Space Debris
27 March 2008In early 2008, a spy satellite weighing over 9000 kg was reported to be out of control and spiralling towards Earth. Events such as these fleetingly focus the attention of the world media upon an important subject.
According to the Wikipedia, space debris is defined as the objects in orbit around Earth created by humans that no longer serve any useful purpose. They consist of everything from entire spent rocket stages and defunct satellites to explosion fragments, paint flakes, dust, slag from solid rocket motors, coolant and other small particles.
A Space Age problem
It is sobering to think that we have created a problem from nothing in the first half century of the Space Age. In October 1957, the USSR launched Sputnik I when there was no such concept as "space debris". Five months after Sputnik, the USA launched a satellite named Vanguard I. After its mission was completed, it became the longest surviving piece of space debris - on 17 March 2008 the oldest human-made object in space "celebrates" 50 years in Earth orbit.
According to Edward Tufte's book Envisioning Information, space debris objects have included a glove lost by astronaut Ed White on the first American space-walk, a camera Michael Collins lost near the spacecraft Gemini 10, garbage bags jettisoned by the Soviet Mir Cosmonauts throughout that space station's 15-year life, a wrench and a toothbrush.
The larger, the better
Most of this debris re-entered the atmosphere of Earth within months. An effect called "residual air drag" (the drag of the Earth's magnetic field) helps keep low Earth orbit (less than 500 km up) clear with this sort of space junk burning up in our atmosphere harmlessly. Only one person has been recorded as being hit by space debris. In 1997 a woman in Oklahama, USA was hit in the shoulder, but uninjured, by a 10x 13 cm. piece of woven metallic material that was later confirmed to be part of the fuel tank of a 1996-launched Delta II rocket.
When dealing with large objects with known trajectories such as spent satellites, rocket stages, cameras and gloves, we are able to track and catalogue them. The first verified collision with catalogued space debris occurred in 1996, tearing off a boom from the French satellite Cerise.
Intermediate-sized objects, between 1 and 10 cm, are harder to detect but can hit spacecraft with enough energy to cause catastrophic damage.
Big bangs in Earth orbit
The majority the problematic, uncataloguable, space debris originally occured because of explosion events in higher orbits. Mission designers have until now have carried extra fuel on board in case it is unexpectedly needed. This extra fuel remains inside pressurised tanks once the rocket stage is discarded. Over time, leaks occur and a sudden explosive release of pressure can result. Each explosion creates thousands of small debris objects and about 100 tonnes of fragments generated during such events are still in orbit.
This problem would be big enough but it is compounded - such debris collides with other objects causing ever more, and smaller, space junk. These tiny undetectable fragments are the main debris issue. According to the European Space Agency, 1 cm is the maximum size of debris that can be defeated by modern shielding technology. Space Shuttle windscreens have been damaged by flecks of paint as small as 0.3 mm in size travelling at a mere 14 400 kph. The fastest debris, at 50 000 kph, are travelling about 17 times faster than a bullet.
A NASA consultant, Donald Kessler, has said that it is possible in the future that the increasing amount of debris in orbit could eventually make satellites too prone to loss to be feasible.
Every satellite, space probe and manned mission has the potential to create space debris. As the number of satellites in orbit grow and older satellites become obsolete, the risk of a cascading "Kessler Syndrome"becomes greater.
In the modern world we have become reliant on satellites. They help forecast the weather, beam television signals across the oceans, route mobile phone signals and may even be providing some of the internet connection that allowed you to read this page. If space debris is going to put these vital tools at risk, solutions need to be found.
Ways forward
Proposals have been made for ways to "sweep" existing space debris back into Earth's atmosphere, including automated tugs, laser brooms to vapourise or nudge particles into rapidly-decaying orbits, or huge aerogel blobs to absorb impacting junk and eventually fall out of orbit with them trapped inside. These methods would be hugely expensive, even if feasable, and somebody would need to provide the finance.
Missions could be designed to be safely disposed of at the end of their lives, minimising the risk of adding themselves to the debris problem. The 2003 CNES Spot-1 satellite for instance has been designed to de-orbit itself after 15 years.
Spent rocket upper stages could release their residual fuel, decreasing the risk of explosions that could generate thousands of additional debris objects. Higher orbit craft such as geostationary satellites could be towed to a "graveyard orbit" where no operational satellites are present.
And as Dr Ruediger Jehn, a space debris specialist working at the European Space Agency has commented: "The amount of debris created during normal operations can be reduced by not discarding, ejecting or detaching anything that does not have to be discarded, ejected or detached".
International solutions
A group of space agency experts from ASI (Agenzia Spaziale Italiana), BNSC (British National Space Centre), CNES (Centre National d'Etudes Spatiales) DLR (Deutsches Zentrum fuer Luft- und Raumfahrt) and ESA (European Space Agency) drew up a Code of Conduct for space debris mitigation in 2002.
The code of conduct represents a consensus between these five space agencies on what needs to be done to mitigate the proliferation of space debris. It details and complements the guidelines already being discussed at international level by the Inter Agency Space Debris Coordination Committee (IADC). The IADC comprises space agencies from China, France, Germany, India, Italy, Japan, Ukraine, the UK, the USA, Russia and ESA, as well as the Scientific and Technical Subcommittee of the UN Committee on the Peaceful Use of Outer Space (UN-COPUOS).
The code indicates measures to be taken during a spacecraft's operational mission and at the end of its life. The main requirements are for objects not to remain in low-Earth orbit for more than 25 years after completing their mission, and to move geostationary satellites to a graveyard orbit.
Implementing such measures and codes of conduct remains controversial within the industry since their adoption as formal policy will invariably raise mission costs. However much effect is now being given to an international code of conduct, worldwide-accepted standards, and international regulations or space law to create a comprehensive framework for reducing space debris and boosting spaceflight safety.
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