Radiation is energy emitted in the form of waves or particles that radiates out from a source. Examples include electromagnetic radiation, cosmic rays, and streams of alpha particles or beta particles. High-energy radiation – in particular, ionizing radiation – can have destructive effects on living tissue and, for example, on electronic devices. However, it but can also be used beneficially in medicine and industry.
Radiation pressure is the minute pressure exerted on a surface at right-angles to the direction of travel of the incident electromagnetic radiation. Its existence was first predicted by James Clerk Maxwell in 1899 and demonstrated experimentally by Peter Lebedev. In quantum mechanics, radiation pressure can be interpreted as the transfer of momentum from photons as they strike a surface. Radiation pressure on dust grains in space can dominate over gravity and explains why the tail of a comet always points away from the Sun.
The Poynting-Robertson effect, also called Poynting-Robertson drag, is an effect of radiation pressure on a small particle orbiting a star that causes it to spiral slowly into the star. The radiation falls preferentially on the leading edge of the orbiting particle and acts as a drag force. For example, a dust grain one micron wide located at the position of Earth would spiral into the Sun in a period of about 3,000 years.
Biological effects of radiation
Radiation therapy is the use of ionizing radiation, in the form of rays from an outside source or from radium or other radioactive metal implants, in treatment of malignant disease – cancer, lymphoma, and leukemia. The principle is that rapidly dividing tumor cells are more sensitive to the destructive effects of radiation on DNA are are therefore damaged by doses that are relatively harmless to normal tissues. Certain types of malignancy indeed respond to radiation therapy but radiation sickness may also occur.
Radiation sickness is a combination of malaise, nausea, loss of appetite, and vomiting occurring several hours after exposure to ionizing radiation in large doses. It occurs as an industrial or war hazard, or more commonly following radiation therapy for cancer, lymphoma, or leukemia. Large doses of radiation may cause bone marrow depression with anemia, agranulocytosis and bleeding, or gastrointestinal disturbance with distension and bloody diarrhea. Skin erythema and ulceration, lung fibrosis, nephritis, and premature arteriosclerosis may follow radiation and malignancy may develop.
Radiation protection in space
High-energy radiation poses a threat to astronauts, especially on long missions beyond the protection of Earth's magnetosphere. The greatest danger comes from two types of radiation: galactic cosmic rays (GCRs) and solar particle events (SPEs). These contain charged particles (mainly protons) that are trapped by Earth's magnetic field so that spacecraft in low Earth orbit, such as the Space Shuttle and International Space Station, are relatively well-protected. But the danger of radiation exposure is very real, for example, in the case of a manned mission to Mars.
GCRs are unpredictable, come from all directions, and may have extremely high energy. However, they tend to be few and far between, consisting of an occasional handful of very-high-speed particles arriving from some random direction. The danger they pose is one of cumulative radiation exposure over the duration of a long mission. An SPE, on the other hand, is quite capable of killing an unprotected person in a single burst. It is associated with the most violent of solar flares and produces X-rays (which reach Earth in minutes), energetic particles (hours), and solar plasma (days). Though the X-rays are certainly not benign, it is the high-energy particles – mostly protons and alpha particles – that are the main concern.
Fortunately, because solar activity is continuously monitored by a number of satellites, a couple of hours' advance warning of potential SPEs is possible. Also, once the particle bombardment starts it takes several hours to reach a peak before fading again. Not that this would have helped the Apollo astronauts once they were on their way to the Moon. The Apollo missions, being relatively brief, relied on the low probability of SPEs and had no extra shielding. By contrast, on a future Mars mission both a solar flare warning system and some form of radiation protection within the spacecraft will be an absolute necessity. The protection could take the form of a small shelter with radiation-resistant walls. However, this approach has limitations. For example, it is not effective against GCRs – in fact, unless the shelter is massive (in which case it places a heavy burden on the propulsion system) it is worse than no shielding at all because the impact of a GCR nucleus on a light shield would be to spawn secondary radiation more intense than the original. Since GCR cumulative doses on missions lasting more than a year may exceed the recommended maximum allowable whole-body radiation dose, mission designers are considering an alternative to the simple shelter in the form of an active electromagnetic shield. This could work like Earth's magnetic field, by bending the trajectory of incoming charged particles away from the region to be protected.