Orbital Debris A Technical Assessment (1995) / Chapter Skim
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2 METHODS FOR CHARACTERIZATION
2 METHODS FOR CHARACTERIZATION
Pages 31-62
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Cataloging space objects requires an expensive network of sensors capable of observing objects periodically to determine any changes in their orbital elements and of continually performing orbit determination computations.
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From page 32... ...
Space Command; the other, the Space Surveillance System (SSS) , is operated by the Russian Unitary (see Box 2-1\ The primary purpose of each system is to detect objects that present ~ Dietary threat thus, although each is capable of detecting certain types of debris neither system is optimized to perform the task of partaking ~ debris catalog.
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In general, objects with larger optical or RCSs are more easily detectable than objects with smaller cross sections. Both the optical and the radar cross sections of particular space objects can vary greatlywhich is not surprising for a collection of irregular-shaped objects.
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Space Command confirmed the SSN's difficulty in cataloging space objects in low-inclination and high-eccentricity orbits (Pearce et al., in press; Clark and Pearce, 1993~. It should be emphasized that these peculiarities do not represent deficiencies in the way the networks perform their normal mission of maintaining a catalog for military reasons, but rather reflect the fact that they were not designed to characterize the space debris population.
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From page 36... ...
As is shown in Figure 2-2, atmospheric drag retardation along the orbital track of medium to large space objects in 300- to 600-km-altitude orbits can range up to hundreds of kilometers per day. The most optimistic estimate of the accuracy with which atmospheric drag can be determined is +15 percent; consequently a prediction error (which cannot be calibrated)
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Finally, increasing the number of sensors available to detect debris would allow for better tracking of cataloged objects and for more searches for uncataloged objects.
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For example, rather than portraying the steady-state small debris population in LEO, in situ measurements of small debris particles acquired by examining returned spacecraft surfaces portray only the average debris flux along a particular orbit during a particular time frame. llemote Sampling from Earth Optical Sensors At first glance, the use of ground-based telescopes to sample the debris population seems like a promising technique.
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From page 39... ...
Although the exact size of debris detectable by these telescopes is not certain since they measure pieces of debris with a variety of unknown reflectivities, the average minimum object size detectable is slightly smaller than 10 cm (Kessler, 1993~. Ground-based telescopes also can be used to sample the space debris population above LEO.
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From page 40... ...
In addition, the longer-wavelength FGAN and MU radars have demonstrated the ability to sample the medium and large debris population, respectively (Mehrholz, 1993; Sato et al., 1992~. In 1989, the Arecibo Observatory's high-power 10-cm-wavelength radar and the Goldstone Deep Space Communications Complex's 3-cmwavelength radar were used (with the assistance of other radars)
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From page 41... ...
Continued sampling efforts with existing radars can increase statistical confidence in existing data and, over time, could provide information on changes in the debris population. However, the Haystack, Goldstone, and Arecibo radars, which were not designed to detect debris, have other users preventing them from being used fulltime for debris detection and are expensive to operate.
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From page 42... ...
Many additional space-based remote sensors to detect debris have been studied and proposed since the late 1970s (Kessler and Cour-Palais, 1978; Neste et al., 1982~. Among these are proposals by Russian experts, who proposed using space-based optoelectronic sensors to detect debris
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has done extensive Work on an inhered system for debris detection and coNi~on Warning for the Space Station and has proposed ~ spacecraR (see Box 2-5) With two Sarah telescopes capable of monitoring the 1m~ and larger environment (Foresee and Loftus/ 19937 In addition/ Kaman Sciences Corporation has proposed an optical (visible and infrared)
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From page 44... ...
can detect debris as small as 2 mm in diameter. Consequently, space-based remote sensors are likely to add significantly to our knowledge of the LEO debris environment only at higher LEO altitudes and in latitude bands that are not adequately characterized by ground-based sensors.
