Tim Furniss/LONDON
Space debris is a perennial problem. About 8,700 man-made objects larger than tennis balls - of which only about 700 are operational satellites - can be tracked in Earth orbit.
Of these, 41% are large fragments left over from explosions, usually caused by unspent propellant on rocket stages. Discarded upper stages represent 17% of debris, defunct satellites 22% and 13% are ejected payload shrouds and covers. In addition, spacewalking astronauts leave tools behind and there is a variety of other items.
Their number is increasing, despite a reduction in the annual launch rate. To make matters worse, there are about 150,000 items of debris smaller than a tennis ball - down to 1cm.
Daily commercial pressures to launch and operate satellites means turning a blind eye to the problem and hoping that space junk will not fatally damage a spacecraft.
The potential for disaster is obvious. At speeds of over 28,000km/h (17,400mph) a 1cm fragment could shatter a $100 million satellite - or the Space Shuttle or an International Space Station module.
Orbiting spacecraft are peppered by debris every day. After 68 months in space, the NASA Long Duration Exposure Facility had been hit by 34,000 particles of up to a micron (a thousandth of a millimetre) in size, the largest of which made a 5.25mm diameter crater. Tiny aluminium oxide particles from solid rocket motors were among the items. A tiny fragment could easily puncture a spacewalk spacesuit.
A window of the Space Shuttle Challenger was chipped by a 0.3mm fleck of paint travelling at 4m/s. The French Cerise satellite's long antenna was severed by a fragment from an Ariane 4 third stage in the first known collision - at 14km/s - of space objects in 1996.
The European Space Agency (ESA) reports the appearance of new space debris problems. Cosmos 2347, a Russian electronic intelligence satellite at the end of its operational life, self-destructed in orbit last November, creating over 130 pieces of debris in a 230 x 410km orbit.
The ESA reported worrying new trends at the second European conference on space debris last year. The first was the planned construction of numerous fleets of mobile communications and high data rate broadband multimedia satellites in low Earth orbit. These will exacerbate a serious build-up in this densely populated region above the atmosphere. If all the projects go ahead, there could be 2,000 or more new satellites operating in Earth orbit within five years.
Evasive actions
The Space Shuttle has made several evasive manoeuvres during missions to keep a wide berth of even the smaller class of debris being carefully tracked by North American Air Defense Command. Debris is tracked worldwide by a variety of means, from radar to high resolution optical telescope cameras.
Two European satellites were also manoeuvred to avoid spent rocket stages. ESA says that recent radar measurements of items in low-Earth orbits show a ratio of uncatalogued to catalogued objects larger than 10-50cm of about 4:1. Optical measurements of objects in circular geostationary orbit, 36,000km over the equator, which is the operational position of hundreds of communications satellites, show a large population of previously undetected objects measuring 20-100cm.
Parts of upper stages previously believed to have burned up completely during a high temperature re-entry into the atmosphere were found intact on the ground, adding new concern to another problem: potential danger to people and property on the ground.
ESA says debris will continue to increase, even with measures to reduce its creation during launches and the operational life of satellites - including the de-orbiting of spent rocket stages.
Strong measures should be taken to prevent the amount of debris from increasing, the agency says, including the compulsory de-orbiting of spent stages and satellites nearing the end of their service life. Such action was taken with seven old ESA satellites in geostationary orbit, including three Meteosat spacecraft, which were boosted several hundred kilometres into a "graveyard orbit", using on-board engines. Another measure to reduce the debris is to vent residual propellant in rockets.
Other ideas include the destruction of spacecraft in orbit using lasers fired from Earth, leaving nothing but vapourised material. This, however, leaves the possibility that the technology could be used for sabotage.
Source: Flight International
