More research needs to be done to protect space travellers and their spacecraft from cosmic-ray radiation and debris, says the US National Research Council (NRC). Two recent NRC reports indicate that NASA does not yet fully understand the effects of long-term exposure to space radiation, and that agencies worldwide need to do more to protect new spacecraft, such as the modules of the International Space Station (ISS), from debris strikes and radiation.
DEBRIS
The NRC says that ISS modules, especially those made in Russia, should be fitted with additional shielding against space debris, and points out that spacewalkers' suits may also need to be provided with extra protection. The NRC says that better planning is required to prevent "loss of life" in the event of a puncture.
Russian ISS modules have been described by one expert as "tin cans". The NRC says that the planned Service Module - which has been delayed - falls far short of requirements, compared with other hardware. It may need to be fitted with a shield in orbit. Fitting it with one on the ground will make it too big for its Proton launcher. Consideration also has to be made to what extent damage can be repaired in orbit.
The NRC says that spacesuits will need more protection against other penetration hazards, such as a glove puncturing, during routine maintenance outside the space station. "Overalls" on top of a spacesuit could be considered.
About 7,600 man-made objects of more than 1m in diameter can be tracked in orbit by ground-surveillance radar. These include over 2,000 satellites, of which only about 350 are operational. Other items include spent rocket stages and payload shrouds.
There are also 40,000 pieces of debris measuring between 10mm and 150mm, some of the larger being able to be detected on radar. These have resulted mainly from rocket stages' residual propellants causing explosions. Other bits include nuts, bolts and wires.
The most treacherous, however, are the 3 million-plus particles, including solid-propellant exhaust dust, flakes of paint and shredded insulation. Mere flakes of paint travelling at very high relative speed have pitted the windows of the Space Shuttle and the Mir 1 space station.
Another hazard, often overlooked, is that of natural space objects - millions of micrometeoroids and meteorites. There is not much that can be done about a meteorite strike, but protection against micrometeoroids is just as important as space-debris shielding.
The assessment and perception of space-debris dangers alter regularly. The European Space Agency once reported that the problem had got out of hand and would result in the cascade effect (in which debris collides, causing more debris) and said that space would be uninhabitable in future years. Other experts, however, believe that the problem is under control and presents no more danger to space travel than a mechanical failure on a spacecraft. What is needed, says the NRC, is a better understanding of the nature of debris, its size, shape and composition to design better protection.
Several space missions, manned and unmanned, have carried experiments to monitor radiation, micrometeoroids and debris hits. One of the first missions to be used to study micrometeoroids and debris was the wing-like Pegasus, launched aboard the Saturn 1 in the 1960s. The Long Duration Exposure Facility, flown in orbit from April 1984 to January 1992, was another such mission.
The Russian Mir space station has had panels deployed outside it to collect "hits" and to be recovered on spacewalks for later inspection. The Mir is subject to a comprehensive micrometeoroid/debris photographic survey during each Shuttle Mir Mission (SMM), while the US Docking Module, which is attached to the Mir, has been fitted with collectable panel plates.
One of these is the Orbital Debris Collector, which is designed to capture hypervelocity particles so that their residues may be characterised mineralogically and chemically. "This will provide empirical verification of potential risks to materials proposed for use on the ISS and to characterise the best-and-worst-case scenarios regarding cumulative kinetic energy," says NASA. The collection medium is aerogel, a highly porous, low-density, glass.
The Passive Optical Sample Assembly consists of trays containing sample ISS external-surface materials to assess dimensions and damage from debris. The Polished Plate Micrometeoroid Debris Collector is used to assess several different materials.
Radiation
If humans are ever going to fly to Mars, much effort will be needed to understand the effects of cosmic rays and solar-particle radiation - and how to shield spacecraft from this radiation. A 300-day round trip to Mars will require much stronger radiation shielding than any now available, says the NRC. The idea of adding "a little more aluminium" is not a satisfactory solution. The NRC says that NASA should spend more on space biology and radiation and stop work on radiation shielding until it has a better understanding of the problem.