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Space Shuttle windows, from materials returned from the Solar Maximum Mission spacecraft, from the Salyut and Mir space stations, from the Palapa and Westar spacecraft, from LDEF, and from the European Retrievable Carrier (EURECA)
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In addition, passive sensors provide only integrated time-exposure data rather than time-dependent data, so little can be determined about the effects of solar activity on the small debris population or the existence and location of debris "swarms." Finally, because the majority of returned surfaces were not designed for debris testing, it is often difficult to distinguish between the impacts of orbital debris and micrometeoroids. The applicability and validity of the damage scaling laws used to interpret the data from passively exposed detector materials are also an issue.
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There are a wide range of active detectors, from simple impact detectors to complex chemical composition sensors. The simplest and cheapest detectors (and the ones most able to be made into large area detection systems)
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Either very large or very long duration in situ sensors, however, have the potential to provide an effective means of sampling the mediumsized debris population by exposing a large enough surface area over a long enough time for it to be impacted by the relatively sparse flux of these particles. There are difficulties with very long duration missions, however: they would obviously not provide data for some time, and their data would be less valuable because they would represent the average flux over a long period of time.
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ST12ATEGIES TO MEASURE THE DEBRIS ENVIRONMENT Figure 2-3 depicts measurements of the LEO debris environment made since 1980. The major gaps that exist in the altitude and size range data are apparent, as is the intermittent nature of most of the data gather
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From page 50... ...
The haphazard nature of the data is a result of the fact that most measurements of the debris environment to date were not part of an overall strategy to understand the environment but rather were gathered whenever measurement op portunities arose. Further ad hoc experiments to measure the debris environment will add to our knowledge of debris, but cost-effective characterization of the debris environment (including understanding the time- and altitude-variant nature of the debris population, the sources of small debris, and the collision hazard in widely used orbital regions)
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From page 51... ...
Population Characterization Models Population characterization models convert data on the orbital elements and other characteristics of space objects into measurable parameters, such as flux, detection rate for an instrument, or collision probability. This conversion is necessary both to help researchers interpret data collected in experiments that sample the uncataloged orbital debris environment and to aid designers in determining the debris hazard to their spacecraft.
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Currently, more complex models are used to predict the growth in the orbital debris population. Such models combine a traffic model, a breakup model, and an orbit propagation model to predict possible future orbital debris population states.
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An orbit propagation model then determines how the orbits of both intact space objects and space object fragments change as a function of time. Traffic Modeling The growth and evolution of the Earth-orbiting space object population will be influenced in large measure by the frequency and character of future space operations.
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The results of these models are typically used to estimate existing debris populations and to predict the future population. Most breakup models use the type and amount of energy causing the breakup of a space object of a given mass to estimate the resulting fragment distribution.
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Propagation Models Orbit propagation models predict how the orbits of space objects change as a function of time. This information is used for two major purposes: determining the location of particular space objects in the relatively near term (typically over a period of a few days or less for purposes of collision avoidance or reentry predictions)
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From page 56... ...
Consequently, only very simple LEO propagation models are normally justified for long-term space object population studies. Although atmospheric drag ceases to be a factor above LEO, space objects at higher altitudes are influenced by solar radiation pressure, lunar and solar perturbations, and irregularities in the Earth's gravity.
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From page 57... ...
and Russian space surveillance networks are able to detect objects down to a size of about 10 cm in LEO. Increasing fractions of larger objects are tracked so that the LEO debris environment in the size range greater than 20 cm is adequately characterized by the catalogs.
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Finding 6: Models predicting the future space object population in Earth orbit draw on traffic, breakup, and orbit propagation models. These component models have large inherent uncertainties; as a result, many characteristics of the future debris population cannot be predicted with precision.
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From page 59... ...
1993. Meteorite and Man-Made Microparticle Impact Detection Methodology and Equipment on the Space Stations "SALYUT" and "MIR." Briefing presented to the National Research Council Committee on Space Debris Workshop, Irvine, California, November 18.
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Pp. 59-64 in Proceedings of the First European Conference on Space Debris, Darmstadt, Germany, 5-7 April 1993.
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From page 61... ...
Pp. 15-18 in Proceedings of the First European Conference on Space Debris, Darmstadt, Germany, 5-7 April 1993.
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