Dual Orion Capsules Studied for Manned Asteroid Missions

Magnetic shielding for spacecraft
by Nancy Atkinson
Monday, January 24, 2005

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The concept of magnetic shielding is not new. As Hoffman says, “The Earth has been doing it for billions of years!” Earth’s magnetic field deflects cosmic rays, and an added measure of protection comes from our atmosphere which absorbs any cosmic radiation that makes its way through the magnetic field. Using magnetic shielding for spacecraft was first proposed in the late 1960’s and early 1970’s, but was not actively pursued when plans for long-duration spaceflight fell by the wayside.

However, the technology for creating superconducting magnets that can generate strong fields to shield spacecraft from cosmic radiation has only recently been developed. Superconducting magnet systems are desirable because they can create intense magnetic fields with little or no electrical power input, and with proper temperatures they can maintain a stable magnetic field for long periods of time. One challenge, however, is developing a system that can create a magnetic field large enough to protect a bus-sized, habitable spacecraft. Another challenge is keeping the system at temperatures near absolute zero, which gives the materials their superconductive properties. However, recent advances in superconducting technology and materials permit superconductive properties to exist at temperatures higher than 120 kelvin (−153° C).

There are two types of radiation that need to be addressed for long-duration human spaceflight, says William S. Higgins, an engineering physicist who works on radiation safety at Fermilab, the particle accelerator near Chicago. The first are solar flare protons, which would come in bursts following a solar flare event, such as those seen last week. The second are galactic cosmic rays, which, although not as lethal as solar flares, represent a continuous background radiation to which the crew would be exposed. In an unshielded spacecraft, both types of radiation would result in significant health problems, or death, to the crew.

The easiest way to avoid radiation is to absorb it, like wearing a lead apron when you get an x-ray at the dentist. The problem is that this type of shielding can often be very heavy, and mass is at a premium for nearly every mission. Also, according to Hoffman, a little bit of shielding can actually make it worse, because the cosmic rays interact with the shielding and can create secondary charged particles, increasing the overall radiation dose.

Hoffman foresees using a hybrid system that employs both a magnetic field and passive absorption. “That’s the way the Earth does it,” Hoffman explained, “and there’s no reason we shouldn’t be able to do that in space.”

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http://www.thespacereview.com/article/308/1

Radiation only becomes dangerous when absorbed in large quantities, particularly so if those doses come over short periods of time. A prompt dose, such as would be delivered by an atomic bomb or a meltdown at a nuclear plant, can be as high as 75 rem without any apparent effects. Longer-term doses have much lower effects: according to the National Academy of Sciences National Research Council, a dose of 100 rem causes a 1.81% increase in the likelihood of cancer in the next 30 years of a person's life. Russian cosmonauts aboard Mir have taken doses as high as approximately 150 rem, with no apparent side effects to date.

There are two types of radiation which concern astronauts: solar flares and cosmic rays. Solar flares, irregular discharges of radiation from the Sun, are made up of particles with roughly 1 million volts of energy, and can be shielded effectively. Astronauts inside a spaceship during any of the last 3 large recorded solar flares would have experienced doses of 38 rem; if they were inside of the storm shelter designed into the Mars Direct habitat, the dose would have been 8 rem. On the surface of Mars, which offers much radiation protection due to its atmosphere, the unshielded dose would have been 10 rem, the shielded dose 3 rem.

Cosmic rays, which constantly bombard space with an average energy of roughly 1 billion volts, are much more difficult to shield against. However, they occur in considerably lower concentrations than the radiation from a solar flare. In fact, on a conjunction-class flight, astronauts would take an average of 31.8 rem from cosmic rays over the course of a year; on a longer opposition-class flight, they would take 47.7 rem over 1.75 years.

In total, radiation doses of 52.0 and 58.4 rem taken on conjunction- and opposition-class missions, respectively, are well below dangerous thresholds -- even were they to come all at once, instead of over the course of years.