At speeds reaching 50 000kph, even the smallest bits of space debris, such as flakes of paint, can cause serious harm to spacecraft.
Near-Earth missions such as the International Space Station have to carry sophisticated armour for protection, and be manoeuvred clear of larger items, such as decommissioned satellites, that will cause instant destruction in a collision.
Walter Flury, the European Space Agency’s (ESA) resident expert on space debris, terms such a collision a ”hypervelocity impact”.
Together with a team of debris specialists in the mission analysis office at the ESA’s operations centre in Darmstadt, Germany, and at the agency’s research centre in Noordwijk, The Netherlands, Flury is the point man in the ESA’s efforts to mitigate the risk to missions.
”There have been cases of damage and destruction caused by hypervelocity impacts. Just last week, we had to do an avoidance manoeuvre,” says Flury.
At the end of 2003, there were about 10 000 catalogued objects in orbit around Earth.
Seven percent are operational spacecraft, 22% are old spacecraft and 17% are rocket bodies, according to the ESA.
The other 54% are either ”mission-related objects” or miscellaneous space junk, including cast-off devices used to stop spacecraft spinning after launch and tools dropped during spacewalks.
There are many thousands more uncatalogued objects larger than 1cm, perhaps more than 500 000. No one really knows the exact count.
The uncatalogued objects include bits of aluminium slag from solid rocket-motor propellant and droplets of sodium-potassium coolant that escaped from Russian nuclear-powered reconnaissance satellites when they ejected their reactor cores.
The really dangerous bits are intermediate in size, between 1cm and 10cm. These are hard to detect, yet pack a kinetic energy punch sufficient to cause catastrophic damage. One centimetre is the maximum size of debris that can be defeated by shielding technology.
Space-shuttle windscreens have been damaged by flecks of paint as small as 0,3mm in size travelling at a mere 14 400kph. The fastest debris, at 50 000kph, is travelling about 17 times faster than a machine-gun bullet, the ESA says.
Daily tracking
Since the 1957 launch of Sputnik 1, humanity has been blasting rockets into orbit — and space debris has been accumulating.
”At that time, space was empty, so no one was thinking about the problem,” says Heiner Klinkrad, one of the debris specialists at the ESA who manages a database of known satellites and debris that is used to predict potential collisions with ESA satellites.
The information is supplied by USSpacecom (United States Space Command), which operates dedicated tracking radars, and by several European sources.
Additional information on objects of sub-catalogue size is provided by Germany’s Research Establishment for Applied Science radar, in Wachtberg, Germany, and by the ESA’s own space debris telescope in Tenerife, Spain.
Each morning, a computer automatically sends an e-mail message to operations staff about likely approaches. Human intervention is only required if the risk of collision exceeds a 1 in 10 000 chance.
Under Flury’s chairmanship, the ESA will host a European conference on space debris next April in Darmstadt.
Gerhard Drolshagen at the ESA has analysed micro-debris impacts on the solar arrays used by the Hubble space telescope. The arrays were supplied by the ESA and retrieved in March 2002 after more than eight years in space.
Thousands of impact craters could be seen on the 41-square-metre solar arrays, the biggest about 8mm in diameter.
”There were 174 complete penetrations of the 0,7-mm-thick array, with impact craters ranging in size from three microns to 8mm,” says Drolshagen.
A chemical analysis of impact crater residue even allowed him to distinguish between impacts due to naturally occurring micrometeoroids and those due to artificial space debris. Despite being peppered with damage, Hubble continued operating normally.
Code of conduct
Flury argues for an international code of conduct and to reduce space pollution, since past efforts by individual space-active nations have not reduced the problem.
The ESA’s policy effort focuses on the Inter-Agency Space Debris Coordination Committee (IADC), comprising itself and the space agencies of China, France, Germany, India, Italy, Japan, Ukraine, Britain, the US and Russia.
The United Nations Committee on the Peaceful Use of Outer Space is also working on the issue.
”Once upper stages are discarded and satellites are turned off, their mission is over. During the first 30 years of space flight, few operators disposed of their spacecraft in a controlled way. This attitude has gradually changed in the past 20 years,” says Klinkrad.
In 2002, the IADC issued guidelines that require spacecraft owners to protect the commercially valuable low-Earth and geostationary orbit zones by not throwing material overboard or deliberately breaking up rockets or payloads.
It recommends spacecraft and spent rockets be moved to ”graveyard” orbits or crashed into the atmosphere, where most burn up.
While the overall risk of a mission ever being hit by an object more than 1cm in size remains small, Flury argues for action now to protect scientific and commercial space activity in the future.
”Debris preventative measures need to be applied now,” he said at a recent conference. — Sapa-DPA