A new report on space debris says that it is a growing hazard, which needs urgent attention.
Tim Furniss/LONDON
A NASA-FUNDED report by the National Research Council (NRC) says that the hazard to spacecraft posed by artificial debris in orbit, although still low, is growing and requires international action (Flight International, 23-29 November).
The space-debris problem needs to be addressed while it is still manageable, and space nations of the world "...should approach the problem with a co-operative, multi-pronged effort", says George Cleghorn, the NRC international debris committee chairman and former TRW chief engineer. "The threat may become significant in some regions of space, with costly and virtually irreversible consequences," he says.
These comments echo earlier related reports. The European Space Agency says that, "space densities in low-Earth orbit have reached critical concentrations which could initiate a self-sustaining process. Even with a zero launch rate, this tendency seems to be irreversible." A UK Defence Research Agency study indicates that conditions between 800km and 1,000km "may already exist for collision cascading to occur".
The NRC report is part of a continuing inter-agency review of orbital-debris programmes and policies being carried out under the direction of the US Government Office of Science and Technology Policy.
The NRC's report suggests several steps in approaching the orbital-debris problem. Firstly, space-faring nations should work together to fill in critical gaps in the data on the effects of collisions between orbiting objects and on the amount and sources of debris in orbit, to better estimate hazards to spacecraft.
Other steps include spacecraft designers being made more aware of the need to protect spacecraft against debris, for example, by shielding critical components. They should also implement ways to prevent future spacecraft from adding to the problem, the report says.
Assessing the danger of orbital debris accurately is difficult because of the inherent problem of studying small, dark, objects travelling at very high speed, or thousands of miles above the Earth. In low-Earth orbit (LEO), only objects larger than 100mm across can be tracked, by ground-based radar and optical sensors. In geostationary orbit (GEO), only objects larger than 1m can be tracked and catalogued. Over 7,580 trackable objects include about 2,000 satellites - of which only about 350 (6%) are still operational. Ninety four per cent is therefore debris.
The most dangerous category of space debris is that of small fragments, which cannot be tracked easily, and some not at all. These are bits of satellites and rockets, which have broken apart in orbit and consist of 40,000 to 150,000 pieces measuring less than 100mm down to 10mm.
These objects, smaller than those now catalogued, "...can destroy spacecraft in a collision. Even collisions that do not destroy a spacecraft can degrade its performance or cause it to fail", says the report.
There are also over 3 million minute particles smaller than 10mm. These consist of dust particles, flakes of paint and pieces of shredded spacecraft insulation. These high numbers of particles have built up into dangerous localised "debris swarms". Removal of existing debris from orbit is neither technically and economically feasible, so international efforts should focus on preventative measures, the NRC says.
One step would be for leftover propellant or other energy sources to be dissipated from spacecraft and rockets at the end of their functional lives to ensure that they do not explode.
Other steps would include minimising the release of debris during launch and on-orbit operation, along with manoeuvring rockets or spacecraft at the end of their functional lifetimes, either out of LEO or into orbits which will carry them back into the Earth's atmosphere.
The NRC suggests that, pending the creation of an international group, to advise the space community about the most important issues concerning debris, interim work should be conducted immediately, with several priorities: careful study of uncatalogued objects in LEO, gaining a better understanding of small debris-by-debris detection spacecraft, studying debris in GEO, through both measurements or modeling and improving models of debris sources.
THE DEBRIS CATALOGUE
There are about 3,000t of man-made debris in Earth orbit, ranging from dead satellites as big as buses to tiny dust particles from the exhaust of solid-rocket motor firings.
Two-thirds of the debris is in LEO, three-quarters of it below 2,000km. One-third is in GEO, where it stays indefinitely. The total in GEO is about 200 operational satellites and 1,000 bits of debris.
Experts estimate that, there is a 1% chance of a debris collision, in ten years of operation of a single satellite. The heaviest concentration of debris is located between 800km and 1,000km altitudes.
Making the danger more acute, much of the smaller debris in the highly populated region of LEO is made up of clouds of radioactive debris from dead nuclear-powered Russian spy satellites, of which 33 were launched to 1987 and fired into junk orbits at the end of their functional life. This radioactive debris is made up of a mixture of sodium and potassium in liquid-metal form, in small droplets travelling at 10km/s.
Source: Flight International
