50th Anniversary of the Apollo 16 Major Solar Flare

April 2022 marked fifty years since Apollo 16, allegedly the penultimate manned mission to the Moon. The mission was launched April 16, 1972, and splashed down on April 27, 1972. A lunar landing supposedly occurred between April 21 and April 24. The anniversary also marked fifty years since another major event: that of the Major Proton Event that the Apollo 16 mission would have encountered.

Fig 1. Happy Anniversary! Artistic illustration by the author of what astronauts would have encountered during the 1972 Apollo 16 mission, scheduled to launch on – yes! April 16th – with splashdown set for April 27th. NASA image AS16-107-17446, dubbed ‘John’s locator’ on the ALSJ website was ascribed to day 6 of the mission timetable: April 22nd. The sky is a composite SOHO image of the Sun by Euclid anderKroew.

Back in 1992, whilst writing his book NASA Mooned America!, Ralph René requested that the National Oceanic and Atmospheric Administration (NOAA) send him the solar x-ray and proton data for the years of the Apollo missions. As he stated in a chapter appropriately named 'Sunstroke': “I hoped to find just one big x-ray and proton exuding flare that took place during any one of the missions. We would have heard about cooked astro-nots, right?”¹

John A. McKinnon of NOAA sent René some pamphlets and floppy disks, together with his 1972 technical memorandum titled August 1972 Solar Activity and Related Geophysical Effect. Despite specifically requesting x-ray and proton data, the material that René received only contained "optical data". Nonetheless from what they sent him, René was able to establish the total number of grouped monthly flares between the years of 1967 and 1991. This allowed him to estimate an average number of flares per day by dividing the total number of flares in a month by the number of days in said month, and he established that a total of ~1,485 flares had occurred during the periods allocated to the nine Apollo Moon missions.

Fig. 2. Graph and listing of the monthly counts of grouped solar flares for solar cycles 20, 21 and the first four years of cycle 22. [NOAA] René reproduced the listing from 1967-1991 on page 126 of his book.

In addition, whilst reading the NASA publication On the Moon with Apollo 17² by Gene Simmons, which can be downloaded on the Apollo Lunar Surface Journal [ALSJ], René learned that at least one of those flares had indeed coincided with the Apollo missions. He wrote:

I later discovered that another big flare started on April 17, 1972 when Apollo 16 was only one day out from Earth on the way to the Moon. Astro-nots Young, Mattingly and Duke should have been fried, but, of course, they weren't.

For this column we are going to discuss this particular solar event and its ramifications in explicit detail. Firstly, here is the passage from Simmons’ publication:

The CRD experiment was flown on Apollo 16 but a solar flare occurred on April 17 and produced many particles. The tracks caused by the solar flare particles are both larger and more numerous than the tracks produced by the normal solar wind particles and cosmic rays, making the readings of the “normal record” very uncertain. So, the CRD experiment will be flown again on Apollo 17 in order to do those things that had been expected for the Apollo 16 flight.

The acronym CRD stands for Cosmic-Ray Detector. Cosmic rays are extremely rarified protons or heavy nuclei with several GeV (billion electron volts) of energy. Such particles require meters of shielding to attenuate.³ But due to their rarity the annual dose rate at the Moon is reported to be 7-12rem,⁴ and a journey to Mars lasting 253 days is expected to accumulate 66rem.⁵ You would need to look in square meters just to find one cosmic ray particle. Similarly, the solar winds are protons, electrons and alpha particles constantly accelerated by the Sun. These are much more prevalent than cosmic rays, but don’t have enough energy to penetrate further than a few millimeters. Solar flares represent the chief radiation hazard in the short term, because such flares can deliver penetrative protons with 10s to 100s of MeV of energy at fluxes far in excess to that of solar wind.

Simmons’ paper is implying that the solar flare of April 17, 1972 delivered so much radiation that the Apollo 16 CRD experiment could not properly measure the ‘normal’ radiation environment. NASA’s Apollo 16 Mission Report makes similar remarks, that the detector was “jammed” by the solar flare radiation.⁶ Before any believers in the veracity of the Apollo missions get carried away and hype that the Apollo astronauts must have gone to the Moon to detect this flare, it needs be emphasised that our knowledge of this event is not dependent on Apollo 16. There were many unmanned missions that detected it.

