Australasian Science Magazine

In 2001, the third stage of a Delta 2 rocket re-entered the atmosphere over the Middle East. The titanium motor casing, weighing about 70 kg, landed in Saudi Arabia about 240 km from the capital of Riyadh.

By Kerrie Dougherty

The orbiting detritus of humanity’s exploration and exploitation of space poses a growing threat to operational space systems and crewed spaceflight activities.

The 2013 space thriller film Gravity, in which two astronauts become stranded in space after a cloud of fragments from an exploded satellite destroys their space shuttle, vividly depicts one of the major issues impacting on current and future space activities – the problem of “space junk”, or orbital debris.

Technically defined as “all man-made objects in orbit about the Earth which no longer serve a useful purpose” (NASA Orbital Debris FAQ at http://www.orbitaldebris.jsc.nasa.gov/faqs.html), orbital debris has been created in many ways and is composed of such items as inactive satellites and other pieces of abandoned or derelict space hardware and spent launch vehicle upper stages that attained orbit. Mission-related debris includes multiple vehicle carriers (used for launching several satellites from the same rocket), reactor fuel cores from radar satellites, refuse from crewed missions, and tools and equipment that have broken off from spacecraft or floated away through open hatches (including nuts, bolts, hand tools and the odd spacesuit glove).

The largest category of space junk is fragmentation debris, shrapnel-like pieces of exploded satellites and rocket stages. This type of debris commonly results from the explosion of residual fuels, bad batteries and high pressure fluids or from spacecraft broken up due to collisions with other pieces of space junk. Also included in this category are solid rocket motor effluents and the products of spacecraft deterioration, such as bits of thermal blanket and even flecks of paint.

Since 2007 there has been a huge increase in the number of fragmentation debris 10 cm or larger, primarily due to two incidents: in 2007, China intentionally destroyed the defunct Fengyun-1C weather satellite in an anti-satellite weapon test, creating 3000 trackable fragments, while in 2009 the accidental collision of the American Iridium 33 satellite with the defunct Russian Cosmos 2251 generated another 2000 pieces of trackable debris. The fragmentation debris from these two events alone now represents one-third of all catalogued orbital debris, in addition to uncountable numbers of smaller pieces.

Fragmentation presents an ongoing, and increasing, problem: as older pieces of space junk break up and create quantities of smaller pieces, the risk of collisions between these smaller fragments increases. Further collisions generate more clouds of fragments, leading to more collisions. This is sometimes referred to as the Kessler Syndrome, after Donald Kessler, who first identified the orbital debris problem in 1978 (http://webpages.charter.net/dkessler/files/KesSym.html).

Since the launch of Sputnik 1 on 4 October 1957, orbital space has become increasingly crowded. Currently approximately 29,000 objects of 10 cm diameter or larger exist in various orbits about the Earth. Of these, only about 6% are actually functioning spacecraft. Smaller debris particles, 1–10 cm in diameter, are estimated to number more than 670,000, while there may be many millions of flecks and fragments smaller than 1 cm.

The LEO (low Earth orbit) and GEO (geosynchronous) graphics above are computer-generated images of objects in Earth orbit that are currently being tracked. Approximately 95% of the objects shown are orbital debris (i.e. non-functional satellites). The dots represent the current location of each item. These images are a good visual guide to where the greatest orbital debris populations exist. Note that the larger population of objects over the Northern Hemisphere is due mostly to Russian objects in high-inclination, high-eccentricity orbits.

While there are debris issues at geostationary orbit altitude (about 36,000 km), most space junk can be found within 2000 km of the Earth’s surface (LEO). The most debris-crowded area is between 700 km and 1000 km, the region in which sun-synchronous orbits occur. These special orbits are primarily used by Earth observation satellites that collect valuable environmental data. Russia has been determined as the source of the largest number of debris items, followed closely by the USA. However, other space-faring nations, such as China, France, India and Japan, as well as the European Space Agency, are also responsible for debris creation.

