Niel Armstrong, the first man on the moon said the thing he feared the most about the
Apollo 11 mission was a solar flare, this solar weather is a much an issue for the upcoming
Artemis program in 2024 as it was for the Apollo missions, in fact in August 1972 between
the Apollo 16 and 17 missions, one of the most powerful and deadliest solar flares of
the space age occurred. So how did they prepare for this back in the
Apollo days and how much better prepared are we for it today
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Just as we have seen how unpredictable and violent the weather here on earth can be,
space is no different though we now know much more about its main cause the sun than we
did 50 plus years ago and this gives hope for better predictions but some of the most
dangerous & damaging events can occur randomly and without warning.
The first time we had an inkling that the sun was doing some other than just providing
us with heat and light came in 1852 when the British astronomer Edward Sabine showed that
there was a connection between magnetic storms on earth with increased auroral activity and
the appearance of sunspots and in 1859 Richard Carrington connected the now so-called “Carrington”
event, an extremely large magnetic storm with a solar flare near sunspots he had observed
the day before. During world war 2 In 1942, the use of radio
and radar was found to be affected by extreme amounts of static and noise when a large solar
flare occurred. Spool on a few years to 1957, the International Geophysical Year and the
first US satellite, Explorer 1 discovered belts of radiation in the form of charged
particles trapped by the Earths magnetic field which we now call the Van Allen belts after
the project leader James Van Allen. All this was revealing space to be far from
just a vacuum where nothing much happened. Charged particles continuously streamed from
the sun in what we now call the solar wind, something that was first observed and measured
by the Soviet satellite Luna 1 in 1959. Whilst the solar wind is a constant stream
flowing from the sun, from over 150 years of solar observations we now know the sun
also goes through phases every 11 years when its magnetic field flips and there is an increase
in sunspots and solar flares which can eject billions of tons of charged particles into
space at high speed and it's just a matter of luck whether these end up heading in our
direction. These ionising particles can damage DNA leading
to cancer or even rip our biological cells apart and damage or destroy delicate electronics
and computer chips which is why they are of so much concern.
So one would think that sending missions during the solar low periods would be the safest
option but there is another type of Galatic radiation that also occurs.
These are Galactic Cosmic Rays or GSR’s, very high energy particles created by events
such as supernova’s or neutron stars thousands or millions of light-years away that can be
many orders of magnitudes greater than anything created by our Sun.
These and are mostly bare nuclei, atoms stripped of their electron shells and high energy protons
travelling at very nearly the speed of light giving them the energy to penetrate even heavily
shielded spacecraft and cause damage to the crew or electronics inside.
So when the sun is in a solar low period it might well be quieter and more predictable
but this weaker activity also weakens its magnetic field allowing these high energy
cosmic intruders to more easily enter the inner solar system.
We here on Earth are protected by the earth's magnetic field and atmosphere. The magnetic
field captures and deflects charged particles to the polar regions creating the northern
and southern auroras while the atmosphere provides us with about 100km of atmospheric
gases of varying densities which absorbs most of the higher energy Galatic Cosmic Rays,
X-rays and Ultraviolet radiation but once you leave the protection of the Earth, such
as when we go the moon, then we are exposed to the full force of these types of radiation.
Whilst we can use the regular cycles to look for the periods of least solar activity, we’ve
discovered that extreme events like the “Carrington event” and the Aug 1972 event which pose
a lethal threat to space missions are random and that can occur out of the blue even in
solar low periods. However, from looking at 150 years of solar observations, we’ve found
they tend to occur later on in odd-numbered solar cycles when they do appear.
This is what concerned Niel Armstrong during the Apollo 11 mission as it took place during
a solar high period where solar flares are more common and intense.
This was a compromise between the political imperative to put a man on the moon by the
end of the decade to beat the Soviets and the risk to the astronauts from the increased
solar activity but as we have seen it also meant the suns stronger magnetic field would
lessen the more damaging GSR’s. The most dangerous part was the moonwalk as
there is only a very weak magnetic field on the moon to deflect incoming particles and
essentially no atmosphere to absorb them, so their only protection was their A7-L Pressure
spacesuits. Although they were used on missions before,
it would be the first time they had been used on the moon's surface. Even though they had
been modelled as accurately as possible at the time, there was no way here on earth to
simulate both the deep vacuum, the wide temperature swings from -127C in the shade to +125C in
direct sunlight, sharp rocks and the one-sixth gravity all at the same time.
