Super Spaceships

Scientists are still searching for a good solution. The trick is to provide adequate shielding without adding lots of extra weight to the spacecraft. Some lightweight radiation-shielding materials are currently being tested in an experiment called MISSE onboard the International Space Station. But these alone won't be enough.

The real bad guy is Galactic Cosmic Radiation (GCR) produced in distant supernova explosions. It consists, in part, of very heavy positive ions - such as iron nuclei - zipping along at great speed. The combination of high mass and high speed makes these little atomic "cannon balls" very destructive. When they pierce through the cells in people's bodies, they can smash apart DNA, leading to illness and even cancer.

"It turns out that the worst materials you can use for shielding against GCR are metals," Bushnell notes. When a galactic comic ray hits a metallic atom, it can shatter the atom's nucleus - a process akin to the fission that occurs in nuclear power plants. The secondary radiation produced by these collisions can be worse than the GCR that the metal was meant to shield.

Ironically, light elements like hydrogen and helium are the best defence against these GCR brutes, because collisions with them produce little secondary radiation. Some people have suggested surrounding the living quarters of the ship with a tank of liquid hydrogen. According to Bushnell, a layer of liquid hydrogen 50 to 100 cm thick would provide adequate shielding. But the tank and the cryogenic system is likely to be heavy and awkward.

A solar flare blasts energetic radiation into space

Here again, nanotubes might be useful. A lattice of carbon nanotubes can store hydrogen at high densities, and without the need for extreme cold. So if our spacecraft of the future already uses nanotubes as an ultra-lightweight structural material, could those tubes also be loaded up with hydrogen to serve as radiation shielding? Scientists are looking into the possibility.

Going one step further, layers of this structural material could be laced with atoms of other elements that are good at filtering out other forms of radiation: boron and lithium to handle the neutrons, and aluminium to sop up electrons, for example.

Earth's surface is mostly safe from cosmic radiation, but other planets are not so lucky. Mars, for example, doesn't have a strong global magnetic field to deflect radiation particles, and its atmospheric blanket is 140 times thinner than Earth's. These two differences make the radiation dose on the Martian surface about one-third as intense as in unprotected open space. Future Mars explorers will need radiation shielding.