Why is orbital debris a matter of concern? As the most used and most useful orbits become increasingly cluttered with junk, the risk of a collision between the debris and a functioning satellite, crewed spacecraft or spacewalking astronaut increases, with the potential for lost revenue (in the case of a commercial satellite), lost data (from a scientific satellite), loss of service (such as GPS or weather monitoring) and loss of life. There is also a risk, albeit small, that re-entering space junk could cause damage and loss of life here on Earth.

In LEO, orbital debris travel at speeds of 7–8 km/s. However, the average impact speed of an orbiting item with another space object will be approximately 10 km/s, meaning that a collision with even a small piece of debris will involve considerable energy. Objects bigger than 10 cm are capable of inflicting catastrophic damage to an operational satellite or crewed spacecraft, so they are regularly tracked and their orbits well-established, enabling operational space vehicles to manoeuvre to avoid them if necessary. The International Space Station (ISS) (and previously the Space Shuttle) will take evasive action and manoeuvre away from an object if the chance of a collision exceeds one in 10,000: this occurs about once a year on average.

Debris that can be attributed to a specific launch and launching state(s) are catalogued in a database of all spacecraft launches and space objects maintained by the US Space Surveillance Network (SSN). Operated by the US Strategic Command, the SSN is the world’s largest tracking network, combining ground-based radar and optical tracking facilities and space-based assets. Russia maintains its own Space Surveillance System and space object catalogue, while civilian radar and optical scientific research facilities also assist in monitoring orbital debris. However, observations are concentrated in the Northern Hemisphere and there is minimal coverage south of the Equator.

Fragments in the 1–10 cm range can be observed with ground-based radar, but they are too small for accurate orbital prediction and thus are not usually tracked. Items this size cannot be manoeuvred around and are therefore particularly dangerous, as they are capable of penetrating and damaging a spacecraft. The ISS is heavily shielded along its leading face to protect the areas with the highest impact risk. Similarly, many satellites now have some level of shielding to protect their vital components.

The millions of fragments smaller than 1 cm are too small to be tracked, but they can still cause significant damage to any spacecraft they might impact upon. Clouds of small particles can cause “corrosive” damage, like sandblasting, to vehicle surfaces, instrument lenses and solar panels.

The estimates for this population of space debris are derived from examining impact features on the surfaces of returned spacecraft such as the Long Duration Exposure Facility, which spent 68 months on orbit, the European Retrievable Carrier, which spent almost a year in space, and the Space Shuttle, as well as observations of the Mir space station, the ISS and the Hubble Space Telescope.

How long a piece of orbital debris remains cluttering up the heavens depends upon the altitude of its orbit. An item circling less than 200 km above the Earth will re-enter the Earth’s atmosphere after a few days, while objects at orbits to around 600 km will take several years to decay. Debris orbiting in the 600–1000 km range can remain in orbit for decades to centuries, while material above 1000 km can be expected to remain aloft for millennia. The oldest satellite still in orbit, Vanguard 1 (launched in March 1958 and defunct since 1964), has an orbital lifespan of approximately 240 years. Satellites in geostationary orbit at 36,000 km will effectively remain there permanently, which makes solving the problem of debris in GEO particularly crucial given the importance of this orbit to communications on Earth and the finite number of positions that can be occupied by satellites in that orbit.

In 2005, a study by Liou and Johnson (Risks in space from orbiting debris, Science 2006, 311(5759), 340–1) indicated that even if no further launches occurred, the orbital debris hazard would continue to increase as collisions between existing space junk will continue to generate further fragments 10 cm and larger faster than atmospheric drag would remove them. This scenario highlights the eventual need for active removal of the existing debris population. It also points to the critical need to reduce the potential for creating future debris as much as possible by practising mitigation measures with all spacecraft launched today.