Cosmic rays are penetrating atomic particles which bombard Earth from deep space. They consist primarily of protons, but also include electrons, positrons, neutrinos and gamma-ray photons. Cosmic rays are more intensive in deep space away from the Earth, so present a greater problem to the interplanetary traveller.
Solar storms and other phenomena result in powerful radiation which reaches the Earth and other parts of the solar system. Their disrupting effect on terrestrial communications, for example, have been well known and, more recently, it has been proved that they have knocked out and "killed" spacecraft (Flight International, 2-8 October, 1996). A "solar storm" in early January coincided with the loss of the Telstar 401 and a "hiccup" on the GOES 8 satellite. Although the loss of the Telstar has not been categorically associated with the storm, it is still the prime suspect. Minor events have even caused damage to lap-top computers being operated by cosmonauts on the Mir 1 space station.
The radiation research is vital because the storms are often linked to the periods of high activity on the Sun which occur every 11 years. The last period of maximum activity was in 1990. That means that the ISS should be up and running when the next high-activity period is due - which some scientists predict will be a less powerful period. Research has indicated, however, that the most dangerous time is not necessarily at maximum activity but at the periods in between, from phenomena associated with coronal-hole events.
In deeper space, tiny particles of cosmic rays "fly" though the human body and may damage its DNA and disrupt nerve cells in the brain. The amount of radiation will be far greater than that experienced by those few who so far have made the briefest of forays into deeper space. Some astronauts who flew to the Moon in Apollo flights were aware of this cosmic-ray radiation which caused brief flashes of light when their eyes were closed and cosmic rays passed through their brains.
It is still unknown how great the danger is. Scientists are aware, however, that cosmic rays can kill cells, mutate them and apparently cause cancer in animals. Cataracts and infertility could also result after long-term exposure. The effect on the brain is of great concern. Long exposure could cause neurons to be disrupted, with the possible result of affecting memory and thought processes. On a long trip in interplanetary space, every cell in the human body could be traversed and affected.
Scientists are unclear about how much shielding a spacecraft will require. If a trip were to be planned now, the margin for error in calculations would have to be extreme to ensure safety, but would also grossly increase costs and the weight of the spacecraft. One estimate puts the cost of shielding at $30 billion. Even the best shielding will not stop all radiation. High-energy protons can pass through shielding and their effect has to be addressed.
The biggest problem may be to develop shielding which not only stops large particles, but protects against the secondary particles which would be created by the initial impact on certain types of shielding. Traditional lead shielding is unsuitable because it would create too much secondary material. Scientists regard liquid hydrogen as the perfect shielding, but its use would be impractical.
The NRC has urged NASA to conduct extensive experiments using particle accelerators on the Earth. Accelerators are limited - and, at $20 million each, are expensive - and the NRC believes that, even if the necessary research could be completed on existing accelerators, it could take 20 years.
Experiments
Japan - which plans to operate an experiment module at the ISS, albeit in Earth orbit - has been flying space-radiation-effects experiments since 1992 in a programme directly associated with the design of its module.
The $500,000 Mitsubishi Real Time Radiation Monitoring Device (RRMD) will be next, flying on the STS86/Shuttle Mir Mission (SMM 6) in May, following its last flight on the STS79/SMM 5 in September 1996. The device includes a detector which measures the energy level of particle radiation, particularly as a result of solar events.
The SMMs are operated at a 51í inclination, where more radiation is likely to be received in the Earth's region than at the 28¹ flown on a previous mission of the RRMD in 1994.
The RRMD uses a linear-energy-transfer spectrometer to measure the elemental composition and energy spectra of cosmic radiation in real time. It also provides information on the effect of radiation on biological samples.
Scientists around the world cannot agree on what constitutes a safe dose of radiation to which a space-station worker can be exposed, or what should be done about the hazard. The research differs from country to country, and no hard data on biological effects of high-energy particles in space have been collected.
Source: Flight International