Wenzel et al. (1973) report that the HEOS-2 satellite first detected protons, electron and alpha particles from the flare on April 17 at 22.00 UT. At the time, HEOS-2 was at an altitude of 22 Earth radii (140,162km). They also report that the particles first struck the Earth’s magnetosphere on April 18 at 04.00 UT, and decreased by 19.00 UT that day.⁷ R.L. Fleischer et al. (1973) report that the April 17 flare was also detected by Explorer 41 “IMP G” (Interplanetary Monitoring Platform), ATS-1, and VELA satellites.⁸ And NOAA’s National Center for Environmental Information webpage concerning Solar Flares has a section for x-ray flare data in .txt format, as recorded by the SOLRAD (low-Earth orbit) satellites between 1968 and 1974.⁹

The data lists three events for April 18, 1972, plus two additional events on April 21 and April 27. The data from these satellites not only proves that that it was not necessary to send astronauts to the Moon in order to collect these data, it also demonstrates that more than one significant flare occurred over the Apollo 16 period from April 17th through to April 27th.

Fig. 3 Readouts of the proton flux rates as recorded by HEOS-2 [upper, Wenzel et al. 1973], IMP G and ATS-1 [lower, Fleischer et al. 1973].

The question becomes what effect would the radiation from these April 1972 solar flares have on the astronauts? The Clavius webmaster, Jay Windley, adamantly denies that any of the solar flares during Apollo were hazardous. Referring to the >1,400 flares that René cited from the publications NOAA sent him, Windley writes:

This number represents the total number of detectable solar events, not major flares that would have posed a danger to the astronauts. The records also show that no major solar flares occurred during the Apollo missions, but the conspiracists don't care to look that closely.¹⁰

The official Apollo 16 Mission Report⁶ published by NASA also attempts to downplay the Apollo 16 flares. The first event is completely missing from the relevant translunar section, and only mentioned in Section 4.9, Lunar Surface Science Experiments, regarding the CRD experiment:

Analysis of the data received on the lunar surface and during translunar and transearth coast will be degraded because of the minor solar flare which occurred during translunar flight.

And the second translunar event relating to three flares is hidden away under section 10.2.4 titled Biomedical Evaluation:

This was the first Apollo mission in which three minor solar flares occurred. Although the nuclear particle detection system registered a slight increase in proton and alpha particle fluxes, no measurable radiation dose increment was received by the crew from these flares.

Whichever way you read those sentences, the NASA flare sums do not add up: Apollo 16 launched on the 16th and effectively that might have given NASA the opportunity to claim 'minor flare status' for the April 17 event. But that was not the case for the three subsequent events which took place while Apollo 16 was scheduled for the translunar coast (18th) nor for the flare that occurred when the mission was scheduled to be on the lunar surface (21st) and the final flare that occurred on the inbound to Earth and last day of the transearth coast (27th).

Whether a solar flare is “major” or “minor” can be quantified by their fluxes of 1-8Å x-rays and high energy protons. The former tells us the class of the solar flare, while the latter we can use to calculate the absorbed and equivalent doses that the astronauts would have received.

NASA claims that the Apollo 16 crew only received a skin dose of 0.51rem. It is important to note that the corresponding body dose is approximately 6-9% of the skin dose. McKinnon’s technical memorandum provides some estimates for what an Apollo crew would have received had they actually encountered the August 1972 solar flares, suggesting a body dose in the order of 9% of the skin dose. A skin dose of 0.51rem would thus imply a body dose between 0.03rem and ~0.05rem.

McKinnon’s technical memorandum also contains the following table defining the Classes of solar flares:

Table 1. Definitions of the x-ray categories for solar flares.

