Cosmic-ray experiment will circle the South Pole
Dec. 17, 1999: If it rains, you put up an umbrella. If you expect a meteor shower, you build the spacecraft with shielding that blunts the blow when particles arrive. But what do you do for radiation?
After almost four decades of human spaceflight, we still are grappling with the challenge of protecting space crews from cosmic rays and other radiation hazards. Part of the problem is understanding fully the composition and energies of cosmic rays.
In the next week or two, two small instruments will piggyback on a ride around the South Pole so they can help scientists develop a better understanding of cosmic radiation that might endanger astronauts on deep space missions. "They're going to record the dose from different cosmic ray energies and particles," said Dr. Leonard Howell, a member of the cosmic ray group at NASA's Marshall Space Flight Center. "The dosimeters will be mounted on a balloon gondola for a 10-day exposure to cosmic rays."
NASA has recently developed the capability to fly high-altitude research balloons on paths that either circle the North or South Pole for about 10 days exposure. These balloon-borne platforms provide frequent and relatively low-cost opportunities to perform experiments utilizing the full cosmic ray spectrum.
The balloon, carrying several radiation-related experiments, will be launched from the U.S. research station at McMurdo, Antarctica, because the polar regions are where the Earth's atmosphere is directly exposed to the space environment, and thus to cosmic rays. At 125,000 ft. altitude (38 km), the balloon will be above most of the Earth's atmosphere.
Despite a similar name, cosmic rays are not electromagnetic radiation like X-rays and gamma rays. Cosmic rays are actually atoms stripped of their electrons and accelerated by supernova explosions and other violent events in the universe.
By far the most dangerous components of cosmic rays are called HZE, meaning high mass (HZ) and high energy (HE). These are fast atomic nuclei zipping along close to the speed of light. If they run into denser matter - like the wall of a spaceship or the flesh of a human body - the particle will shatter itself and its target, and create a cascade of secondary particles. It's like a musket ball plowing into a wall, shattering, and leaving a trail of debris. Secondary particles also present a hazard.
Because the particles are electrically charged, all but the most energetic are deflected by the Earth's magnetic field, and what does get through is absorbed by the upper atmosphere. But around the magnetic poles, the magnetic field lines are vertical, and the full spectrum of particles can barrel straight in from space.
Howell explained that the experiments being launched this month are a modest start for a larger effort to acquire the knowledge that will let scientists devise better shielding and strategies to protect Mars crews.
The experiment comprises two packages, each housing three identical sets of dosimeters - film emulsions, plastic sheets, and thermoluminescent detectors - each covering about 2 square inches. The film emulsions, made at NASA/Marshall, will be developed after the flight to reveal cosmic ray tracks. The plastic sheets are CR-39 Plexiglas which will be chemically etched to reveal tiny holes bored by cosmic rays. The thermoluminescent detectors (TLDs) will record the total does. When heated, TLDs radiate visible light that is proportional to the absorbed dose of ionizing radiation. Thermistors in the packages will keep a record of the temperature ranges during the flight. (A fourth set will stay on the ground as a control unit.)
"The three dosimeters in each tube are positioned behind different amounts of shielding in order to understand more fully the effects of the shielding," Christl said. "Two different materials were chosen as shielding at this stage of the investigation, aluminum and polyethylene."
The housings for the dosimeters is decidedly low-tech, as balloon experiments often are: a 4-inch-diameter length of PVC tubing from a hardware store, with end caps screwed in place and made air tight by Teflon tape. One end of each tube has a bicycle-like valve sticking out so the tube can be pressurized to 1 atmosphere just before flight.
Then the tubes are placed inside shielding sleeves - one made of aluminum, to stand in for the spacecraft wall, and the other made of polyethylene, a type of plastic - and mounted on the sides of the balloon gondola.
"These are two shielding material types that might be used in a space mission," said Dr. Mark Christl, a cosmic-ray scientist at NASA/Marshall. "The main purpose of this experiment is to characterize the environment and get some data for comparison with computer simulations."
Aluminum was selected because its properties are well known, "but we don't want to give the impression that we have chosen it over anything else" as a spacecraft structure. The aluminum shielding on this experiment is 7.6 cm (3 inches) thick.
"A major constraint in the design of space transit vehicles and surface habitats for a manned Mars mission," said Dr. Tom Parnell, the former director of high-energy astrophysics at NASA/Marshall, "is the thickness of material required to protect flight crews from the hazards of radiation resulting from the galactic cosmic rays (GCR) and solar flare particles." Parnell explained that studies are underway to define the best shielding approaches and the accuracy with which the hazards can be predicted. The fastest approach to test the calculations and the effectiveness of shielding is to use the cosmic ray flux itself.
"Some experiments have already been carried out on spacecraft," Parnell continued. "However, the full cosmic ray spectrum is only available near the Earth's magnetic poles or in deep space because of the shielding effects of the Earth's magnetic field."
The launch date for the experiment will depend on weather conditions in the Antarctic. The dosimeter packages should be returned to NASA/Marshall sometime in January 2000 for analysis.
The project is supported by NASA's Human Exploration and Development of Space (HEDS) enterprise. Balloon launches are managed by the Wallops Flight Facility of NASA's Goddard Space Flight Center.