ApolloHoax.net
Apollo Discussions => The Hoax Theory => Topic started by: Gazpar on June 24, 2015, 07:45:11 PM
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What does this means? Didnt NASA solve the radiation problem already in 1969?
Or is more problem for electronics than human safety? I though they had the data about radiation levels of the belts.
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As I understand it, it's a duration question: Orion is intended for longer-duration missions. The Apollo capsule was exclusively intended for a short-duration journey to the Moon and back, and nothing else. It also may not have had much margin in the event of a major solar flare (as higher risk levels were acceptable not only in the Apollo program, but in society as a whole at the time).
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As I understand it, it's a duration question: Orion is intended for longer-duration missions. The Apollo capsule was exclusively intended for a short-duration journey to the Moon and back, and nothing else. It also may not have had much margin in the event of a major solar flare (as higher risk levels were acceptable not only in the Apollo program, but in society as a whole at the time).
What about the electronics part of the apollo spacecraft of that time? Maybe that part was not too susceptible as Orion cutting edge computers onboard.
I heard also that Orion will pass through the thickest parts of the belt(deep space travel) as opposed to apollo, which went through the thin parts of it.
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Both duration and sophistication. The electronics from the Apollo era was much less susceptible, as you imaging (look up "rope core" some time); however, mission duration (and solar flares) are again a factor.
I suspect the Orion electronics, by the way, will not be cutting edge. Spacecraft electronics needs to be robust, not cutting edge.
I understand from other posts here (and from the Planetary Society's recent LightSail test) that modern satellite electronics is expected to have failures. The hardware must be designed to be fault-tolerant, i.e. recover from faults and continue to operate.
I don't know about the transfer orbits, though; perhaps someone else can answer that.
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It is my understanding that Orion's trajectory took it through regions of the Van Allen Belts that were far more intense than the regions through which Apollo flew. I don't know Orion's trajectory exactly, but I do know that the Apollo trajectories reduced the radiation exposure by about 95% versus flying right through the heart of the radiation belts. Furthermore, Orion was in orbit and moving more slowly than Apollo, thus it spent more time in the higher intensity lower regions of the belts. Apollo was on a high speed outward trajectory that carried it through the worst part of the belts very quickly. Orion's computer is also more sensitive to radiation than Apollo's.
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The Apollo Guidance Computer used RTL (resistor-transistor logic) integrated circuits. These were not susceptible to static, cosmic rays or EM radiation to the same degree that the later CMOS chips were.
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The Apollo Guidance Computer used RTL (resistor-transistor logic) integrated circuits. These were not susceptible to static, cosmic rays or EM radiation to the same degree that the later CMOS chips were.
Yeah, that would make sense but is there any reason why RTL are not susceptible to static, cosmic rays or EM radiation than CMOS chips?
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The actual part of the component which makes the connection is much larger, and the erosion from radiation will have to destroy much more material, before the component is inoperative.
Like killing a bridge with rifle fire versus killing a power line.
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I understand from other posts here (and from the Planetary Society's recent LightSail test) that modern satellite electronics is expected to have failures. The hardware must be designed to be fault-tolerant, i.e. recover from faults and continue to operate.
The LightSail mission was saved by this. Its computer shut down because of an overlooked problem with the software, but a few days later it got a radiation hit that caused a re-boot.
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The Orion flight was an engineering test. No crew was on board, so they could afford to take larger risks to stress the systems than would be encountered in a normal mission. One of those greater risks was the radiation environment, which was considerably more intense than Apollo saw on a trip to the moon.
And yes, the Apollo hardware was inherently more radiation-resistant than modern electronics. Much of the logic was done purely in hardware with diodes, discrete transistors, mechanical switches and relays. The SSI (Small Scale Integration) RTL (Resistor-Transistor Logic) integrated circuits in the guidance computer had very large transistors, which means radiation has to deposit considerably more energy to cause a temporary upset or destroy the device.
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Didnt NASA solve the radiation problem already in 1969?
Yes, for a short mission of two weeks' duration.
I though they had the data about radiation levels of the belts.
Would you want to rely on shipping forcasts from fifty years ago when you embark on a trip across the sea?
A few bullet points to consider:
1: All data are valuable, and the more up-to-date the better.
2: Modern electronics respond differently to radiation than the hardware used in Apollo does.
3: Radiation damage is cumulative.
4: The probability of encountering a major solar event increases with duration of the mission.
5: Orion is planning missions of much longer duration than Apollo.
All of which boils down to the simple fact that the data collected about the radiation problem, and the solutions devised, for the Apollo programme are inadequate for the very different planned Orion missions.
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Part of the concept of Orion is to make it reusable. That is a significant difference between the mostly one time use design of the Apollo missions. Although IIRC there was some reuse of Apollo equipment. Designing in longevity of human rated equipment from the start means more a more robust design than was necessary 40 years ago and more testing. What better way to do the testing than to put it through a harsh environment. Orion is really a quite different system from either the Apollo CSM or the Shuttle, with different mission goals and requirements. Besides, nothing is ever permanently solved.
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Many of Apollo's critical control systems (e.g., Earth Landing System) were relay logic. This is due in part to its being impervious to radiation and a host of other hostile influences, but also in part to the long experience in designing such systems. Solid-state logic was still in relative infancy and not fully trusted. Relay logic at that point had a 60-year design pedigree and had all the bugs (literal, in fact) worked out.
The Apollo shielding factor was almost entirely accidental. Which is to say, no part of the Apollo spacecraft was designated as radiation shielding. All the shielding effects came from things that were already there for other reasons. They added up to a robust 7 g cm-2, which turns out to be sufficient for an Apollo-type mission barring catastrophic solar activity. Orion is intended for more extensive missions.
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Could it be that orion passed through the more dense parts of the belts, opposed to apollo? Im trying to find a source for this
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Van Allen belt radiation consists of high-energy electrons and protons in the inner belt, and high-energy electrons in the outer belt.
The most energetic electrons encountered along the trajectories flown by Apollo have an energy of about 7 MeV. The 7 g/cm shielding provided by the CM hull is about twice that needed to completely stop a 7 MeV electron. Electrons, therefore, were of no concern to Apollo as they were virtually all blocked by the hull.