To which McKinnon specifies:

Since class C events are often accompanied by insignificant ionospheric effects and very small cm-wavelength radio bursts, it is to this category that the term “non-energetic” applies. On the other hand class M and X flares produce large to very great bursts at short and long wavelengths and consequently are considered “energetic”.¹¹

NASA’s Dr. Tony Phillips regarded Class X solar flares as “the most powerful kind.”¹² Thus a ‘minor solar flare’ would be considered a Class C event, a ‘moderate solar flare’ would be Class M, and a ‘major solar flare’ would be Class X. The SOLRAD x-ray flare data mentioned previously contains a column for 1-8Å Max x-ray flux data. I have reproduced the relevant data in the following table:

Table 2: x-ray solar flares during the Apollo 16 mission as recorded by the SOLRAD series of satellites.
‡As an aside, in addition to the SOLRAD data, Dodson and Hedeman reported two "lesser events" during Apollo 16. One between 0950-2326 on April 20, and the other between 0957-2328 on April 21.¹³ These events are not listed here, because their 1-8Å x-ray fluxes are unspecified.

At first, the start and end times do not seem to match those recorded by HEOS-2, which indicated that the particles struck Earth’s magnetosphere at 04.00 UT on April 18, some three hours after the first SOLRAD detection. However, it is important to note that HEOS-2 was detecting particle radiation, while SOLRAD was detecting x-rays. Being photons, x-rays travel at the speed of light and will arrive much sooner than protons, electrons and alpha particles – which all have mass.

A quick look at Table 2 shows that out of all the Apollo 16 flares as recorded by SOLRAD: two of them have x-ray fluxes that put them in the category of Class M events; and the remaining three have fluxes corresponding to Class C. Are these Class C events the 'three minor solar flares' that NASA mentioned in their Apollo 16 Mission Report? If so, why did they neglect to mention the more pertinent Class M solar flares also listed in this data? Class M flares pose far greater threats to astronauts than Class C flares. For comparison, the major solar flares of August 1972, which nobody disputes would have been lethal to astronauts, had x-rays fluxes that put them in the category of either Class X or Class M.

Still, just being a Class X or M event doesn’t tell us how dangerous they were. While we could calculate the x-rays doses from these fluxes, wavelengths between 1-8Å correspond to an energy range of 1.5keV to 12.4keV. Such x-rays are readily shielded by a few millimeters of material and thus cannot penetrate the spacecraft. To determine what kind of doses we’d expect from such events, we need the proton data.

NOAA’s online data for proton fluxes only goes back to 1976.¹⁴ Given the number of satellites recording the April 1972 event, you’d think there would be records of the proton data. And you’d be right! Returning to Fleischer’s 1973 paper we are told that the April 17, 1972 event delivered a time integrated flux (or fluence) for >5MeV protons of ~10⁸ protons/cm². However, these fluxes again can be ignored, because such protons do not have enough energy to penetrate the Apollo spacecraft.

J.W Keller et al. (1963) provide a graph plotting the proton energy as a function of their range in various materials.¹⁵ NASA claims the Apollo CSM walls, which were composed of sheets and brazed honeycomb of stainless steel and aluminum topped with a layer of epoxy resin as an ablator,¹⁶ were rated at 8g/cm². Looking at J.W. Keller et al.’s graph, we can see that 8g/cm² of aluminum or copper (which is similar in density to stainless steel) would only attenuate protons with energies up to 8 MeV. Any protons with energies greater than that would pass clean through the spacecraft walls and bury themselves in the astronauts’ bodies.

Fig. 4. Proton attenuation in material plotted as a function of proton energy and areal density of materials. Note that the Apollo CSM hull was made of aluminum, stainless steel and ablative resin rated at 8g/cm². [J.W. Keller et al. 1963] annotated by J. White.

For the higher energy protons sufficient to penetrate the CM walls, I found the fluxes in a 1990 paper by M.A. Shea & D.F. Smart. Their paper, titled A summary of major solar proton events, contains a table listing solar proton events and their corresponding time integrated fluxes for >10MeV and >30MeV protons between the years of 1955 and 1985.¹⁷. The solar flares of April 17-18, 1972 are on the list. This alone contradicts the claim that no major solar flares occurred during the Apollo missions. And if the Apollo 16 transcoast solar flares were minor, why are they listed in this table for major proton events?