The threat posed by orbital debris to ongoing space activities has been recognised for several decades: since 1988, the US has had a declared policy of minimising orbital debris to protect the orbital “resources” of the Earth. In 1995, NASA became the first space agency to issue a comprehensive set of orbital debris mitigation guidelines, which became the basis of the US government’s Orbital Debris Mitigation Standard Practices, which covers the design and operation of spacecraft and upper stages to reduce the potential for explosive fragmentation. These guidelines include the use of better batteries and the venting or burning of remaining fuel in rocket stages and satellites to prevent later explosions. Many US commercial firms voluntarily follow these guidelines, and similar ones are in place with other space-faring nations and space agencies, such as Russia, France, Japan and the European Space Agency.

In 2007, the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space adopted a consensus set of space debris mitigation guidelines that were endorsed by the United Nations in January 2008. These included limiting the production of debris during routine operations, minimising the potential for accidental on-orbit breakups, prevention of on-orbit collisions, prohibition of the intentional destruction of satellites and disposal of spacecraft post-mission.

Post-mission disposal is increasingly used as a way to mitigate the creation of future space debris. Satellites in geostationary orbit are now moved to “graveyard orbits”, approximately

300 km above GEO, at the end of their operational life, while spacecraft and rocket stages in lower orbits can either be directly de-orbited (if they are in a low enough orbit) or moved into a lower orbit that will decay within 25 years. For example, in 2013 the US Geological Survey deactivated Landsat 5, which had been in orbit since 1984. The satellite was “safed”, with all sources of energy that might lead to a future explosion expended or released, and residual propellant was used to lower the spacecraft from an orbit of 705 km to one that would reduce the remaining orbital lifetime of Landsat 5 from several decades to less than 25 years.

However, these measures alone will not be enough to contain the growth of orbital debris: only the removal of existing large objects from orbit can prevent long-term problems for the commercialisation and research use of space. Any successful debris removal concept must be technologically feasible, economically viable and politically acceptable to the international community. Debris removal activities should also be carried out in ways that do not unduly increase hazards to people and property on Earth from re-entering objects.

Over the past decade, various remediation techniques have been suggested, such as attaching tethers or other drag-enhancement structures to large pieces of debris to hasten their orbital decay, or attaching small ion engines or other thrusters to drive objects down to re-entry. But attaching such devices, either by robotic means or via crewed space missions, requires difficult and dangerous rendezvous procedures and would be prohibitively expensive, even if each mission could service several debris objects. In addition, there is currently no crewed spacecraft available that can reach the critical debris regions above 600 km. These techniques could, however, be viable options for future orbital debris reduction if they were routinely fitted to spacecraft during construction.

More exotic but currently unfeasible suggestions for removing space junk have included using huge aerogel chunks to absorb impacting debris (with the aerogel then being de-orbited with the trapped particles), catching pieces of space junk in large mesh nets strung between inflatable booms, and somehow herding larger objects into an “orbital junkyard” where they can remain out of the way while potentially being available as sources of on-orbit materials.

Using ground-based lasers to perturb a satellite’s orbit, thus leading it to decay, was first proposed in the 1990s, but the concept was not feasible at the time for both technical and political considerations: the high-powered lasers being considered also had the potential to become anti-satellite weapons. In recent years, however, an Australian company, EOS Space Systems (http://www.eos-aus.com), has pioneered research in both high-precision laser tracking of space objects, including fragments as small as 5 mm, and the use of low-powered lasers to deflect pieces of debris away from a collision course with another object. Together, these techniques potentially represent the most cost-effective and technologically feasible means of reducing the orbital debris problem in the near future.The research has been supported by NASA and the Australian Space Research Program.

The orbital debris threat is a serious issue for the international space community, and it is one on which concerted action must be taken now to prevent the loss or degradation of space-based services and hazards to crewed spacecraft in the future.

Kerrie Dougherty is Curator of Space Technology at the Powerhouse Museum.