At a given energy, protons are less penetrating than electrons. Therefore, virtually all protons on the low end of the energy spectrum were easily blocking by the spacecraft shielding. The only real concern from a radiation standpoint were the most highly energetic protons, which can exceed 100 MeV. These protons could penetrate Apollo's shielding and potentially cause damage. Fortunately, high-energy particles are far fewer in number than low-energy particles (by many orders of magnitude). Apollo also used a highspeed injection that allowed it to pass by the inner radiation belt in just a manner of minutes. Also note that the inner radiation belt is a torus (doughnut shaped) that is most intense near the center and weaker near the edges. Apollo flew inclined trajectories that bypassed the intense middle regions and grazed along the weak edges. All of this adds up to a radiation condition that was not of significant concern. Apollo passed through the region quickly and through the weak outer edges where there just weren't enough high-energy protons to cause damage.
The Orion test flight is an entirely different animal. After performing one low orbit of Earth, Orion fired its engine to alter its trajectory. The apogee was raised to 5790 km and the perigee was lowered to -29.8 km (below the surface of Earth). This orbit allowed Orion to pass through the inner Van Allen belt to test the spacecraft's radiation resistance, and then to make a high-speed reentry to test the spacecraft's thermal protection.
The inner Van Allen belt's altitude above Earth's surface ranges from about 1000 km to 6000 km. Therefore, the entire part of Orion's orbit above 1000 km was within the radiation belt. From Orion's orbital parameters, I calculate that it spent 118 minutes in this region of space. For comparison, Apollo passed through the same range of altitudes in just 17 minutes, once on the way to the Moon and once on the way back, for a total of 34 minutes. The duration of Orion's passage through the inner radiation belt was about 3.5 times longer than Apollo's.
It must also be noted that Orion's inclination was 28.8 degrees, which is lower than the inclinations used by Apollo. Orion was therefore closer to the more intense middle region of the radiation belt. More importantly, the portion of Orion's orbit above 1000 km spanned an arc of 229 degrees. This means that Orion had to pass through at least one of its nodes while within the radiation belt. The significance of this is that at some point during its passage through the radiation belt, Orion had to cross the latitudes at which the radiation belt is its most intense.
I don't have all the specific data needed to make a detailed quantitative analysis of Orion versus Apollo. However, based on the orbital data that I do have and my past experience studying the radiation belts, I think it is safe to say that Orion's radiation exposure was dozens of times greater than what Apollo was exposed to.
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Thanks for the info Bob B. Very interesting. I read somewhere shortly after the recovery, that they also put a new type of radiation detector on board. just wondering if you know anything about that and if it met expectations?
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You should also be aware, Gazpar, that the unmanned boilerplate test flight, as mentioned above, intentionally went into more intense regions of the belts. Crewed missions will probably take a different flight profile, although I don't know what the plans are for those.
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It is important to remember that we are talking about a Orion test object designed to gather information on a limited number of situations. So the comparisons to a functional crewed Apollo mission will be indirect and highly technical in nature. It may be more informative to look at Apollo capsule development (https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Development_history) and test flights. The Apollo capsule contract was first awarded in 1961 for use in the direct assent mode. The Block 2 was proposed in 1964 due to the mission change to LOR and to incorporate the knowledge gained in the intervening time. Similarly we can expect the Orion to change as research and mission plans develop. Since it is envisioned for longer duration use, Orion has to meet radiation challenges that are not just from the Van Allen belts but from longer term risks outside of the belts.
We also have to consider that the Apollo program benefited from the Gemini flights, which included trips into the Van Allen belts. It's not that this information is useless, but that 40 to 50 year old information is incomplete by today's standards because it was not acquired with the current knowledge requirements in mind.
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Van Allen belt radiation consists of high-energy electrons and protons in the inner belt, and high-energy electrons in the outer belt.
The most energetic electrons encountered along the trajectories flown by Apollo have an energy of about 7 MeV. The 7 g/cm shielding provided by the CM hull is about twice that needed to completely stop a 7 MeV electron. Electrons, therefore, were of no concern to Apollo as they were virtually all blocked by the hull.
At a given energy, protons are less penetrating than electrons. Therefore, virtually all protons on the low end of the energy spectrum were easily blocking by the spacecraft shielding. The only real concern from a radiation standpoint were the most highly energetic protons, which can exceed 100 MeV. These protons could penetrate Apollo's shielding and potentially cause damage. Fortunately, high-energy particles are far fewer in number than low-energy particles (by many orders of magnitude). Apollo also used a highspeed injection that allowed it to pass by the inner radiation belt in just a manner of minutes. Also note that the inner radiation belt is a torus (doughnut shaped) that is most intense near the center and weaker near the edges. Apollo flew inclined trajectories that bypassed the intense middle regions and grazed along the weak edges. All of this adds up to a radiation condition that was not of significant concern. Apollo passed through the region quickly and through the weak outer edges where there just weren't enough high-energy protons to cause damage.
The Orion test flight is an entirely different animal. After performing one low orbit of Earth, Orion fired it engine to alter its trajectory. The apogee was raised to 5790 km and the perigee was lowered to -29.8 km (below the surface of Earth). This orbit allowed Orion to pass through the inner Van Allen belt to test the spacecraft's radiation resistance, and then to make a high-speed reentry to test the spacecraft's thermal protection.
The inner Van Allen belt's altitude above Earth's surface ranges from about 1000 km to 6000 km. Therefore, the entire part of Orion's orbit above 1000 km was within the radiation belt. From Orion's orbital parameters, I calculate that it spent 118 minutes in this region of space. For comparison, Apollo passed through the same range of altitudes in just 17 minutes, once on the way to the Moon and once on the way back, for a total of 34 minutes. The duration of Orion's passage through the inner radiation belt was about 3.5 times longer than Apollo's.
It must also be noted that Orion's inclination was 28.8 degrees, which is lower than the inclinations used by Apollo. Orion was therefore closer to the more intense middle region of the radiation belt. More importantly, the portion of Orion's orbit above 1000 km spanned an arc of 229 degrees. This means that Orion had to pass through at least one of its nodes while within the radiation belt. The significance of this is that at some point during its passage through the radiation belt, Orion had to cross the latitudes at which the radiation belt is its most intense.
I don't have all the specific data needed to make a detailed quantitative analysis of Orion versus Apollo. However, based on the orbital data that I do have and my past experience studying the radiation belts, I think it is safe to say that Orion's radiation exposure was dozens of times greater than what Apollo was exposed to.
That makes sense.
Orion has modern electronics (more susceptible to radiation) and NASA wanted to test them on high radiation because Orion is designed to make deep space travel with longer duration than Apollo (less susceptible than radiation). So Orion went to the worst parts of the belts to test its resistance against high radiation (radiation Apollo didnt had to be protected with in the first place)
It all part of collecting data, I understand now.