They also offered little in the way of radiation protection from the high energy particles
of a large solar flare or GSR’s. In fact all the Apollo missions were gambling
that an extreme event wouldn’t happen during their transit or time on the moon and luckily
none of them were caught out, but it was just a matter of fortuitous timing.
On the 4th August 1972 about midway between safe return of Apollo 16 in April and the
launch of Apollo 17 in December, what would become the largest solar storm of the space
age occurred creating 63 normal solar flares and four “X” class solar flares whilst
it was facing the Earth. It was so strong that it caused widespread
disruption of the power and communications grid across the US. It damaged satellites
as the earth's magnetosphere was severely constricted by the buffeting from the coronal
cloud and increased solar wind which exposed many of them to the high energy bombardment
causing 2 years worth of wear to the solar panels in a few days and also caused the accidental
detonation of up to 4000 US Naval magnetic mines off the coast of North Vietnam.
It also had the fastest transit time from the Sun to the Earth for a coronal cloud ever
recorded at just 14.6 hours Its been calculated that if this had occurred
whist a crew had been outside of the lander on the surface of the moon they would have
been exposed to a radiation dose of 10 Gy, that’s 10 times the normal human dosage
of radiation expected in a lifetime. Inside the lander would have offered only marginally
more protection as was thin-skinned compared to the command module.
This would be a lethal dose that would not only require an emergency return to earth
whilst suffering acute radiation sickness but also a bone marrow transplant when they
arrived back at earth to save their lives. Of course, we would have seen the flare and
would have known that a particle event was on its way and been able to give them a warning
to get back the command module but as we didn’t have the sun monitoring satellites that we
have now, we wouldn’t have known how quickly it was travelling.
Back in the more heavily shielded command module, the aluminium skin would have reduced
the exposure to about 1/10th of that compared to their spacesuits on the surface. That would
still give them sickness and nausea but they would probably avoid the bone marrow transplant
when they got back but either of these scenarios would have given them a much greater chance
of cancer in later years. Now all the conspiracy theorists out there
will no doubt point out, well how did they avoid the deadly van allen radiation belts,
well they didn’t, they flew through the least dense areas and only for a few hours,
that and the attenuation provided by the command module shielding meant they were exposed to
a moderate dose but for a short time, that and the days between outgoing belt transition
and the return one gave their bodies more time to recover.
In fact, when the dosimeters that the crews wore during all the Apollo missions were checked
afterwards, they had received much less than the yearly limit of 5 rem set for radiation
workers. Radiation still poses a major problem for
any future missions outside of the earth’s protective envelope. The use of new lightweight
and yet more effective high hydrogen content shielding materials means that new spacecraft
are safer than those of the Apollo era but it's still a compromise between the extra
shielding needed and the weight it adds to the launch vehicle.
For future spacecraft that will be built in space where launch weights are not an issue,
active shielding using superconducting electromagnets to create an artificial magnetic field around
the craft to deflect charged particles could be used.
Another extremely good, simple and cheap radiation shield is water due to its very high hydrogen
content. Each 7 cm of water reduces ionising radiation by half.
Putting a 1-meter thick water envelope around the crew quarters would provide enough shielding
to reduce even the most extreme particle events to the equivalent of less than background
radiation levels here on earth. The problem with this is that it adds a huge amount of
mass which then greatly increases the amount of fuel required to go anywhere even using
gravity assist for long journeys. So until such time that we can build spacecraft
like that in space using space sourced materials and fuels, we’ll have to be a bit more creative.
Its been suggested that if we really want to protect future crews during the transit
to and from permanent moon bases we should have more solar monitoring satellites to give
more accurate warnings and an orbiting safe haven where crews can stop off to weather
out any high energy particle events that may occur.
On the moon, bases built either in caves or with thick regolith formed walls will protect
crews on the surface. But further research into the sun is probably
our best hope. Programs like the Birmingham Solar-Oscillations Network (BiSON) which have
been running for 30 years and are giving us a view of the sun’s interior.
Using this helioseismology data we have been able to see structural changes that occurred
under the surface in the run-up to the most recent solar cycle and this may well lead
to a better prediction of the most dangerous solar storms in the future.
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