Can anything be done about the radiation hazard to astronauts?
The cover story for the June, 2006, issue of Discover magazine says "Are We Trapped On Earth? Why Cosmic Rays Could Prevent Us From Leaving."
There are severe radiation hazards in space that would be damaging or lethal to unprotected astronauts. These hazards exist due to the curse that God put on nature in response to the sin of Adam and Eve. Three types of radiation pose the bulk of the threat. These are the Van Allen radiation belts, Solar Particle Events and Galactic Cosmic Rays.
The Van Allen belts are regions of highly energetic protons and electrons which come from the sun and get trapped inside earth's magnetic field. But astronauts on the International Space Station and in earth orbit travel beneath them. The Apollo astronauts were sent on trajectories through low-density parts of the belts and moved quickly through them. So the Van Allen belts can essentially be avoided.
Solar Particle Events, or SPEs, are caused by eruptions on the sun, either solar flares or Coronal Mass Ejections (CMEs). They consist of mostly protons plus electrons and atomic nuclei, and, at least in the case of CMEs, they travel far into interplanetary space.
Galactic Cosmic Rays, or GCRs, are primarily high-energy protons and atomic nuclei. They come from the galaxy outside our solar system, and possibly some of them come from distant galaxies. Possibly some come from supernova explosions.
Earth's magnetic field and atmosphere protect us from SPEs and GCRs. But the earth's magnetic field does not extend out to the moon. Mars has no magnetic field. Astronauts on the ISS and in other earth orbiting spacecraft are also protected by the magnetic field, but they experience more exposure to SPEs and GCRs than people on the ground.
The solar events are immediately dangerous, threatening the life of any exposed astronauts in the short term. Astronauts on the Apollo missions to the moon had no protection from SPEs. All the missions occurred while there were no solar events by sheer luck. But NASA has solutions to the SPE problem that can be implemented on future trips. The spacefarers could be shielded by two to four inches of polyethylene in an aluminum shell, according to the Discover magazine article. Other possible approaches include having the astronauts surrounded with their own supplies.
The real showstopper for the time being is the galactic cosmic rays. Buzz Aldrin noticed "fireworks" when his eyes were closed during his trip to the moon. These were retinal flashes caused by GCRs slicing through his brain. These rays do not kill in the short term. But over weeks, they can cause brain damage and life-threatening cancer. On the surface of Mars, the problem would be compounded by rays striking the ground and generating secondary radiation.
Regular shielding that we would think of, such as the lead plates used to keep x-rays out of non-targeted body parts in medical examinations, would not only not work, but would make the problem worse. Some of the incoming cosmic rays would collide with the atoms in the shielding and generate showers of secondary radiation. The astronauts inside the craft or shelter would get both cosmic rays and secondary radiation.
Various solutions to the GCR problem are being considered. One is to generate an artificial magnetic field around the vehicle or shelter. But Francis Cucinotta, chief scientist of NASA's space radiation program, calls this "pie in the sky" and says it will not stop all cosmic rays. The solutions under review address the proton radiation, but do not all deal with the heavy ion component of GCRs.
As to stopping nuclear particles (protons and neutrons), a number of suggestions have been made. One general idea is that it is best to have shielding whose atomic components are about the same size as the incoming particles. Hydrogen, consisting of one proton and one electron, is thought to be a good bet for protecting against proton radiation. So, materials rich in hydrogen are proposed. These include water and plastics. Nanotech solutions may be helpful here, also. For instance, NASA is experimenting with one artificial material that may prove useful. In a NASA online article about shielding Mars astronauts, it says:
"Hydrogenated boron nitride nanotubes - known as hydrogenated BNNTs - are tiny nanotubes made of boron, carbon and nitrogen, with hydrogen interspersed throughout the empty spaces left in between the tubes. Boron is also an excellent absorber of secondary neutrons, making hydrogenated BNNTS an ideal shielding material.
'This material is really strong - even at high heat, meaning that it's great for structure,' said Thibeault.
"Remarkably, researchers have successfully made yarn out of BNNTs, so it's flexible enough to be woven into the fabric of spacesuits, providing astronauts with significant radiation protection even while they're performing spacewalks in transit or out on the harsh Martian surface. Though hydrogenated BNNTs are still in development and testing, they have the potential to be one of our key structural and shielding materials in spacecraft, habitats, vehicles and spacesuits that will be used on Mars."
But not everyone seems to seriously propose that we can deal with the heavy ion component of galactic cosmic rays. Though this is a minority part of the total GCR problem, it is a very damaging one to biological systems. Heavy ions are called HZE particles: (H) High, (Z) Atomic number and (E) Energy. These are atomic nuclei that have had their electrons stripped off. These include nuclei of carbon, oxygen, magnesium, silicon and iron.
The HZE problem means that we can only do short trips to the moon. Mars is out and so is the rest of the solar system.
I have two ideas about this, one of which is assuredly undoable and the other is only doable if we really get serious about human exploration of the solar system.
The first idea is to have spacecraft travel at extremely high speeds - say millions or tens of millions of miles per hour. This would be to shorten the lengths of missions to other worlds to a few weeks apiece, at the outside. Besides the propulsion challenges, this idea is impractical due to the shielding concerns. The craft would have to be protected from collisions with interplanetary debris at such high speeds. If nothing else could be done about GCRs, then, maybe, theoretically, something could be done about the debris. But this sounds highly unlikely. Nevertheless, I bring this up to make a related point. If we wanted to have spacecraft accelerate at high Gs - say ten Gs - there would be a way to make this survivable for the astronauts on board. Scientists have already levitated a frog using a magnetic field. The frog, while inside earth's one G gravitional field, experienced zero G. Since this has been done, then it should be possible to suspend astronauts in zero G while being accelerated/decelerated by a muiltiple G force.
The second idea calls for spending enough money on human space exploration to allow for heavy shielding to block out the cosmic ray heavy ions. How many feet of dirt, water, ice or other material is necessary to protect the crew compartment from GCRs? Whatever it takes, spend the money to do it. See Trillions.
There are proposals about to provide massive shielding, but due to the expense, these tend to get discounted.
On one blog, it has been suggested that fifteen feet of ice would take care of the SPEs and the GCRs. In the Discover article mentioned above, it says to "surround the entire crew quarters with five feet of water in a tank - daunting to engineers because of the expense of getting that weight, or mass, of water off Earth." I don't know if they expect five feet of water to take care of all the heavy ions. Soil-based shielding is discussed in some places, such as one indicating six feet of earth. (I've lost that article.) A blog suggests two-and-half meters of lunar regolith to protect astronauts on the moon. It has been proposed to have a Mars Ice Dome to protect visitors to the Red Planet. It has also been proposed to use near-earth asteroids as the bodies of interplanetary craft An article on Engadget.com, which I link to in Lunar Tourism? Solar System Tourism?, says that "meters" of shielding would be necessary to protect astronauts from GCRs.
Even with heavy shielding to protect astronauts on the voyage out and back home, and to protect their living quarters on the surface of another world, there is the problem of GCRs affecting astronauts while at work "in the field" - on the surface of the planet, not in a sheltered base. It seems that the total outside exposure of the space workers will need to be limited to, say, a few weeks, at most. Unless a nanotech solution for spacesuits can be found, it looks like future voyagers to other worlds in the solar system may spend months in transit but only days or weeks on the target world itself. There are no doubt space cowboys and cowgirls that would be willing to go on such missions.
But, again, heavy shielding means heavy expense. Is it worth it? Check out the article pointed to by the "Trillions" link, above.
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