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I read somewhere shortly after the recovery, that they also put a new type of radiation detector on board. just wondering if you know anything about that and if it met expectations?
I have not read or heard any results from that.
Orion has modern electronics (more susceptible to radiation) and NASA wanted to test them on high radiation because Orion is designed to make deep space travel with longer duration than Apollo (less susceptible than radiation). So Orion went to the worst parts of the belts to test its resistance against high radiation (radiation Apollo didnt had to be protected with in the first place)
It all part of collecting data, I understand now.
That sums it up pretty well.
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I read somewhere shortly after the recovery, that they also put a new type of radiation detector on board. just wondering if you know anything about that and if it met expectations?
I have not read or heard any results from that.
I looked into it more and this is what I recall reading about. I didn't realize that it had already been tested on the ISS:
http://www.uh.edu/news-events/stories/2014/November/111914SpaceRadiation
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Van Allen belt radiation consists of high-energy electrons and protons in the inner belt, and high-energy electrons in the outer belt.
The most energetic electrons encountered along the trajectories flown by Apollo have an energy of about 7 MeV. The 7 g/cm shielding provided by the CM hull is about twice that needed to completely stop a 7 MeV electron. Electrons, therefore, were of no concern to Apollo as they were virtually all blocked by the hull.
At a given energy, protons are less penetrating than electrons. Therefore, virtually all protons on the low end of the energy spectrum were easily blocking by the spacecraft shielding. The only real concern from a radiation standpoint were the most highly energetic protons, which can exceed 100 MeV. These protons could penetrate Apollo's shielding and potentially cause damage. Fortunately, high-energy particles are far fewer in number than low-energy particles (by many orders of magnitude). Apollo also used a highspeed injection that allowed it to pass by the inner radiation belt in just a manner of minutes. Also note that the inner radiation belt is a torus (doughnut shaped) that is most intense near the center and weaker near the edges. Apollo flew inclined trajectories that bypassed the intense middle regions and grazed along the weak edges. All of this adds up to a radiation condition that was not of significant concern. Apollo passed through the region quickly and through the weak outer edges where there just weren't enough high-energy protons to cause damage.
The Orion test flight is an entirely different animal. After performing one low orbit of Earth, Orion fired its engine to alter its trajectory. The apogee was raised to 5790 km and the perigee was lowered to -29.8 km (below the surface of Earth). This orbit allowed Orion to pass through the inner Van Allen belt to test the spacecraft's radiation resistance, and then to make a high-speed reentry to test the spacecraft's thermal protection.
The inner Van Allen belt's altitude above Earth's surface ranges from about 1000 km to 6000 km. Therefore, the entire part of Orion's orbit above 1000 km was within the radiation belt. From Orion's orbital parameters, I calculate that it spent 118 minutes in this region of space. For comparison, Apollo passed through the same range of altitudes in just 17 minutes, once on the way to the Moon and once on the way back, for a total of 34 minutes. The duration of Orion's passage through the inner radiation belt was about 3.5 times longer than Apollo's.
It must also be noted that Orion's inclination was 28.8 degrees, which is lower than the inclinations used by Apollo. Orion was therefore closer to the more intense middle region of the radiation belt. More importantly, the portion of Orion's orbit above 1000 km spanned an arc of 229 degrees. This means that Orion had to pass through at least one of its nodes while within the radiation belt. The significance of this is that at some point during its passage through the radiation belt, Orion had to cross the latitudes at which the radiation belt is its most intense.
I don't have all the specific data needed to make a detailed quantitative analysis of Orion versus Apollo. However, based on the orbital data that I do have and my past experience studying the radiation belts, I think it is safe to say that Orion's radiation exposure was dozens of times greater than what Apollo was exposed to.
This is why I love this site. Information like this
Further reading Gazpar
http://image.gsfc.nasa.gov/poetry/tour/AAvan.html
Note especially the section on Human Impacts about 3/4 of the way down the page
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I'm really thinking about writing about the radiation environment in space and putting to bed the myths and utter BS of Ralph Rene and the BFDU. Dwight suggested this to me a long time ago, but sometimes I feel it is such an esoteric subject that it would not be worth the while. Bob has produced a great analysis of the van Allen belts in context of Apollo.
I'm yet to see the BFDU actually quote this NOAA page:
http://www.swpc.noaa.gov/noaa-scales-explanation
The real problem for any manned space flight beyond the VABs is the Solar Proton Event, and sadly for historical reasons, the term solar flare and SPE are interchanged. The BFDU and other prominent CTs cite every NOAA source they can gain mileage from, but will not address the information in the above link that actually shows the real problems astronauts will encounter.
Back to finding the wrong negative in the differentiation of a trigonometric nightmare. :(
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Van Allen belt radiation consists of high-energy electrons and protons in the inner belt, and high-energy electrons in the outer belt.
The most energetic electrons encountered along the trajectories flown by Apollo have an energy of about 7 MeV. The 7 g/cm shielding provided by the CM hull is about twice that needed to completely stop a 7 MeV electron. Electrons, therefore, were of no concern to Apollo as they were virtually all blocked by the hull.
At a given energy, protons are less penetrating than electrons. Therefore, virtually all protons on the low end of the energy spectrum were easily blocking by the spacecraft shielding. The only real concern from a radiation standpoint were the most highly energetic protons, which can exceed 100 MeV. These protons could penetrate Apollo's shielding and potentially cause damage. Fortunately, high-energy particles are far fewer in number than low-energy particles (by many orders of magnitude). Apollo also used a highspeed injection that allowed it to pass by the inner radiation belt in just a manner of minutes. Also note that the inner radiation belt is a torus (doughnut shaped) that is most intense near the center and weaker near the edges. Apollo flew inclined trajectories that bypassed the intense middle regions and grazed along the weak edges. All of this adds up to a radiation condition that was not of significant concern. Apollo passed through the region quickly and through the weak outer edges where there just weren't enough high-energy protons to cause damage.
The Orion test flight is an entirely different animal. After performing one low orbit of Earth, Orion fired its engine to alter its trajectory. The apogee was raised to 5790 km and the perigee was lowered to -29.8 km (below the surface of Earth). This orbit allowed Orion to pass through the inner Van Allen belt to test the spacecraft's radiation resistance, and then to make a high-speed reentry to test the spacecraft's thermal protection.
