A typical dictionary definition of debris is "the remains of something broken down or destroyed". To this is often added "ruins, fragments, rubbish". A more geological orientation may elicit "an accumulation of fragments of rocks".

Space debris is often thought of as fragments of material moving in space. While this may be true for natural space debris, it is not always true for artificial space debris, as we shall see. And space debris may not always be the result of a destruction or inorganic catabolism. Although much debris does result from catastrophic processes, some may occur as the byproduct of another activity. And like weeds, what some regard as debris others may regard as an opportunity, or even an artwork.

Natural space debris is ubiquitous throughout the universe. The most energetic events in space, supernovae, spew enormous quantities of dust and gas into the interstellar and probably even the intergalactic medium. Many people regard the resulting nebulae as the most attractive and colorful of all astronomical sights. Much closer to home, we might view solar system debris each clear night when a meteor streaks across the sky. An occasional comet or asteroid gives us an opportunity to view the larger debris bodies gravitationally bound to the Sun.

From 1957 man started to add to the space debris population with the launch of artificial satellites. These spacecraft, at the end of their useful life, become debris. Launching rockets and other pieces of equipment (such as shrouds) that remain in orbit are debris peripheral to the main objective of orbiting a useful space object. Fragmentation of objects, both derelict and functioning, is another source of man-made or artificial space debris.

Space debris has not only been created since the start of the space age, but it (both natural and artificial) has assumed a growing importance because of the hazard that it may present to man and his activities on space and even on the Earth itself.

Space Debris Classification Scheme

We know little about the interstellar medium and even less about the intergalactic medium. We do know that the former is filled with large amounts of dust and gas. Looking in the night sky we can see that large areas of the milky way are blocked to our view by dust. In the southern hemisphere there is one very dark small region near the southern cross that is called the coal-sack. This is due to dust blocking the light of stars behind it. Although this dust is augmented and often accompanied by large areas of gas and plasma, we shall restrict our definition of space debris to macroscopic particles and leave discussion of atomic and molecular debris throughout the universe for another discussion.

Although we know nothing about larger bodies (eg meteoroids, asteroids and comet-type bodies) in the interstellar medium, we believe them to be present.

In the solar system itself we have two classes of body that are not only often regarded as debris in their own right, but that also contribute to the smaller particles (meteoroids) that constantly pepper the Earth. Comets can be classified into long period and short period bodies. However, it is believed that all comets originate from a region known as the Oort cloud, a spherical shell of space way beyond the orbit of Pluto. Perturbing forces, possibly due to passing stars, sometimes send comets on highly eccentric orbits (so eccentric that they can be very accurately approximated by a parabolic orbit) into the inner solar system. Sometimes such comets are then perturbed (or captured) into short period orbits by close passage of a planet (usually Jupiter). Just under 200 short period comets are currently known.

Asteroids are more dense bodies than comets (with typical densities around 3500 kg/m³) and the majority of them are found in orbits between Mars and Jupiter, although a significant number have been found all over the solar system. The current majority theory is that asteroids are left over fragments from the formation of the solar system - bodies that never accreted to a size large enough to form a planet. However, there is some evidence to support the earlier and minority theory that they are the result of a massive collision between two or more larger planet sized bodies. Due to recent concerted searches for asteroids there are currently almost 200,000 objects known in this category.

Meteoroids of all sizes are continually bombarding the Earth. Smaller sizes are more prolific than larger sizes. Most meteoroids burn up in the process of traversing the Earth's atmosphere, but a few of the larger bodies reach the surface of the Earth and are then called meteorites.

We know that there is a large amount of very fine dust in the solar system because it scatters sunlight and thus reveals itself in the zodiacal light. When particles this small (less than a few hundred microns in diameter) encounter the Earth's atmosphere they do not burn up. The ratio of their mass to cross sectional area becomes so small that they are decelerated without undergoing substantial heating. They then float slowly down, over a period of a few months to a few years, to the surface of the Earth. It is estimated that the daily influx of interplanetary dust particles (IDPs) to the Earth could be around 1000 tons. These particles are known as micrometeorites.

The above diagram shows artificial debris (often referred to as orbital space debris) classified according to whether it is a byproduct of man's space activities (eg booster in orbit) or a fragment resulting from degradation, explosion, collision or similar event. However, many workers in the field adopt a much more exhaustive classification. One such scheme is shown below:

Orbital Debris Classification
After "Orbital Debris: A Technical Assessment"

• BURTON G COUR-PALAIS (1925 - 2004)
  Internationally recognised for contributions to hypervelocity research, he helped establish the NASA hypervelocity laboratory at the Johnson Space Center. His early work was concerned with development of meteoroid environment models.
• JOSEPH P LOFTUS JR (1930 - 2005)
  An international spokesman for the understanding and mitigation of orbital debris. He was instrumental in establishing the NASA Orbital Debris Program Office and one of the guiding forces in the formation of the Inter-Agency Space Debris Coordination Committee.
• JOHN L AFRICANO (1951 - 2006)
  Involved in a diverse range of operational and observational space debris projects including the (US) Air Force Mauii site (AMOS), the Liquid Mirror Telescope and the CCD Debris Telescope.
• DONALD A KESSLER
  Widely credited with being the first to raise awareness of the future problems that will result from uncontrolled proliferation of space debris - through a paper (with Burt Cours-Palais) to the Journal of Geophysical Research. This paper described the creation of an artificial debris belt around the Earth that would increase in time to pose a significant impediment to satellite operations. The "Kessler Syndrome" is a term now used colloquially to describe this problem.

• ARTIFICIAL SPACE DEBRIS
  by Nicholas Johnson and Darren McKnight [Orbit Book Company, 1987]
• ORBITAL DEBRIS : A Technical Assessment
  by the (US) National Research Council [National Academy Press, 1995]
• FUNDAMENTALS OF ASTRODYNAMICS AND APPLICATIONS
  by David A Vallado [Springer (Microcosm Press), 2007 (3rd ed)]

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