Author Topic: Radiation mitigation strategies for early SpaceX Mars missions  (Read 40645 times)

Online Robotbeat

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...and before anyone says "but magnetosphere," remember that the magnetosphere isn't NEARLY as important for shielding from radiation as the atmosphere is, and the magnetosphere doesn't even shield the REALLY high energy GCRs beyond the geomagnetic cutoff...
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline scienceguy

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I just wanted to point out, that in a paper by Sanin et. al. (2015), it says that Galactic Cosmic Rays hitting Mars’ surface have an energy of greater than 100 MeV (p. 117), and that these are all stopped by the atmosphere and turned into energetic neutrons.

The actual wording in the paper is as follows:

Besides the pulsing DAN PNG, there are two other permanent sources of neutrons in the vicinity of the rover: the secondary emission of the soil due to irradiation by MSL MMRTG and the secondary emission of the soil due to bombardment by energetic particles of Galactic Cosmic rays (GCRs). The GCR flux varies in time due to solar activity. These variations are generally slow and smooth with 11-year period or fast during Solar Particle Events with time scales from minutes to days. The GCR particles are primarily protons with energies > 100 MeV. They generate neutrons in the Martian atmosphere and regolith by spallation reactions. Most of these secondary neutrons have initially high energy (>20 MeV) and are able to produce additional charged particles and next generation of neutrons by nuclear interactions with the regolith and rover material.

Perhaps I missed it or am not reading the paper correctly, but it seems to say that GCR’s do not even make it to the surface of Mars: they are just turned into energetic neutrons which can be stopped by water or any other hydrogen rich material (or borosilicate glass).

https://en.wikipedia.org/wiki/Neutron_radiation

Reference
Sanin, A. B. et. al. (2015) Data processing of the active neutron experiment DAN for a Martian Regolith investigation. Nuclear Instruments and Methods in Physics Research A. 789: 114-127

edit: oops I realized the paper says that some GCR's reach the regolith
« Last Edit: 07/07/2017 06:16 PM by scienceguy »
e^(pi)i = -1

Online Bynaus

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I just wanted to point out, that in a paper by Sanin et. al. (2015), it says that Galactic Cosmic Rays hitting Mars’ surface have an energy of greater than 100 MeV (p. 117), and that these are all stopped by the atmosphere and turned into energetic neutrons.

The actual wording in the paper is as follows:

Besides the pulsing DAN PNG, there are two other permanent sources of neutrons in the vicinity of the rover: the secondary emission of the soil due to irradiation by MSL MMRTG and the secondary emission of the soil due to bombardment by energetic particles of Galactic Cosmic rays (GCRs). The GCR flux varies in time due to solar activity. These variations are generally slow and smooth with 11-year period or fast during Solar Particle Events with time scales from minutes to days. The GCR particles are primarily protons with energies > 100 MeV. They generate neutrons in the Martian atmosphere and regolith by spallation reactions. Most of these secondary neutrons have initially high energy (>20 MeV) and are able to produce additional charged particles and next generation of neutrons by nuclear interactions with the regolith and rover material.

Perhaps I missed it or am not reading the paper correctly, but it seems to say that GCR’s do not even make it to the surface of Mars: they are just turned into energetic neutrons which can be stopped by water or any other hydrogen rich material (or borosilicate glass).

https://en.wikipedia.org/wiki/Neutron_radiation

Reference
Sanin, A. B. et. al. (2015) Data processing of the active neutron experiment DAN for a Martian Regolith investigation. Nuclear Instruments and Methods in Physics Research A. 789: 114-127

edit: oops I realized the paper says that some GCR's reach the regolith

Yes, most of them actually. See e.g. this paper:

http://science.sciencemag.org/content/343/6169/1244797
(paywalled, but google the title)

As I mentioned above, radiation shielding is to first order the amount of matter between the radiation source and the target. On the martian surface, shielding amounts to about 20 g/cm2 (you can imagine a column with a mass of 20 g above each square centimeter - if the material is water, then the column would thus be 20 cm high, and about 13 cm if it is CO2; note that this shielding also varies with atmospheric pressure, with the height of the ground, and with solar activity). This is enough to shield some GCRs and SCRs (with energies <150 MeV/nucleon), and to produce some secondary neutrons, but the shielding is still far from the one provided by the Earths atmosphere (which corresponds to about ~1000 g/cm2 or 10 m of water).