The inner Van Allen belt's altitude above Earth's surface ranges from about 1000 km to 6000 km. Therefore, the entire part of Orion's orbit above 1000 km was within the radiation belt. From Orion's orbital parameters, I calculate that it spent 118 minutes in this region of space. For comparison, Apollo passed through the same range of altitudes in just 17 minutes, once on the way to the Moon and once on the way back, for a total of 34 minutes. The duration of Orion's passage through the inner radiation belt was about 3.5 times longer than Apollo's.
It must also be noted that Orion's inclination was 28.8 degrees, which is lower than the inclinations used by Apollo. Orion was therefore closer to the more intense middle region of the radiation belt. More importantly, the portion of Orion's orbit above 1000 km spanned an arc of 229 degrees. This means that Orion had to pass through at least one of its nodes while within the radiation belt. The significance of this is that at some point during its passage through the radiation belt, Orion had to cross the latitudes at which the radiation belt is its most intense.
I don't have all the specific data needed to make a detailed quantitative analysis of Orion versus Apollo. However, based on the orbital data that I do have and my past experience studying the radiation belts, I think it is safe to say that Orion's radiation exposure was dozens of times greater than what Apollo was exposed to.
This is why I love this site. Information like this
Further reading Gazpar
http://image.gsfc.nasa.gov/poetry/tour/AAvan.html
Note especially the section on Human Impacts about 3/4 of the way down the page
Reading it. thx
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Yeah, that would make sense but is there any reason why RTL are not susceptible to static, cosmic rays or EM radiation than CMOS chips?
Most electronics are susceptible to radiation, but some components (mainly semiconductors) are far more susceptible than others.
Radiation is energy, so what really matters is how much energy it delivers to a device (e.g., a transistor in an integrated circuit) compared to what that device normally handles or can handle. Until Apollo, all computers had been made from discrete components (transistors, etc) that tend to be physically large. (The Apollo Guidance Computer was the first computer made with integrated circuits.) Computers occupied large rooms and had to be continuously cooled, yet their processing speeds and memory sizes were quite low. Each component therefore could (and usually did) handle a lot of power, and with everything else the same, such a computer would probably be fairly radiation tolerant.
A modern commercial computer uses integrated circuits with many orders of magnitude more transistors than those used in the Apollo AGC, so each transistor has to be quite tiny. This means it can't handle much power without damage, and a very small amount of injected energy can overwhelm the energy of the data bits it is processing. The latter phenomenon causes transient errors without necessarily also causing permanent damage.
But it's possible to mitigate the effects of radiation on even these devices. One part is to use fabrication methods known to be rad-hard. Another is to add redundancy and cross-checking so that if one device (e.g., an entire CPU) has a transient error caused by a charged particle, it will be detected. Typically you have three devices working in parallel, with logic to compare their results and 'vote out' a CPU if it doesn't agree with the other two.
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I read somewhere shortly after the recovery, that they also put a new type of radiation detector on board. just wondering if you know anything about that and if it met expectations?
I have not read or heard any results from that.
I looked into it more and this is what I recall reading about. I didn't realize that it had already been tested on the ISS:
http://www.uh.edu/news-events/stories/2014/November/111914SpaceRadiation
Ha, I know Larry! Nice article.
Anyway, in this thread (http://www.apollohoax.net/forum/index.php?topic=515.msg17621#msg17621) we talked about the similar voyages of Apollo 4 and 6.
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Anyway, in this thread (http://www.apollohoax.net/forum/index.php?topic=515.msg17621#msg17621) we talked about the similar voyages of Apollo 4 and 6.
As a comparison to the image showing the trajectories of Apollos 4 and 6, the following shows the outbound trajectory of Apollo 11.
(http://www.braeunig.us/apollo/pics/AP100MeV.gif)
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I just watched the launch of the first Orion mission. And currently watching the Orbital ATK Cyngus launch. I noticed in both a bright orange flare some distance from the vehicle. Is this H2 burn off? I never noticed this from the STS launches or Saturn V. Is this a new procedure to eliminate possible accumulation and fire at the booster?
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Yes, all the launch sites I've seen that support hydrogen-fueled vehicles have burn ponds or flares to safely burn off vented hydrogen. When pure hydrogen (i.e., not pre-mixed with air) burns, it produces an orange flame.
See some Periodic Videos clips on hydrogen:
Note that in the second (newer) video the Professor revised his hypothesis on why the flame is orange. It appears to be the hydrogen itself.
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I never noticed any in the Shuttle launches, where were they/it?
I do remember watching challenger replays and there was a small igniter at the base of the vehicle
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The hydrogen burn ponds at Pads 39A and B are some distance from the pad itself. They did show up sometimes in certain TV shots.
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OK, never knew that.
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This probably isn't the best map of LC39A, but it's the best one I could find in a few minutes of searching:
http://www.wpusa.dynip.com/space/LC39A.html
J8-1611 is the hydrogen burn stack. It's northeast of the pad, about halfway to the perimeter.
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It pays to be cautious with the highly flammable H2 as those around the Hindenburg found out!
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Looks like NASA is still reviewing and refining the capsule in preparation for the first crewed launch. I wonder what the CT's will have to say about that flight.
http://www.nasa.gov/feature/a-year-after-maiden-voyage-orion-progress-continues
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It pays to be cautious with the highly flammable H2 as those around the Hindenburg found out!
Hydrogen safety is a timely topic for me, as I mentor a local high school ham radio club that builds and flies high altitude balloon payloads. Helium has become extremely expensive and scarce over the past few years, forcing many balloon enthusiasts to switch to hydrogen. I have yet to hear of an accident.
I've researched the safety issues extensively, and we've come to the conclusion that if you're willing to handle gasoline, then you should be willing to handle hydrogen. Both must be handled with respect, but I think hydrogen is actually less hazardous than gasoline. It's certainly less poisonous.
The main thing is to work in the open air so small leaks will quickly dissipate.
You also must avoid mixing it with air to form an explosive H2/O2 mixture, but that actually takes a deliberate effort; tiny amounts of air in a balloon will not be a problem because the hydrogen will be outside its flammability range.
We take the extra step of grounding the tank with a ground stake and working on a damp tarp to dissipate any static electricity charges. And it should go without saying that only the adults handle the gas and flames are kept far away. Fortunately, nobody smokes.
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The hydrogen burn ponds at Pads 39A and B are some distance from the pad itself. They did show up sometimes in certain TV shots.
