Author Topic: How could the radiation doses for Apollo 8 and 11 be so low?  (Read 2650 times)

Offline jfri

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I have seen this information in table 2
https://history.nasa.gov/SP-368/s2ch3.htm?utm_source=pocket_saves
Here Apollo 8 is given a radiation dose of 0.16 rad. That is equal to Apollo 7 and less than Apollo 9. Also Apollo 11 has 0.18 which is less than 0.20 for Apollo 9.
How is this possible considering Apollo 11 and Apollo 8 went to the moon and was subjected to deep space radiation outside earths protective  magnetic field unlike Apollo 7 and 9?

Offline eric z

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 Welcome to the Forum. Certainly an interesting question. Hopefully the experts will start weighing in!

Offline Hobbes-22

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The radiation level in space is not constant: it depends on solar activity.

Offline jfri

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The radiation level in space is not constant: it depends on solar activity.

I know that. But from what I understand so are particles from the sun rather low energetic (keV) unless there are a solar eruption. And low energetic particles is the Apollo capsule able to block.
But what about GCR is it not fairly constant ? It has more energetic particles. Why do they not raise measured doses in translunar space ?

Offline whitelancer64

The radiation level in space is not constant: it depends on solar activity.

I know that. But from what I understand so are particles from the sun rather low energetic (keV) unless there are a solar eruption. And low energetic particles is the Apollo capsule able to block.
But what about GCR is it not fairly constant ? It has more energetic particles. Why do they not raise measured doses in translunar space ?

The rate of GRC is strongly influenced by solar activity, it is not constant. A good predictor of GCR intensity is the number of sunspots.

If you were outside of the heliosphere, GCR would be pretty much constant. This is one of the things that we have found from the radiation measurements by Voyagers 1 and 2 as they have left the heliosphere.
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Offline jfri

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The radiation level in space is not constant: it depends on solar activity.

I know that. But from what I understand so are particles from the sun rather low energetic (keV) unless there are a solar eruption. And low energetic particles is the Apollo capsule able to block.
But what about GCR is it not fairly constant ? It has more energetic particles. Why do they not raise measured doses in translunar space ?

The rate of GRC is strongly influenced by solar activity, it is not constant. A good predictor of GCR intensity is the number of sunspots.

If you were outside of the heliosphere, GCR would be pretty much constant. This is one of the things that we have found from the radiation measurements by Voyagers 1 and 2 as they have left the heliosphere.

That leads to the question. Did the solar activity vary during the timeperiod when Apollo 8-11 took place ? Was it lower during the Apollo 8 and 11 mission ?

Offline laszlo

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That leads to the question. Did the solar activity vary during the timeperiod when Apollo 8-11 took place ? Was it lower during the Apollo 8 and 11 mission ?

It certainly did and it certainly was. You can find this info at https://www.spaceweatherlive.com/en/solar-activity/solar-cycle/historical-solar-cycles.html. I've attached an image showing the solar activity during the Apollo period. The black jagged line is the monthly mean total sunspot number at the time while the red line is the 13-month smoothed monthly total.

The Apollo period, from 1965 - 1975 is lower than the 2 periods before and after. Apollo 11 occurred as the sunspot number was declining to a local minimum during the peak. Apollo 8, December 1968, was actually at a local minimum in the sunspot number. For both Apollo 8 & 11, the monthly mean sunspot numbers were actually below the 13-month smoothed numbers.

So 8 and 11, while flying during the maximum solar activity for 1965-1975, managed to squeeze in during periods when the activity was lower than the average for the peak. Note also that Apollo 8 did not have a LM, so all the crew members were within the thickest shielding for the entire flight, whereas 2 of the 11 crew spent some hours in the much thinner walls of the LM. That was also bound to have some effect on the relative numbers for 8 and 11.

Offline LittleBird

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That leads to the question. Did the solar activity vary during the timeperiod when Apollo 8-11 took place ? Was it lower during the Apollo 8 and 11 mission ?

It certainly did and it certainly was. You can find this info at https://www.spaceweatherlive.com/en/solar-activity/solar-cycle/historical-solar-cycles.html. I've attached an image showing the solar activity during the Apollo period. The black jagged line is the monthly mean total sunspot number at the time while the red line is the 13-month smoothed monthly total.

