How do astronauts survive travel through the potentially lethal Van Allen Radiation Belts? 

The "conspiracy theory" programs that pop up on TV every now and again always seem to draw a hefty audience. Let's face it, negative news generates higher ratings than does positive news. Some people shrug off such programs, but surprisingly many believe that just because it is on the "tube," there must be something to it. For example, a recent show aired on the Fox Network debated whether or not NASA astronauts actually made it to the moon. Some participants argued that they did not, that our government faked the moon-landing pictures and hoodwinked the public. As evidence, they claimed that it would be impossible to protect astronauts traveling through the Van Allen Radiation Belts, and therefore that radiation would surely have killed them. Let's take a closer look.

The Van Allen Radiation Belts lie within the Earth's magnetosphere, the name given to Earth's magnetic environment in space. A popular way of representing the structure of this magnetized region ("magnetic field") is by "magnetic field lines" (or "lines of force"), imaginary lines which at every point give the direction of the magnetic force. It is a familiar pattern, also produced by iron filings on a sheet of paper placed above a bar magnet: the lines emerge from near the southern magnetic pole, bend away near the equator and converge again near the north magnetic pole.

The lines are imaginary, an aid to mapping (like lines of longitude and latitude on a globe), but in the magnetosphere they have an extra role, describing the way charged particles move and are trapped. Space away from Earth is filled with such particles: negative electrons knocked off atoms by sunlight or collisions, and "positive ions" which are atoms that have lost one or more electrons. Some are quite fast, able to penetrate material and damage tissue. Similar particles are emitted by radioactive substances and are commonly named "radiation," so when energetic particles were discovered to be magnetically trapped around Earth, they were named "radiation belts." They spiral around field lines and are attached to them, like beads to a wire, though they can slowly jump from line to line and work their way around the Earth. It is the magnetic field which holds them near Earth--not gravity, which is much too weak.

Are they dangerous? Yes, you would not want to stay within them too long. Are they an unknown hazard? No, they have been studied from satellites for decades. To mention just one study, the CRRES (Combined Radiation and Release Explorer Satellite) of NASA and the USAF in 1990-1 was designed to map them--and to release some artificial ions, for studying their motion.

The lowest, densest and maybe most dangerous is the inner belt, consisting of high-energy protons, a byproduct of the drizzle of celestial ions known as the cosmic radiation. It was this belt that was discovered in 1958 by the Iowa University team, led by James Van Allen and using the first US satellites Explorer 1 and 3. Beyond this lies the "outer belt," lower in energy but with some "killer electrons" as well. The Sun itself lets loose now and then, unpredictably, blasts of energetic particles; for astronauts on their way to Mars, those would pose the greatest danger.

Scientific spacecraft which spend years in the magnetosphere may accumulate damage, though some last surprisingly long. The space station's orbit is below the belts, and is protected from solar particles by the Earth's own magnetic field. The Apollo astronauts, on the other hand, avoided risk by whizzing through the belt at maximum speed.

David Stern, a physicist at Goddard Space Flight Center, has assembled web sites providing extensive information about the magnetosphere, space, the sun, spaceflight and much more (see below). He also answers questions, and some with their replies are on those sites as well. The airing of "We Never Landed on the Moon" produced quite a few inquiries.
As Dr. Stern states, the Van Allen Radiation Belt is a tough neighborhood, and like most such neighborhoods, if you hang around too long, you're bound to get hurt. But knowing what to expect, you can get by quite well if you go through it as quickly as possible.

Our old atmosphere does a pretty good job of shielding us from anything the sun emits, and it stops most cosmic radiation (whose intensity is low anyway). It's like having a 10 foot thick concrete shield surrounding the Earth. However, solar eruptions are a special concern during spaceflight, as interplanetary space can be flooded with energetic particles.

The chances that a solar eruption will occur are higher during the peak (solar maximum) of the 11-year solar sunspot cycle, which is where we are now in the current solar cycle. Within the last few days, the Sun has been particularly active. In fact, one of the largest sunspots and one of the largest solar flares ever seen were detected by astronomers observing the Sun. Like most such eruptions, they've produced vivid auroras and interfered with communications. Though they've never been fatal to humans in space, solar eruptions are not taken lightly. If a solar storm were in progress, the launch of a manned mission would be postponed. Even if we can protect astronauts from most all of the energetic particles, it makes no sense to put them at an increased risk by launching during a solar eruption. By the way, the active sun also shortens the orbital lifetimes of low flying satellites: it emits x-rays which heat the upper atmosphere, making it swell upwards to where satellites orbit. The space station "Skylab," a quarter century ago, re-entered ahead of schedule because of the Sun's activity.

Dr. Stern calculated that the solar cells of a satellite passing through the inner Van Allen belt, shielded by only 1 mm of glass, would receive about 25 rad of radiation per pass. Particles in the outer belt are less penetrating. Anything over about 200 rad is dangerous to humans and about 500 rad can be lethal. Fortunately, it doesn't take much shielding to deter the particles, and the shielding doesn't need to be constructed of rare or exotic metals.

The bottom line is that even though radiation from the Sun can be lethal to humans in space, it has not prevented us from going to the moon or anywhere else.

One of Dr. Stern's websites is "The Great Magnet Earth," http://www-spof.gsfc.nasa.gov/earthmag/demagint.htm