Somebody posted video of those in the not too distant past. Permit me to rummage in the memory bank, they were spectacular.
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It pays to be cautious with the highly flammable H2 as those around the Hindenburg found out!
Hydrogen safety is a timely topic for me, as I mentor a local high school ham radio club that builds and flies high altitude balloon payloads. Helium has become extremely expensive and scarce over the past few years, forcing many balloon enthusiasts to switch to hydrogen. I have yet to hear of an accident.
I've researched the safety issues extensively, and we've come to the conclusion that if you're willing to handle gasoline, then you should be willing to handle hydrogen. Both must be handled with respect, but I think hydrogen is actually less hazardous than gasoline. It's certainly less poisonous.
The main thing is to work in the open air so small leaks will quickly dissipate.
You also must avoid mixing it with air to form an explosive H2/O2 mixture, but that actually takes a deliberate effort; tiny amounts of air in a balloon will not be a problem because the hydrogen will be outside its flammability range.
We take the extra step of grounding the tank with a ground stake and working on a damp tarp to dissipate any static electricity charges. And it should go without saying that only the adults handle the gas and flames are kept far away. Fortunately, nobody smokes.
I never worked around it except in chemistry labs. In those controlled environs it was interesting to burn, but not too dangerous. I wonder why helium has become so expensive? Perhaps the fields producing it have decline in production/reserves.
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I never noticed any in the Shuttle launches, where were they/it?
I do remember watching challenger replays and there was a small igniter at the base of the vehicle
You may be thinking of the hydrogen burn-off igniters. Hydrogen is a powerfully small molecule, and it's very difficult to design large gas-tight valves for it. So some leakage is expected. You really don't want hydrogen collecting in the nozzles after propellant pressurization and suddenly going off with a bang at SSME ignition.
But yes, the burn ponds are to dispose of boiled hydrogen from the LH2 tanks, prior to close-off and pressurization. It's normally collected via vent hoods.
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Hydrogen safety is a timely topic for me...
Several branches of engineering take hydrogen very, very seriously. Nuclear physics experiments, for example, can suddenly liberate vast amounts of hydrogen. At one place I worked, we had routine releases and subsequently routine evacuations and visits from HazMat guys to inspect and clear the spaces. Got to know them by their first names.
The main thing is to work in the open air so small leaks will quickly dissipate.
This is actually a plus for hydrogen-fueled vehicles, should any be built. In a wreck, gasoline and other liquid hydrocarbons spill out of their tanks and generally stay at the premises, ready to fuel horrible and difficult-to-extinguish fires. In a similar wreck, should the hydrogen tank burst, its contents would disperse rapidly.
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Many of Apollo's critical control systems (e.g., Earth Landing System) were relay logic. This is due in part to its being impervious to radiation and a host of other hostile influences, but also in part to the long experience in designing such systems. Solid-state logic was still in relative infancy and not fully trusted. Relay logic at that point had a 60-year design pedigree and had all the bugs (literal, in fact) worked out.
The Apollo shielding factor was almost entirely accidental. Which is to say, no part of the Apollo spacecraft was designated as radiation shielding. All the shielding effects came from things that were already there for other reasons. They added up to a robust 7 g cm-2, which turns out to be sufficient for an Apollo-type mission barring catastrophic solar activity. Orion is intended for more extensive missions.
...and now that I have moved (reason for my abscence over the last few weeks) and have finally dusted off my old desktop, I can now access a small library of research that I downloaded when I had access to Oxford University's Library facilities. I'm trying to dig out the paper that discusses the August 1972 flare. I recall that had the astronauts been on the moon then they would have been rendered ill (or dead), but inside the CM the shielding was enough to reduce their dose to less harmful levels. The CM did offer some protection against SPEs. I'll try and find the article.
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Many of Apollo's critical control systems (e.g., Earth Landing System) were relay logic. This is due in part to its being impervious to radiation and a host of other hostile influences, but also in part to the long experience in designing such systems. Solid-state logic was still in relative infancy and not fully trusted. Relay logic at that point had a 60-year design pedigree and had all the bugs (literal, in fact) worked out.
The Apollo shielding factor was almost entirely accidental. Which is to say, no part of the Apollo spacecraft was designated as radiation shielding. All the shielding effects came from things that were already there for other reasons. They added up to a robust 7 g cm-2, which turns out to be sufficient for an Apollo-type mission barring catastrophic solar activity. Orion is intended for more extensive missions.
Luke's reposting of your post gave me another thought. In the event of a catastrophic solar burst, I have read somewhere that the contingency plan was to turn the CSM around and face the oncoming burst to allow the liquids in the SM to shield much of the event. Is this correct?
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Luke's reposting of your post gave me another thought. In the event of a catastrophic solar burst, I have read somewhere that the contingency plan was to turn the CSM around and face the oncoming burst to allow the liquids in the SM to shield much of the event. Is this correct?
That's my understanding, and contrary to popular CT ideas the proton flux does not all arrive in one devastating front. There would be a lead edge of high energy protons which are detected, thus allowing the crew to turn the ship around as the flux rises to its maximum.
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Luke's reposting of your post gave me another thought. In the event of a catastrophic solar burst, I have read somewhere that the contingency plan was to turn the CSM around and face the oncoming burst to allow the liquids in the SM to shield much of the event. Is this correct?
That's my understanding, and contrary to popular CT ideas the proton flux does not all arrive in one devastating front. There would be a lead edge of high energy protons which are detected, thus allowing the crew to turn the ship around as the flux rises to its maximum.
Have you had time to find the data on the Aug 72 flare? If so, please post.
