Galactic Cosmic Rays (GCR) – The 800 Pound Gorilla

The Augustine Commission identified Galactic Cosmic Radiation, or Galactic Cosmic Rays (GCR), as one of the high priority technical challenges facing the Deep Space program.

Synopsis

The GCR problem arises from interstellar atomic nuclei traveling near the speed of light striking the structure of a spacecraft. The resulting shower of secondary particles cause radiation damage. The Earth is protected by the Van Allen belts and a deep atmosphere. Brief journeys such as an Apollo mission does not expose the astronaut to dangerous dosages. However, astronauts on such a journey are at risk from Solar flares (Solar Particle Events – SPE). SPEs can be mitigated with layers of hydrogen rich materials such as polyethylene or water. GCRs, however, require spaceships on long journeys of more than 100 days, or habitats on the Lunar or Martian surface, to be surrounded by tens of meters of water for passive protection, or magnetic shields for active protection. Either solution is extremely heavy and makes space flight prohibitive in terms of propellant requirements.

The following sections discuss each aspect and provide references for further reading about the problem

The Source of GCR

Galactic Cosmic Rays come from outside our Solar System, but from within our galaxy, the Milky Way. They are comprised of atomic nuclei that have been stripped of their electrons. These nuclei can be any element. Common elements are carbon, oxygen, magnesium, silicon, and iron with similar abundances as the Solar System. Lithium, Berylium and Boron are overabundant relative to the Solar System ratios.

The Shielding Problem

Early on, it was suggested that cosmic rays could penetrate the Apollo spacecraft. From “Biomedical Results of Apollo” section IV, chapter 2, Apollo Light Flash Investigations we have the following account:

Crewmembers of the Apollo 11 mission were the first astronauts to describe an unusual visual phenomenon associated with space flight. During transearth coast, both the Commander and the Lunar Module Pilot reported seeing faint spots or flashes of light when the cabin was dark and they had become dark-adapted. It is believed that these light flashes result from high energy, heavy cosmic rays penetrating the Command Module structure and the crew members’ eyes. These particles are thought to be capable of producing, visual sensations through interaction with the retina, either by direct deposition of ionization energy in the retina or through creation of visible light via the Cerenkov effect.

When Galactic Cosmic Rays collide with another atom, such as those contained in the Aluminum, Stainless Steel or Titanium structures of a spacecraft, they can create a shower of secondary particles, These secondary particles cause radiation damage in living organisms (humans).

The problem is creating sufficiently powerful barriers to these extremely energetic nuclei.

Researching Solutions

  • Passive Shielding – At least for solar flares (SPE), some solutions are easier than the GCR problem.
  • Active Shielding
  • Fast Passage to avoid exposure (VASIMR propelled craft). A proposal for vapor core reactors integrated with VASIMR engines.
  • A proposal for studying radiation and other factors associated with long term human occupation of space.
  • NASA’s Space Radiation Program in association with the Brookhaven National Laboratories.
  • In 2008, the National Academies of Science published Managing Space Radiation Risk in the New Era of Space Exploration, which included chapter 6: Findings and Recommendations
  • From the Summary in Radiation Shielding Simulation For Interplanetary Manned Missions
      Inflatable Habitat + shielding

    • Hadronic interactions are significant, systematics is under control
    • The shielding capabilities of an inflatable habitat are comparable to a conventional rigid structure – Water / polyethylene are equivalent
    • Shielding thickness optimisation involves complex physics effects
    • An additional shielding layer, enclosing a special shelter zone, is effective against SPE
      Moon Habitat

    • Regolith shielding limits GCR and SPE exposure effectively
    • Its shielding capabilities against GCR can be better than conventional Al structures as in the ISS

See also the recent article in New Scientist about radiation hazards. A tip of the hat to ParabolicArc.

5 thoughts on “Galactic Cosmic Rays (GCR) – The 800 Pound Gorilla

  1. I believe mass drivers and space manufactured light sails are the long term solutions to the problem of solar and galactic radiation. Lunar mass drivers would be able to cheaply deliver hundreds and even thousands of tonnes of mass shielding for orbiting space stations and interplanetary vehicles in the form of regolith or water (lunar oxygen plus hydrogen from Earth or water from the lunar poles). Much simpler and cheaper regolith exporting mass drivers could be placed on the surface of the moons of Mars. Maybe even polyethylene could be manufactured on the Martian moons or at the lunar south pole if there are sources of carbon and hydrogen. Polyethylene would have only 80% of the mass of a water shield for the same amount of radiation protection.

    But a properly shielded manned interplanetary vehicle is still going to require several hundred tonnes of shielding which means that we’re going to need a space craft that can transport that kind of mass through interplanetary space. Space manufactured light sails would appear to be the best solution to this problem. A 20 kilometer in diameter light sail capable of transporting nearly three thousand tonnes to Mars in less than a year would weigh less than 36 tonnes if it were composed of aluminized carbon nanotube sheets rigged to a rigged support structure about twice as massive as the sail material.

    If there was a light sail manufacturing plant at L1, such a facility might be operated tele-robotically from Earth. We already know how to manufacture carbon nanotube sheets on Earth. So the question is could a solar or nuclear powered nanotube sheet factory be placed at L1?.

    • One way of shielding the crew is to build a “Bus” that orbits between Earth and Mars (looking for references). From EML-2, a crew vehicle burns and matches the Bus. The crew transfers to the Bus (several thousands of tons of shielding, thanks to your mass driver launch regolith shielding blocks) and ride the Bus to Mars. Re-board the crew vehicle and decelerate into Mars orbit (Phobos?). Return crews then burn to catch up with the Bus and ride it back to Earth and EML-2.

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