Fig. 5. A sample of Shea & Smart’s list of Major Proton Events. The flares listed on April 17th and 18th are highlighted, as they occurred when the Apollo 16 mission was scheduled for its translunar coast.

In Shea and Smart’s table the fluxes for the April 17, 1972 flare is left blank. But the fluxes for the April 18, 1972 event are listed: 3x10⁷ protons/cm² for E >10 MeV; and 7.8x10⁶ protons/cm² for E >30 MeV. To find the absorbed dose: we simply convert the flux to Joules/m²; multiply by the average surface area of a human (2m²) to establish how many protons struck each astronaut; and then divide by the body mass of a human to find out how much proton energy was deposited. I could not find the respective masses of each individual crew member. But since NASA’s weight restrictions state the astronauts must weigh between 50-95kg, we’ll use an average of 72.5kg.

For >10MeV protons, the resulting absorbed doses becomes:

For >30MeV protons, the resulting absorbed doses becomes:

To find what effect it would have on humans, we need to convert the absorbed dose to equivalent dose by multiplying by the appropriate radiation weight factor (W_R). The International Commission on Radiological Protection (ICRP) considered W_R = 5 for protons >2 MeV. This was reduced to W_R = 2 from 2007 onward.¹⁸ But in cross-comparing my calculations with the peer-reviewed literature, W_R = 5 seems more appropriate for solar protons. Using the August 4, 1972 flare as an example, E.E. Kovalev¹⁹ estimated a dose as high as 5,300rem from 30 MeV protons at a flux of 8x10⁹ protons/cm². Following the mathematical steps shown above and then multiplying by 5, I calculate ~5,296.6rem. That’s a 99.94% match with Kovalev’s estimate. So let’s just go with 5.

By comparison, a single chest x-ray delivers 0.1 mSv (0.01rem) and residents in Chernobyl relocated after the 1986 nuclear disaster received <100 mSv (10rem), which is the recommended limit for radiation workers every five years.²⁰ Writing about the radiation belts he discovered, James Van Allen stated “A human being exposed for two days to even 10 roentgens would have only an even chance of survival.”²¹ 11.75rem is equal to 13.4 roentgens. Looking at Shea and Smart’s table, we can see the Apollo 16 flare that began on April 18 reached its maximum intensity the following day. If a two-day exposure to 10 roentgens only gives humans a 50/50 chance of living, then less than a day’s exposure to 13.4 roentgens can’t be good.

Even if we assume the astronauts’ 50% chance of living prevailed, NASA claims the Apollo 16 crew only received a skin dose of 0.51rads (in humans 1rad = 1rem). Remember, the body dose is about 6-9% of the skin dose. And we can calculate it too. Because the total mass of an adult’s skin is about 4.5kg,²² we can redo the calculations above with the skin mass in mind rather than the body mass to find the skin dose.

For >10MeV protons, the resulting absorbed skin doses becomes:

For >30MeV protons, the resulting absorbed skin doses becomes:

The equivalent skin dose becomes:

Our calculated dose is over 372 times NASA’s reported 0.51rem skin dose for Apollo 16. To put a skin dose of nearly 190rem into perspective, the textbook Pain Procedures in Clinical Practice contains the following passage on page 36:

Transient skin erythema can result from as little as 200 rad, and at 300 rad temporary hair loss may occur. The threshold for permanent injury is 700 rad, and doses >1800 rad can cause dermal necrosis.”²³

Erythema is the reddening of the skin associated with sunburn. At the very minimum, even if their 50% chance of survival prevailed, we would have expected the Apollo 16 crew to have suffered horrendous skin burns from the radiation exposure. But they did not.

In conclusion, the claim that no major solar flares occurred during the Apollo missions is demonstrably false. Taking only one mission, Apollo 16, we have demonstrated that of the 144 detectable flares that were recorded by NASA between April 17, and April 27, two (1.4%) were of the large and energetic Class M variety, and at least one of them was a major proton event (one of two major proton events between April 17 and April 18) – but propagandists don’t care to look that closely.