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1972 SOLRAD (8-20A) DAILY BACKGROUND LEVELS (WATTS/METER*2)
=======================================================================
Day Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
--------------------------------------------------------------------------------------
1 7.4E-6 4.2E-6 1.2E-5 2.9E-6 2.9E-6 7.0E-6 6.8E-6 4.6E-5 2.9E-5 1.4E-5 1.3E-5 4.5E-6
2 4.2E-6 4.4E-6 1.2E-5 3.2E-6 2.8E-6 6.4E-6 7.3E-6 1.0E-4 2.6E-5 1.0E-5 1.2E-5 4.3E-6
3 3.5E-6 5.0E-6 9.5E-6 3.9E-6 3.4E-6 8.5E-6 1.0E-5 3.5E-5 2.1E-5 9.3E-6 1.0E-5 3.9E-6
4 2.4E-6 4.3E-6 1.1E-5 4.4E-6 3.7E-6 8.6E-6 1.3E-5 4.3E-5 1.6E-5 7.0E-6 9.4E-6 3.2E-6
5 2.8E-6 5.2E-6 1.3E-5 5.1E-6 4.3E-6 1.4E-5 1.1E-5 2.8E-5 1.7E-5 6.2E-6 9.5E-6 3.4E-6
6 3.6E-6 4.6E-6 1.6E-5 6.0E-6 6.1E-6 1.1E-5 1.4E-5 2.7E-5 1.9E-5 7.0E-6 7.7E-6 3.2E-6
7 3.3E-6 4.7E-6 1.3E-5 6.7E-6 6.0E-6 9.6E-6 1.1E-5 3.9E-5 1.1E-5 5.8E-6 4.6E-6 6.6E-6
8 2.8E-6 6.9E-6 1.1E-5 6.3E-6 6.5E-6 1.3E-5 9.6E-6 3.0E-5 1.6E-5 7.1E-6 2.8E-6 8.5E-6
9 3.3E-6 7.8E-6 1.0E-5 8.6E-6 7.9E-6 1.5E-5 8.6E-6 2.8E-5 1.4E-5 8.9E-6 3.3E-6 7.0E-6
10 2.3E-6 8.9E-6 8.2E-6 6.5E-6 9.6E-6 1.1E-5 7.0E-6 3.1E-5 1.2E-5 8.8E-6 4.7E-6 8.7E-6
11 4.4E-6 8.9E-6 8.7E-6 6.1E-6 1.4E-5 1.0E-5 7.9E-6 3.3E-5 8.6E-6 8.3E-6 5.6E-6 9.6E-6
12 5.2E-6 8.0E-6 8.0E-6 5.3E-6 1.4E-5 2.2E-5 7.8E-6 3.0E-5 6.8E-6 6.9E-6 5.4E-6 1.9E-5
13 6.1E-6 1.1E-5 9.5E-6 5.5E-6 1.7E-5 1.7E-5 7.9E-6 1.7E-5 5.9E-6 7.3E-6 4.8E-6 1.6E-5
14 7.0E-6 9.9E-6 8.6E-6 6.6E-6 1.8E-5 1.2E-5 1.1E-5 1.1E-5 6.4E-6 1.2E-5 4.3E-6 1.3E-5
15 5.2E-6 1.8E-5 1.0E-5 6.8E-6 1.7E-5 1.3E-5 9.3E-6 9.0E-6 7.9E-6 1.0E-5 3.5E-6 1.2E-5
16 8.8E-6 1.5E-5 8.9E-6 6.6E-6 1.3E-5 1.5E-5 6.6E-6 7.8E-6 8.1E-6 1.2E-5 4.3E-6 1.0E-5
17 4.7E-6 2.1E-5 8.1E-6 7.4E-6 1.1E-5 1.6E-5 6.5E-6 8.8E-6 7.3E-6 1.3E-5 5.5E-6 1.3E-5
18 4.5E-6 2.5E-5 8.4E-6 8.6E-6 1.2E-5 1.1E-5 4.3E-6 1.0E-5 1.2E-5 1.1E-5 8.2E-6 1.5E-5
19 8.4E-6 1.9E-5 9.0E-6 6.3E-6 8.3E-6 1.0E-5 4.6E-6 1.5E-5 1.2E-5 1.0E-5 8.9E-6 1.2E-5
20 6.7E-6 2.1E-5 9.5E-6 4.3E-6 8.5E-6 1.0E-5 4.9E-6 2.1E-5 1.6E-5 1.5E-5 1.0E-5 9.2E-6
21 6.5E-6 1.9E-5 9.5E-6 4.1E-6 7.6E-6 9.8E-6 7.3E-6 1.7E-5 1.8E-5 2.6E-5 1.0E-5 1.0E-5
22 9.6E-6 2.1E-5 8.5E-6 4.4E-6 8.3E-6 7.8E-6 6.1E-6 1.7E-5 2.6E-5 3.5E-5 9.9E-6 1.3E-5
23 1.3E-5 1.5E-5 9.1E-6 4.4E-6 1.3E-5 7.0E-6 7.6E-6 1.9E-5 1.8E-5 5.7E-5 1.3E-5 8.7E-6
24 1.3E-5 1.3E-5 8.7E-6 4.4E-6 3.1E-5 7.9E-6 5.6E-6 1.9E-5 1.5E-5 5.7E-5 1.4E-5 7.3E-6
25 1.1E-5 1.1E-5 5.8E-6 4.4E-6 1.5E-5 7.0E-6 4.9E-6 2.1E-5 1.2E-5 6.6E-5 1.3E-5 9.0E-6
26 1.0E-5 1.2E-5 6.3E-6 4.6E-6 1.0E-5 6.1E-6 5.1E-6 3.2E-5 1.2E-5 4.9E-5 1.3E-5 7.3E-6
27 8.5E-6 1.1E-5 5.4E-6 8.0E-6 1.2E-5 6.4E-6 6.8E-6 3.1E-5 1.0E-5 4.7E-5 9.2E-6 8.1E-6
28 7.7E-6 1.0E-5 5.8E-6 5.3E-6 2.2E-5 7.2E-6 8.8E-6 3.5E-5 1.1E-5 5.3E-5 6.3E-6 7.8E-6
29 8.1E-6 1.2E-5 4.0E-6 4.2E-6 1.0E-5 9.6E-6 8.1E-6 3.8E-5 1.1E-5 3.4E-5 4.6E-6 1.0E-5
30 8.1E-6 2.9E-6 3.9E-6 7.3E-6 8.1E-6 7.2E-6 4.5E-5 1.2E-5 5.4E-5 5.8E-6 1.3E-5
31 7.7E-6 2.8E-6 8.1E-6 9.5E-6 2.9E-5 2.3E-5 9.6E-6
--------------------------------------------------------------------------------------
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OK, it looks like Aug 2 was 1.0 E-4 with a few E-5's. Not being a expert on the values what would be a dangerous level? From past questions regarding this subject, Jay has indicated it is Not necessarily but some other characteristic.
