, at just barely under the 50 REM
annual limit, for the case of no
sleeping quarters shielding. With sleeping
quarters shielding, this reduces well
under the limit. Worst case monthly
exposures are well under the limit for both cases. Two such missions will approach career
limits, in either case.
Note that if there is no shielded place for SFE events, then during any event in the same class as
the 1972 event, the exposure will be
fatal at 500 REM received over a matter of hours. There
simply must be a solar flare shelter somewhere.
This is true during the transits and on Mars.
Note also that during times when the GCR is under 60
REM/year out in space, exposures inside
the ship (either case) will be much lower.
It is only the worst-case 60 REM/year space environment that is analyzed
here.
Note also that it is the shorter-than-a-year transit time that reduces
in-transit unshielded exposure to 45 accumulated REM! Extending the transit time by using
repeated aerobraking passes to capture at Mars, instead of a one-time rocket burn, will quickly violate the annual exposure limit! The same thing applies to electric propulsion
using spiral-out/spiral-in flight plans.
(Not to mention exposure times passing through the Van Allen Belts at
Earth.)
Once the unshielded one-way flight time exceeds 10 months, the annual exposure limit gets exceeded in a
max GCR year. Once that long-transit
situation obtains, you must shield
the entire habitable volume of the ship,
not just a designated shelter and perhaps the sleeping quarters.
Finally, if the
entire habitable volume of the spacecraft could be shielded at the 15-20 cm
water-equivalent level, exposures would
be cut essentially in half, to only around 30-something REM per radiation-exposure
year in the transits, and 15 REM/year on
Mars, even in a 60 REM/year part of the
solar cycle. Year 1 would be 32.08
REM, year 2 would be 17 REM, and year 3 would be 33.3 REM, for a mission total of 83.4 REM. Three, or possibly even 4, such missions in 60 REM GCR years might be
feasible, before hitting career exposure
limits.
Final Comments:
First, this kind of radiation
shielding will inevitably prove to be absolutely necessary, but it will look nothing at all like what
we have ever before done with our spacecraft designs. When “they” show you spacecraft design
concepts that look like what we have done before, you already know that ”they” have not thought
this problem through!
Second, the shield
design concept shown in Figure 6 above is entirely compatible with a “long”
ship design that is spun end-over-end like a rigid baton for artificial
gravity. A design like that is also entirely
unlike anything we have ever before done,
but it is rather well-understood from an engineering viewpoint, and would require far less technology
development and demonstration than any sort of cable-connected spin gravity design.
Third, it is
quite evident that worst-case GCR risks are slight over-exposure for late-in-life
cancer, while the SFE risks really are
lethal doses leading to an ugly death within hours. Thus, when “they” point to GCR as the radiation risk
that precludes humans going into deep space,
you already know that (1) they are lying for nothing but fear-mongering
purposes, and (2) “they” are truly ignorant
of the real radiation risk. Such claims
are simply not credible.
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