The Apollo period, from 1965 - 1975 is lower than the 2 periods before and after. Apollo 11 occurred as the sunspot number was declining to a local minimum during the peak. Apollo 8, December 1968, was actually at a local minimum in the sunspot number. For both Apollo 8 & 11, the monthly mean sunspot numbers were actually below the 13-month smoothed numbers.

So 8 and 11, while flying during the maximum solar activity for 1965-1975, managed to squeeze in during periods when the activity was lower than the average for the peak. Note also that Apollo 8 did not have a LM, so all the crew members were within the thickest shielding for the entire flight, whereas 2 of the 11 crew spent some hours in the much thinner walls of the LM. That was also bound to have some effect on the relative numbers for 8 and 11.

Nice answer. See also the interesting summary  in the NASA SP reference that the OP cited, https://history.nasa.gov/SP-368/s2ch3.htm, boldfacing is mine.

Quote
Radiation was not an operational problem during the Apollo Program. Doses received by the crewmen of Apollo missions 7 through 17 were small because no major solar-particle events occurred during those missions. One small event was detected by a radiation sensor outside the Apollo 12 spacecraft, but no increase in radiation dose to the crewmen inside the spacecraft was detected. Solar-particle releases are random events, and it is possible that flares, with the accompanying energetic nuclear particles, might hinder future flights beyond the magnetosphere of the Earth.

I think this translates as "we were lucky". The famous event of August 1972 illustrates just how lucky: https://svs.gsfc.nasa.gov/13275

This found its way, iirc, in fictionalised form into James Michener's Space as a threat to "Apollo 18".
« Last Edit: 11/05/2023 11:52 am by LittleBird »

Online Emmettvonbrown

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Mike Jenne also use the same massive solar storm as a plot device in his Gemini trilogy. Together with the hurricane that ravaged the missiles facilities at Johnston island, the same month of August 1972.

Offline gwiz

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How is this possible considering Apollo 11 and Apollo 8 went to the moon and was subjected to deep space radiation outside earths protective  magnetic field unlike Apollo 7 and 9?
Radiation isn't only present outside the magnetosphere.  The Van Allen belts are inside and the inner belt, particularly the South Atlantic Anomaly, would have provided the doses for Apollo 7 and 9.

Offline jfri

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That leads to the question. Did the solar activity vary during the timeperiod when Apollo 8-11 took place ? Was it lower during the Apollo 8 and 11 mission ?

It certainly did and it certainly was. You can find this info at https://www.spaceweatherlive.com/en/solar-activity/solar-cycle/historical-solar-cycles.html. I've attached an image showing the solar activity during the Apollo period. The black jagged line is the monthly mean total sunspot number at the time while the red line is the 13-month smoothed monthly total.

The Apollo period, from 1965 - 1975 is lower than the 2 periods before and after. Apollo 11 occurred as the sunspot number was declining to a local minimum during the peak. Apollo 8, December 1968, was actually at a local minimum in the sunspot number. For both Apollo 8 & 11, the monthly mean sunspot numbers were actually below the 13-month smoothed numbers.

So 8 and 11, while flying during the maximum solar activity for 1965-1975, managed to squeeze in during periods when the activity was lower than the average for the peak. Note also that Apollo 8 did not have a LM, so all the crew members were within the thickest shielding for the entire flight, whereas 2 of the 11 crew spent some hours in the much thinner walls of the LM. That was also bound to have some effect on the relative numbers for 8 and 11.

Just one detail. Activity seem to be around top during late sexties when Apollo 7-11 happened. Then dropping after 1970 when later Apollo missions happened which measured higher doses.

Offline Proponent

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Longer missions with astronauts spending much more time either in the lightly shielded LM or outside it protected only by their suits?
« Last Edit: 11/09/2023 12:54 am by Proponent »

Offline apollolanding

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Longer missions with astronauts spending much more time either in the lightly shielded LM or outside it protected only by their suits?

Later missions were longer duration with astronauts on multiple EVAs.  Even the CMP performed a spacewalk on the return to Earth to retrieve canisters outside of the CSM. 
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