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1972 SOLRAD (8-20A) DAILY BACKGROUND LEVELS (WATTS/METER*2)
=======================================================================
Day Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
--------------------------------------------------------------------------------------
1 7.4E-6 4.2E-6 1.2E-5 2.9E-6 2.9E-6 7.0E-6 6.8E-6 4.6E-5 2.9E-5 1.4E-5 1.3E-5 4.5E-6
2 4.2E-6 4.4E-6 1.2E-5 3.2E-6 2.8E-6 6.4E-6 7.3E-6 1.0E-4 2.6E-5 1.0E-5 1.2E-5 4.3E-6
3 3.5E-6 5.0E-6 9.5E-6 3.9E-6 3.4E-6 8.5E-6 1.0E-5 3.5E-5 2.1E-5 9.3E-6 1.0E-5 3.9E-6
4 2.4E-6 4.3E-6 1.1E-5 4.4E-6 3.7E-6 8.6E-6 1.3E-5 4.3E-5 1.6E-5 7.0E-6 9.4E-6 3.2E-6
5 2.8E-6 5.2E-6 1.3E-5 5.1E-6 4.3E-6 1.4E-5 1.1E-5 2.8E-5 1.7E-5 6.2E-6 9.5E-6 3.4E-6
6 3.6E-6 4.6E-6 1.6E-5 6.0E-6 6.1E-6 1.1E-5 1.4E-5 2.7E-5 1.9E-5 7.0E-6 7.7E-6 3.2E-6
7 3.3E-6 4.7E-6 1.3E-5 6.7E-6 6.0E-6 9.6E-6 1.1E-5 3.9E-5 1.1E-5 5.8E-6 4.6E-6 6.6E-6
8 2.8E-6 6.9E-6 1.1E-5 6.3E-6 6.5E-6 1.3E-5 9.6E-6 3.0E-5 1.6E-5 7.1E-6 2.8E-6 8.5E-6
9 3.3E-6 7.8E-6 1.0E-5 8.6E-6 7.9E-6 1.5E-5 8.6E-6 2.8E-5 1.4E-5 8.9E-6 3.3E-6 7.0E-6
10 2.3E-6 8.9E-6 8.2E-6 6.5E-6 9.6E-6 1.1E-5 7.0E-6 3.1E-5 1.2E-5 8.8E-6 4.7E-6 8.7E-6
11 4.4E-6 8.9E-6 8.7E-6 6.1E-6 1.4E-5 1.0E-5 7.9E-6 3.3E-5 8.6E-6 8.3E-6 5.6E-6 9.6E-6
12 5.2E-6 8.0E-6 8.0E-6 5.3E-6 1.4E-5 2.2E-5 7.8E-6 3.0E-5 6.8E-6 6.9E-6 5.4E-6 1.9E-5
13 6.1E-6 1.1E-5 9.5E-6 5.5E-6 1.7E-5 1.7E-5 7.9E-6 1.7E-5 5.9E-6 7.3E-6 4.8E-6 1.6E-5
14 7.0E-6 9.9E-6 8.6E-6 6.6E-6 1.8E-5 1.2E-5 1.1E-5 1.1E-5 6.4E-6 1.2E-5 4.3E-6 1.3E-5
15 5.2E-6 1.8E-5 1.0E-5 6.8E-6 1.7E-5 1.3E-5 9.3E-6 9.0E-6 7.9E-6 1.0E-5 3.5E-6 1.2E-5
16 8.8E-6 1.5E-5 8.9E-6 6.6E-6 1.3E-5 1.5E-5 6.6E-6 7.8E-6 8.1E-6 1.2E-5 4.3E-6 1.0E-5
17 4.7E-6 2.1E-5 8.1E-6 7.4E-6 1.1E-5 1.6E-5 6.5E-6 8.8E-6 7.3E-6 1.3E-5 5.5E-6 1.3E-5
18 4.5E-6 2.5E-5 8.4E-6 8.6E-6 1.2E-5 1.1E-5 4.3E-6 1.0E-5 1.2E-5 1.1E-5 8.2E-6 1.5E-5
19 8.4E-6 1.9E-5 9.0E-6 6.3E-6 8.3E-6 1.0E-5 4.6E-6 1.5E-5 1.2E-5 1.0E-5 8.9E-6 1.2E-5
20 6.7E-6 2.1E-5 9.5E-6 4.3E-6 8.5E-6 1.0E-5 4.9E-6 2.1E-5 1.6E-5 1.5E-5 1.0E-5 9.2E-6
21 6.5E-6 1.9E-5 9.5E-6 4.1E-6 7.6E-6 9.8E-6 7.3E-6 1.7E-5 1.8E-5 2.6E-5 1.0E-5 1.0E-5
22 9.6E-6 2.1E-5 8.5E-6 4.4E-6 8.3E-6 7.8E-6 6.1E-6 1.7E-5 2.6E-5 3.5E-5 9.9E-6 1.3E-5
23 1.3E-5 1.5E-5 9.1E-6 4.4E-6 1.3E-5 7.0E-6 7.6E-6 1.9E-5 1.8E-5 5.7E-5 1.3E-5 8.7E-6
24 1.3E-5 1.3E-5 8.7E-6 4.4E-6 3.1E-5 7.9E-6 5.6E-6 1.9E-5 1.5E-5 5.7E-5 1.4E-5 7.3E-6
25 1.1E-5 1.1E-5 5.8E-6 4.4E-6 1.5E-5 7.0E-6 4.9E-6 2.1E-5 1.2E-5 6.6E-5 1.3E-5 9.0E-6
26 1.0E-5 1.2E-5 6.3E-6 4.6E-6 1.0E-5 6.1E-6 5.1E-6 3.2E-5 1.2E-5 4.9E-5 1.3E-5 7.3E-6
27 8.5E-6 1.1E-5 5.4E-6 8.0E-6 1.2E-5 6.4E-6 6.8E-6 3.1E-5 1.0E-5 4.7E-5 9.2E-6 8.1E-6
28 7.7E-6 1.0E-5 5.8E-6 5.3E-6 2.2E-5 7.2E-6 8.8E-6 3.5E-5 1.1E-5 5.3E-5 6.3E-6 7.8E-6
29 8.1E-6 1.2E-5 4.0E-6 4.2E-6 1.0E-5 9.6E-6 8.1E-6 3.8E-5 1.1E-5 3.4E-5 4.6E-6 1.0E-5
30 8.1E-6 2.9E-6 3.9E-6 7.3E-6 8.1E-6 7.2E-6 4.5E-5 1.2E-5 5.4E-5 5.8E-6 1.3E-5
31 7.7E-6 2.8E-6 8.1E-6 9.5E-6 2.9E-5 2.3E-5 9.6E-6
--------------------------------------------------------------------------------------
That's X-ray data, not proton data. The SPE event occurred in August 1972. Here is one paper on the subject:
http://emmrem.unh.edu/papers/general/parsons.pdf
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I'll keep this for future reference in dealing with the radiation issues during Apollo.