In the paper, they find that the radiation level on the martian surface is at about 44% of the value in interplanetary space. The bulk mass of Mars provides most of the additional shielding (89% of the total reduction), with the rest being due to the martian atmosphere.

Offline LMT

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I just wanted to point out, that in a paper by Sanin et. al. (2015), it says that Galactic Cosmic Rays hitting Mars’ surface have an energy of greater than 100 MeV (p. 117), and that these are all stopped by the atmosphere and turned into energetic neutrons.

The actual wording in the paper is as follows:

Besides the pulsing DAN PNG, there are two other permanent sources of neutrons in the vicinity of the rover: the secondary emission of the soil due to irradiation by MSL MMRTG and the secondary emission of the soil due to bombardment by energetic particles of Galactic Cosmic rays (GCRs). The GCR flux varies in time due to solar activity. These variations are generally slow and smooth with 11-year period or fast during Solar Particle Events with time scales from minutes to days. The GCR particles are primarily protons with energies > 100 MeV. They generate neutrons in the Martian atmosphere and regolith by spallation reactions. Most of these secondary neutrons have initially high energy (>20 MeV) and are able to produce additional charged particles and next generation of neutrons by nuclear interactions with the regolith and rover material.

Perhaps I missed it or am not reading the paper correctly, but it seems to say that GCR’s do not even make it to the surface of Mars: they are just turned into energetic neutrons which can be stopped by water or any other hydrogen rich material (or borosilicate glass).

https://en.wikipedia.org/wiki/Neutron_radiation

Reference
Sanin, A. B. et. al. (2015) Data processing of the active neutron experiment DAN for a Martian Regolith investigation. Nuclear Instruments and Methods in Physics Research A. 789: 114-127

edit: oops I realized the paper says that some GCR's reach the regolith

Yes, most of them actually. See e.g. this paper:

http://science.sciencemag.org/content/343/6169/1244797
(paywalled, but google the title)

As I mentioned above, radiation shielding is to first order the amount of matter between the radiation source and the target. On the martian surface, shielding amounts to about 20 g/cm2 (you can imagine a column with a mass of 20 g above each square centimeter - if the material is water, then the column would thus be 20 cm high, and about 13 cm if it is CO2; note that this shielding also varies with atmospheric pressure, with the height of the ground, and with solar activity). This is enough to shield some GCRs and SCRs (with energies <150 MeV/nucleon), and to produce some secondary neutrons, but the shielding is still far from the one provided by the Earths atmosphere (which corresponds to about ~1000 g/cm2 or 10 m of water).

In the paper, they find that the radiation level on the martian surface is at about 44% of the value in interplanetary space. The bulk mass of Mars provides most of the additional shielding (89% of the total reduction), with the rest being due to the martian atmosphere.

The cosmic ray spectrum is certainly a key to shielding.  For a modeled spectrum of cosmic rays at the martian surface, see for example:

Wilson, J. W., Kim, M. Y., Clowdsley, M. S., Heinbockel, J. H., Tripathi, R. K., Singleterry, R. C., ... & Suggs, R. (1999, January). Mars surface ionizing radiation environment: Need for validation. In Workshop on MARS 2001: Integrated Science in Preparation for Sample Return and Human Exploration (p. 112).

That GCR proton flux peaks around 70 MeV, and the SPE proton flux peaks around 100 MeV.  Fortunately, magnetostatic shielding can deflect at higher energies.  Our simple artificial geomagnetic field design deflects 500 MeV protons well, and 1 GeV protons partially.  It can't block all cosmic rays, but it can block solar storms and even solar flares, apparently.

Quote
The Urbanization of Mars

HP Mars Home Planet Urbanization Concept Challenge

Artificial Geomagnetic Field to Protect a Crewed Mars Facility from Cosmic Rays









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