On Earth we apply sunscreen to shield our skin from ultraviolet waves. On the Space Station, the astronauts use a four-tiered approach to limiting their exposure. Under a level one alert, the radiation monitor on board the station (called a TEPC) issues an alarm, and astronauts begin monitoring the situation closely. For a 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. Under a level three alert, astronauts would find a means within the station to shield themselves. These options are described below. And finally, under a level four alert, astronauts can take one final and possibly life-saving measure. They would either deorbit the station, taking it in closer to the Earth, or they would escape in the crew return vehicle and come back to the surface.

The Shielding Challenge
In December of 2000, NASA brought three teams of people together to discuss radiation shielding. One team was asked to develop shielding to be built in to the habitation module-scheduled to be installed in 2005. Another team was asked to develop a shorter-term solution to limit exposure by finding a way to shield astronauts while they sleep. A third team was asked to find an immediate means to limit exposure in the existing sleeping station by sending up a portable shield on the very next flight in three months. There is more on this story in the article titled, "The Real Story Behind the Christmas Bricks."

In the search for effective shielding materials, these teams had to take into consideration a wide range of requirements. First there was the problem of efficiency. Effective radiation shielding had to either reflect or absorb the most dangerous radiation. Then there were issues of practicality. Too much shielding would take up too much space and be too bulky to transport to the Space Station. The shielding materials could not be too heavy. The shielding had to fit within special racks on the shuttle which transports all materials to the station. Finally, scientists had to consider the wide range of electromagnetic energies to which the astronauts were exposed. During testing it was found that different shielding materials are effective with different intensities of radiation. Finding a shielding that could be designed to form comfortable sleeping quarters and that would protect the astronauts from the specific ionizing radiation found in space was a challenge.

Materials used for shielding on Earth
Scientists, nurses, technicians, and engineers whose work exposes them to ionizing radiation can use heavy materials such as concrete and lead. These materials are used to shield people while they perform their work, such as: conducting medical treatments and experiments, working in nuclear power plants, or testing military weapons. Lead and concrete are ideal shielding materials because their molecular structures reflect or totally absorb dangerous radiation. These materials, however, are far too heavy to transport into space.

Discovering a "Down-Home" Solution
The astronauts' shielding had to be effective yet very light. Researchers have known that materials made of hydrogen-rich molecular structures make very good radiation shielding. One such substance is water. Consequently, in the initial design of the Space Station water reserves were positioned strategically around work and living spaces. In this way they serve a dual purpose on Space Station Alpha.

Another material rich in hydrogen is polyethylene. There are thousands different polyethylene substances made of a molecular structure consisting of C2H4 (two carbon atoms linked to two hydrogen atoms). The Food and Drug Administration (FDA) has approved some 800 types which are in use in everyday life.

[Illustration of a cutting block and of a "shielding brick".]

NASA engineers, not wanting to waste precious time and funding on intensive research and development with so many options for polyethylene made a practical choice. They chose to buy huge sheets of polyethylene "right off the shelf" from a manufacturer who makes kitchen cutting boards. You may have seen these boards. They are about 5/8 of an inch thick and made of a light, white, plastic-like material. Polyethylene bricks measuring 1" x 14" x 14" were created and transported to the space shuttle. These small bricks can, in turn, be strapped together by the astronauts to fit their bodies and their sleeping needs, while one side remains open to provide ventilation. Furthermore, tests proved that blocks of this thickness provided adequate shielding, if not total protection to the astronauts.

[Illustration of the astronauts sleeping in their "new shielded sleeping quarters."]