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http://emmrem.unh.edu/papers/general/parsons.pdf
Thanks for the link; interesting.
Some comments on terminology, since the paper doesn't fully explain it.
"Deterministic effects" refers to acute radiation sickness. "Stochastic effects" refers to increasing the probability of getting cancer some time later, possibly years. The "blood forming organs", i.e., bone marrow, are an important target of radiation because they're among the most rapidly dividing cells in the body. A typical healthy adult forms about 3.5 million red blood cells/sec and 3,500 white blood cells/sec. An acute radiation dose will kill some or all of these newly forming blood cells and result in at least temporary anemia and reduced immunity to infection, which could be what actually kills you. The infection doesn't have to be from an external source; we all carry huge numbers of bacteria (e.g. in our gut) that are normally benign or even helpful.
The classic "stochastic effect" of a sublethal radiation dose also involves the blood forming organs: an increased chance of certain forms of leukemia.
Having been through a stem cell transplant (as treatment for lymphoma, a blood cancer) where this was done deliberately (though by chemicals rather than radiation) I can tell you it's not very pleasant. I survived because I received a carefully calibrated dose over 6 days, then a re-injection of my own uninjured stem cells that had been harvested and preserved, followed by constant medical care for 3 weeks to keep my blood chemistry stable and my body uninfected while those stem cells "took" and repopulated my bone marrow. An uncontrolled radiation dose to an astronaut crew far from good medical care might not do as well.
This does raise the interesting possibility of harvesting stem cells from the astronauts going on an interplanetary flight and storing them frozen (and well shielded) on the spacecraft in the event of a radiation emergency. At least they'd be completely isolated from the world and its diseases, and being in zero gravity would certainly have helped me while I was recovering from anemia. But I'm an engineer, not an oncologist or radiation specialist, and it would be interesting to hear the opinions of some professionals in those fields.
"RBE" is Radio-Biological Effectiveness, the relative amount of actual biological damage for a given amount of absorbed energy. The latter quantity is measured in grays (joules/kg) while the former is in sieverts:
RBE = sieverts / grays
The paper is saying that while they can calculate the amount of energy deposited in body tissue by solar protons, high energy protons are such an unusual radiation hazard that we don't really know how much damage that a given amount of deposited energy would cause. Most radiation exposures (intentional or accidental) are from gamma, which is much better understood. Some are from neutrons; those result from proximity to a small nuclear bomb (Hiroshima or Nagasaki) or to a nuclear criticality accident.
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http://emmrem.unh.edu/papers/general/parsons.pdf
Thanks for the link; interesting.
Some comments on terminology, since the paper doesn't fully explain it.
"Deterministic effects" refers to acute radiation sickness. "Stochastic effects" refers to increasing the probability of getting cancer some time later, possibly years. The "blood forming organs", i.e., bone marrow, are an important target of radiation because they're among the most rapidly dividing cells in the body. A typical healthy adult forms about 3.5 million red blood cells/sec and 3,500 white blood cells/sec. An acute radiation dose will kill some or all of these newly forming blood cells and result in at least temporary anemia and reduced immunity to infection, which could be what actually kills you. The infection doesn't have to be from an external source; we all carry huge numbers of bacteria (e.g. in our gut) that are normally benign or even helpful.
The classic "stochastic effect" of a sublethal radiation dose also involves the blood forming organs: an increased chance of certain forms of leukemia.
Having been through a stem cell transplant (as treatment for lymphoma, a blood cancer) where this was done deliberately (though by chemicals rather than radiation) I can tell you it's not very pleasant. I survived because I received a carefully calibrated dose over 6 days, then a re-injection of my own uninjured stem cells that had been harvested and preserved, followed by constant medical care for 3 weeks to keep my blood chemistry stable and my body uninfected while those stem cells "took" and repopulated my bone marrow. An uncontrolled radiation dose to an astronaut crew far from good medical care might not do as well.
This does raise the interesting possibility of harvesting stem cells from the astronauts going on an interplanetary flight and storing them frozen (and well shielded) on the spacecraft in the event of a radiation emergency. At least they'd be completely isolated from the world and its diseases, and being in zero gravity would certainly have helped me while I was recovering from anemia. But I'm an engineer, not an oncologist or radiation specialist, and it would be interesting to hear the opinions of some professionals in those fields.
"RBE" is Radio-Biological Effectiveness, the relative amount of actual biological damage for a given amount of absorbed energy. The latter quantity is measured in grays (joules/kg) while the former is in sieverts:
RBE = sieverts / grays
The paper is saying that while they can calculate the amount of energy deposited in body tissue by solar protons, high energy protons are such an unusual radiation hazard that we don't really know how much damage that a given amount of deposited energy would cause. Most radiation exposures (intentional or accidental) are from gamma, which is much better understood. Some are from neutrons; those result from proximity to a small nuclear bomb (Hiroshima or Nagasaki) or to a nuclear criticality accident.
Its a great event that you survived and shared the experience. And to be such a valued member of the society. Very nice explanation of the terms of the report.
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Its a great event that you survived and shared the experience. And to be such a valued member of the society. Very nice explanation of the terms of the report.
Thank you. Hematopoietic (blood forming) stem cell transplants (formerly known as "bone marrow" transplants) have become pretty routine and the risks have been minimized through long and hard-fought experience, but it's still a rather unpleasant experience. Nevertheless I have nothing but admiration for my surgeons and nurses who helped me through it.
The interesting thing is how similar my experience was to that of radiation poisoning even though I received no radiation. Apparently the effects of chemo are remarkably similar. From what I've read, my experience was roughly comparable to receiving 2 Sv of whole body radiation; enough to make you pretty sick but survivable with good medical care. You feel reasonably well through the chemo infusions, just as people who receive serious (but not overwhelming) radiation exposures do. You start feeling ill only a few days later as your blood counts begin to drop, just as radiation survivors become ill only some time later (the bigger the dose, the earlier the illness). And I had to accept that my risk of eventually developing a leukemia (unrelated to my lymphoma) is about 5% higher, just as with survivors of acute radiation sickness.
I'm not exactly sure why chemo is preferred over radiation for my condition, but there is one difference between the two: the chemo I had did not penetrate the blood-brain barrier, so my central nervous system was unaffected. Radiation would not have been as selective.