Jarrah White

Aulis Online, June 2022
Updated January 2025: The original publication of this column erroneously reported the five Apollo 16 flares recorded by SOLRAD to be Class X. This error has been corrected.

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About the Author

Jarrah White is an Australian filmmaker, astrophysicist and geologist. He has Certificate III & IV qualifications with distinctions in Screen and Media at the Sydney Institute of TAFE NSW, Australia; and a BSc with Major in Geology and a Minor in Astrophysics completed in November 2017 and July 2019 respectively.

 

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References

1. R. René (1992) NASA Mooned America!, self-published
2. G. Simmons (1972) On the Moon with Apollo 17: A Guidebook to Taurus-Littrow, NASA Document EP-101
   
3. J.H. Mauldin (1992) “Prospects For Interstellar Travel”, AAS Science and Technology Series, Vol. 80
   
4. E.N. Parker (2006) “Shielding Space Travelers”, Scientific American, Vol. 294, No. 3, pp40-47
   
5. E. Howell (2018) “What are cosmic rays?”, Space.com
   
6. NASA (1972) “Apollo 16 Mission Report”, Manned Spacecraft Center, NASA Document MSC-07230
   
7. K.-P. Wenzel, V. Domingo, D.E. Page, B.G. Taylor (1973) “The April 17/18, 1972 Solar Event: Spectral and Directional Measurements of Protons, Alpha-Particles, and Electrons. (Abstract)”, Proceedings of the 13th International Conference on Cosmic Rays, Vol. 2, p.1438
   
8. R.L. Fleischer, H.R. Hart Jr., A. Renshaw, R.T. Woods (1973) “Enrichment of Heavy Nuclei in the April 17, 1972 Solar Flare”, Proceedings of the 13th International Conference on Cosmic Rays, Vol. 2, pp. 1486-1491
   
9. NOAA, “Solar x-ray Flares” (In Solar Flares), National Centers for Environmental Information [seen April 19, 2022]
   
10. J. Windley, “here comes the sun”, Clavius [Accessed April 19, 2022]
    
11. J.A. McKinnon (1972) “August 1972 solar activity and related geophysical effects”, NOAA technical memorandum, ERL SEL-22
    
12. T. Phillips (2005) “Sickening Solar Flares”, Science@NASA
    
13. H.W. Dodson, E.R. Hedeman (1975) “Experimental comprehensive solar flare indices for certain flares, 1970-1974”, NOAA Report UAG-52
    
14. NOAA (1976-2011) Solar Proton Events Affecting the Earth Environment.
    
15. J.W. Keller, R.D. Shelton, M.O. Burrell, J.A. Downey III (1963) “Problems in Radiation Shielding of Space Vehicles” (In Astronautical Engineering and Science: From Peenemünde to Planetary Space, edited by E. Stuhlinger, F.I. Ordway III, J.C. McCall, G.C. Bucher), McGraw Hill Book Company, pp241-260
    
16. NASA Space Science Data Coordinated Archive, “Apollo 16 Command and Service Module (CSM)”, NSSDCA/COSPAR ID: 1972-031A, [Accessed April 19, 2022]
    
17. M.A. Shea, D.F. Smart (1990) “A summary of major solar proton events”, Solar Physics, Vol. 127, pp297-320 “Radiation Weighting Factor”, Nuclear Power [Accessed May 26, 2022]
    
18. E.E. Kovalev (1983) “Radiation Protection During Spaceflight”, Aviation Space and Environmental Medicine, Vol. 54, No. 12 Pt 2, pp16-23
    
19. The Chernobyl Gallery, “Radiation Levels”, [Accessed April 19, 2022]
    
20. J.A. Van Allen (1959) “Radiation Belts around The Earth”, Scientific American, Vol. 200, No. 3, pp39-47
    
21. Fulom, “Understanding your skin – Skin Facts”, [Accessed May 26, 2022]
22. K.P. Botwin, P. Ceraulo and C.P. Shah (2011) “Radiation Safety for the Physician” (In Pain Procedures in Clinical Practice, Third Edition edited by T.A. Lennard, A.K. Singla, S. Walkowski, D.G. Vivian), Elsevier Inc, pp31-36
    
    

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