Magnetic shielding for spacecraft
by Nancy Atkinson
|The concept of magnetic shielding is not new. As Hoffman says, “The Earth has been doing it for billions of years!”|
The principal investigator for this concept is former astronaut Dr. Jeffrey Hoffman, now a professor at the Massachusetts Institute of Technology (MIT). Hoffman’s concept is one of 12 proposals that began receiving funding last October from the NASA Institute for Advanced Concepts (NIAC). Each gets $75,000 for six months of research to make initial studies and identify challenges in developing it. Projects that make it through that phase are eligible for as much as $400,000 more over two years.
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.”
|A little bit of shielding can actually make the situation worse, because the cosmic rays interact with the shielding and can create secondary charged particles, increasing the overall radiation dose.|
One of the most important conclusions to the second phase of this research will be to determine if using superconducting magnet technology is mass effective. “I have no doubt that if we build it big enough and strong enough, it will provide protection,” Hoffman said. “But if the mass of this conducting magnet system is greater than the mass just to use passive (absorbing) shielding, then why go to all that trouble?” That’s the challenge, and the reason for this study. “This is research,” Hoffman said. “I’m not partisan one way or the other; I just want to find out what’s the best way.”
Assuming Hoffman and his team can demonstrate that superconducting magnetic shielding is mass effective, the next step would be doing the actual engineering of creating a large enough—but lightweight—system, in addition to the fine-tuning of maintaining magnets at ultra-cold superconducting temperatures in space. The final step would be to integrate such a system into a Mars-bound spacecraft. None of these tasks are trivial.
The examinations of maintaining the magnetic field strength and the near-absolute zero temperatures of this system in space are already being studied in an experiment that is scheduled to be launched to the International Space Station for a three-year stay. The Alpha Magnetic Spectrometer (AMS) will be attached to the outside of the station and search for different types of cosmic rays. It will employ a superconducting magnet to measure each particle’s momentum and the sign of its charge. Peter Fisher, a physics professor also from MIT, works on the AMS experiment, and is cooperating with Hoffman on his research of superconducting magnets. A graduate student and a research scientist are also working with Hoffman.
NIAC was created in 1998 to solicit revolutionary concepts from people and organizations outside the space agency that could advance NASA's missions. The winning concepts are chosen because they “push the limits of known science and technology,” and “show relevance to the NASA mission,” according to NASA. These concepts are expected to take at least a decade to develop.
|“When it comes to doing things in space, if you’re an astronaut, you go and do it with your own hands,” Hoffman said. “But you don’t fly in space forever, and I still would like to make a contribution.”|
Hoffman flew in space five times and became the first astronaut to log more than 1,000 hours on the space shuttle. On his fourth space flight, in 1993, Hoffman participated in the first Hubble Space Telescope servicing mission, an ambitious and historic mission that corrected the spherical aberration problem in the telescope’s primary mirror. Hoffman left the astronaut program in 1997 to become NASA’s European representative at the US Embassy in Paris, and then went to MIT in 2001.
Hoffman knows that to make a space mission possible, there’s a lot of idea development and hard engineering which precedes it. “When it comes to doing things in space, if you’re an astronaut, you go and do it with your own hands,” Hoffman said. “But you don’t fly in space forever, and I still would like to make a contribution.” Does he see his current research as important as fixing the Hubble Space Telescope? “Well, not in the immediate sense,” he said. “But on the other hand, if we ever are going to have a human presence throughout the solar system we need to be able to live and work in regions where the charged particle environment is pretty severe. If we can’t find a way to protect ourselves from that, it will be a very limiting factor for the future of human exploration.”