In the Kitchen with Poly
What protects us on earth from the sun's most lethal forms of electromagnetic radiation? Much of the sun’s radiation is deflected by gases in the atmosphere. Think about it. In areas where the atmosphere is thinner, like the Rocky Mountains, you are more susceptible to sunburns. There is also a protective area in portions of the Earth’s upper atmosphere called the Van Allen belts in which radioactive particles are captured and held until they lose their energy. A final degree of protection is offered by the earth’s magnetosphere which surrounds the earth thousands of miles out into space and serves to deflect much dangerous solar radiation.

The space station, on the other hand, is much more vulnerable. For instance, it orbits above our atmosphere’s protective gases. Also, instead of being protected by the Van Allen belts, the space station may pass through them at times and be exposed to the captured radioactive particles. Fortunately, the space station’s orbit still places it within the magnetosphere, for some degree of protection.

The astronauts and the station’s sensitive electronic equipment are susceptible to the effects of the most violent solar storms called solar proton events. The space station's aluminum skin deflects or absorbs various low-energy forms of radiation such as infrared and ultraviolet rays, but high-energy radiation such as X-rays and gamma rays and radioactive solar particles can penetrate the station’s thin outer layers.

On earth we can use sunscreen, clothing, and the shade of trees and umbrellas to shield ourselves from ultraviolet waves. On the space station, the astronauts protect themselves from dangerous doses of radiation in a number of ways. NASA uses a four-tiered approach:

    • Level One Alert: The radiation monitor on board the station (called a TEPC) issues an alarm, and astronauts begin monitoring the situation closely.
    • Level Two Alert: Mission Control asks the astronauts to change the "attitude" or orientation of the station. The station is placed in a position where the bulk of its mass is placed to face the sun and the astronauts move to the rear, using the station itself as a shield.
    • Level Three Alert: Astronauts find a means within the station to shield themselves. These options are described below.
    • Level Four Alert: Astronauts must consider two extreme options. They must either change the orbit of the station, moving it closer to the earth, or escape and return to earth in the crew’s return vehicle.

Meeting the Shielding Challenge
In December 2000, NASA assembled three teams of engineers to discuss using radiation shielding for crew sleeping areas. That way, for at least a few hours a day, the crew would be at least partially shielded. This would make it possible for them to remain on the station for longer periods of time before exceeding their career radiation limits. One team, the long-range planning team, was asked to develop shielding for the habitation module scheduled to be installed in 2005. The second team was asked to develop a mid-term solution. The third team, the short-term team, was given three months to find a way to limit the exposure of the next crew scheduled for space.

Each team had to meet very specific requirements, but the third team had to present its proposed solution in a matter of days. First, there was the issue of shielding efficiency. Effective shielding must either reflect or absorb the most dangerous forms of radiation, X-rays, gamma rays, and high-energy protons. At first, the scientists considered the wide range of electromagnetic energies to which the astronauts would be exposed. They knew that different shielding materials are effective at blocking different forms of electromagnetic radiation. Designing a form of shielding that would form comfortable sleeping quarters and still protect the astronauts from the specific ionizing radiation found in space was a real challenge.

Then, there was the challenge of size and weight. Bulky shielding could not be transported to the space station, nor would it fit in the close quarters on board. In addition, the shielding had to fit within the special materials-transport racks on the shuttle. If the shielding materials were too heavy or large, they would limit the shipment of other important items needed to resupply the astronauts during a shuttle mission.

Earth-bound Shielding Materials
Scientists, nurses, technicians, and engineers frequently work in environments in which they are exposed to ionizing radiation. Heavy materials such as concrete and lead are used to shield them while they conduct medical treatments and experiments, or work in nuclear power plants, or test military weapons. Lead and concrete make ideal shielding materials because their molecular structures reflect or totally absorb dangerous radiation. Unfortunately, these materials are far too heavy to transport into space.

Cooking Up a Quick Solution
The astronauts' shielding had to be effective, yet very light. The NASA scientists knew that materials made of hydrogen-rich molecular structures also make very good radiation shielding. One such substance is water; and during the initial design of the space station, pliable water containers, like big square-ish duffle bags, were strategically placed around the walls of the station’s work and living spaces. These containers can be unfastened, moved, and reattached with straps and Velcro into a variety of convenient positions within the space station’s various modules.

Another material rich in hydrogen is polyethylene (don’t confuse this with "polyurethane" which is a very hard shellac type-material). There are thousands of different polyethylene substances. Basic polyethylene consists of molecules that have two carbon atoms linked to four hydrogen atoms. The chemical symbol for this molecule is C2H4. The Food and Drug Administration (FDA) has approved almost 800 types of polyethylene for our everyday use.

The third-team of NASA engineers, not having any time or money to waste, could not afford to test all the available forms of polyethylene. They had to make a choice based upon common sense. They picked simple kitchen cutting boards and purchased several huge, pre-cutting-board sheets of "poly" from a local manufacturer. These massive sheets were about an inch thick (2.5 cm), thin enough, if cut down, to fit into the shuttle’s racks. From these sheets they cut squares. They then glued two squares together to form a "brick" with lap joints. Each brick measured 2" x 14" x 14" (5 x 35.5 x 35.5 cm). The bricks could interlock and straps were attached. They reasoned that the astronauts could select the number of bricks that would fit their bodies and their sleeping needs, strap them together, and still leave one side of their polyethylene cocoon open for ventilation.

When the third team presented its solution, just days after they received the assignment, the members of all the teams agreed that this was the best possible solution for each of their assignments. The "winning" team then proceeded with a testing phase and found that these "poly" blocks, christened the "Christmas Bricks," because the idea for the bricks was hatched over Christmas, would indeed provide adequate shielding, if not total protection to the astronauts during dangerous periods of solar radiation.