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)."