Digging in and taking cover
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Lunar and Martian dirt could provide radiation shielding
for crews on future missions.
The challenge of finding out is being added to the role of materials scientists who previously had been using the space environment as a tool to explore the nature of matter. Last week, at the 1998 Microgravity Materials Sciences Conference in Huntsville, a small group of scientists discussed how to use materials in space to make space exploration safer.
The problem is not new. As early as 1952, Dr. Wernher von Braun and other space visionaries suggested using lunar soil to protect a manned expedition from space radiation and meteors.
But how much is enough? And what do you use for protection on the way out and back?
"The problem is that we need to figure out what needs to be improved," said Dr. Jim Adams of the Naval Research Laboratory in Washington. Although space radiation has been measured extensively since the 1950s, its intensity changes, and our knowledge of how it reacts with materials still lags in many areas.
Astronauts exploring Mars may dig soil for self-protection as well as for scientific research. (links to 598x405, 139KB JPG of painting by Paul Hudson.)
Adams and several other scientists discussed directions research
takes. Current materials work in this field, in support of NASA's
Human Exploration and Development of Space (HEDS) initiative,
was started under an NRA released in December 1996.
One of the problems in radiation shielding is that a little can be worse than none. Radiation actually comprises electromagnetic radiation - X-rays and gamma rays - and particulate radiation - high-speed particles like electrons, protons, neutrons, and atomic nuclei. Low-energy radiation can be stopped by a spacecraft wall, but at higher energies the wall helps produce showers of secondary radiation, like splinters from a wall hit by a bullet. So, even more shielding is needed to absorb that, until eventually the radiation is worn down.
Oddly, one of the better ways to stop radiation is with lightweight materials - hydrogen, boron, and lithium. The nuclei of heavy elements in cosmic rays can be shattered by lightweight atoms without producing additional hazardous recoil products like neutrons.
Thus, composites and other materials using low-mass atoms might provide good shielding.
Still, as Adams noted, "We need a tool that lets engineers compare radiation doses inside the spacecraft they are designing."
Learning how to manufacture materials also is lagging, said Dr. John Wilson of NASA's Langley Research Center. He soon will start radiation tests using simulated Mars soil, samples of Earth soil whose chemical makeup has been changed to match the findings of the Viking and Mars Pathfinder spacecraft. Even if it is an ideal radiation shield for crews staying on the surface, it has to be prepared.
"We need to learn a lot on mining, processing, packing, and using in situ [local] materials," he said. "There's an enormous energy penalty for using rocks [which have to be crushed], so we've turned our attention to using regolith," the soil and rubble found on the surface.
His team is looking at using polyamide binders that also could be manufactured from local materials. NASA is studying technologies to make methane that could be used as fuel for the trip back to Earth. Variations on the equipment could make polymers that would be mixed with Mars dirt and cured in sunlight to make shielding bricks or mats.
"Hopefully, maybe you can do all of this on the surface and take very little material with you," he added.
Extraterrestrial Processes and Technology Development, Section 6 in NASA Research Announcement on Microgravity Materials Science: Research and Flight Opportunities:
"The provision of shielding for a Mars mission or a Lunar base from the hazards of space radiations (solar flares, galactic cosmic radiation) is a critical technology since astronaut radiation safety depends on it and shielding safety factors to control risk uncertainty appear large. Thus, the development and evaluation of high performance radiation shield materials is a research area where materials science can play a pivotal role. Understanding the basics physics of the shielding process should allow the tailoring of materials performance through control of structure. Empirical evidence indicates that favorable characteristics include: high electron density per unit mass, minimum nuclear cross section per unit mass, and high hydrogen content. High performance shielding materials would be particularly useful for protecting crews during the protracted journey to Mars.
"A recent workshop on Shielding Strategies for Human Space Exploration made several observations and recommendations. Many past lunar missions have identified the possible use of in situ resources such as regolith or regolith-derived compounds for space radiation shielding. Possible hybrid shielding concepts require greater investigation. New combinations of materials, each possessing favorable performance related characteristics (shielding, structural, etc.) may markedly improve synergistic possibilities for reduced launch mass. Some possible candidate materials include the layering of various materials, regolith/epoxy mixtures, borated composites, and novel dual use materials (e.g. magnesium hydride as a hydrogen storage medium)."
- Spaceweather.com -- information and predictions about space radiation and other solar effects
- Microgravity.com -- information and tutorials
- Homesteading the planets with local materials (story we posted April 28)
- The Space Environmental Effects group at NASA/Marshall describe Ionizing radiation (cosmic and other rays)
- Microgravity Research Program Office at Marshall has a wealth of information and background on various microgravity projects
- Life and Microgravity programmatic information from NASA headquarters.
- In Situ Resource Utilization studies at the Space Engineering Research Center, University of Arizona