Phillip D. Anz-Meador, Ph.D.
Department of Physics
Embry-Riddle Aeronautical University

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1. Introduction

The non-benign nature of spaceflight had been recognized well before the first on-board sensors detected the van Allen radiation belts about the Earth or piezoelectric sensors, meant to measure strain and flexure within the structure of a rocket or spacecraft, first noted the impacts of micrometeoroids on the craft (Explorer 1, Alexander, personal comm.). Indeed, the recognition that meteoroids were "burning" in the upper atmosphere implied the requirement of a source of particles to burn, hence the studies of both photographic and radar/radio meteors. Observations of the sun, as well as such terrestrial phenomena as the Aurora Borealis and Australis, and such celestial phenomena as comets, asteroids, and the faint reflection provided by the meteoroid complex (the so-called "Zodiacal light"), provided further evidence, if needed, as to the significant constituents (and potential hazards) in the space environment.

Means of protecting spacecraft from this natural environment were required, and a significant amount of laboratory and on-orbit testing was conducted in order to protect and preserve spacecraft functions. Almost all measures were passive in nature, e.g. shielding was deployed to protect electronics from cosmic rays and micrometeoroids, designs were optimized to prevent static discharge, and "rad-hard" (radiation hardened) electronics were developed to cope with the ambient radiation environment. Time passed, and space became a place to explore, to do business, and to protect global security.

During that time, an appreciation of the many and varied components of the space environment grew. A large body of literature developed to characterize, explain, and predict the effect of the ambient environment upon spacecraft and space materials. It is not the intent of this paper to review that portion of the environment.

The near static nature of some of the components was noted, as well as the dynamic nature of others. Yet this was not the only categorization possible. For example, some (notably Mr. John Gabbard) noticed oddities in the catalogs of space objects tracked by the North American Aerospace Defense Command (NORADCOM). Certainly it wasn't common knowledge that not only had we launched the LANDSAT 1 spacecraft aboard a Delta rocket, but evidently hundreds of other small objects with this launch. Further analysis indicated that these objects were debris associated with the accidental fragmentation of the Delta's second stage. Again, time passed. Military tests were conducted in space, including intentional explosions and collisions, and the list of accidental explosions grew.

Debris began to accumulate and, with a maturity of thought not present at the dawn of the Space Age, scientists and engineers in the United States came to realize that spacecraft must not only be protected from the "natural" environment on-orbit, but also the induced, or "man-made" environment. Finally, a burgeoning sense of environmental stewardship led to the modern international consensus that not only must spacecraft be protected from their environment, but that same environment must be protected from spacecraft.

This, then, is the subject of this paper: what is being done to protect spacecraft from the macroparticle (to include both anthropogenic debris and meteoroids) environment, and what is being done to protect the environment from man's presence. Only a holistic view of these processes can ensure a future environment safe for its navigation and capable of sustaining continued growth and exploitation of the unique natural resource offered us by space. Thus, in this paper we shall review the international guidelines being formulated to protect both spacecraft and the environment. To place these in context for the general reader, we shall start by providing an overview of the current space environment and environmental effects upon spacecraft.

2. Environmental Overview

2.1 The Man-Made Space Environment

The man-made component of the overall space environment is usually categorized into five types of objects, and as well by the object's active or inactive status. The five types are spacecraft or payloads, rocket bodies or rocket boosters, operational debris, fragmentation debris, and anomalous debris. To be more explicit, we may define the types as follows:

• Spacecraft or payloads: active or inactive (in storage, or derelict) vehicles or objects whose purpose was the primary goal of their respective launch. While the term "spacecraft" is usually reserved for relatively complex vehicles, the broader term "payloads" describes all levels of sophistication, including such inert objects as calibration spheres and dipoles.
• Rocket bodies (or boosters; usually abbreviated as "R/B"): these vehicles provide the means of launch, orbital transfer, and orbital insertion to the payloads. Thrust is provided by liquid fuel engines, solid fuel motors, or gaseous and/or electric/ionic thrusters. Size ranges from over ten meters in length (e.g. the Commonwealth of Independent State's [CIS] Zenit, or SL-16 [US Dept. of Defense designation], R/B) to small ullage motors used to settle liquid propellants and ejected by the CIS Proton's (SL-12) fourth stage.
• Operational debris: debris released during stage separation, payload deployment, or payload operations. These may include, respectively, straps and bolts; adapters, clamp bands, and spin/de-spin weights ("yo" weights); and retention or hold-down straps and radiator or sensor covers.
• Fragmentation debris: debris created during the planned or accidental explosion of, or collision between, payloads and/or R/B. Though not cataloged due to their size, debris produced by collisions of small objects with large targets could logically fit into this category.
• Anomalous debris: debris created by unknown means, usually long after payload deployment or end-of-mission. While the majority of instances have produced one or two anomalous objects, some (such as the Cosmic Background Explorer [COBE] or the SNAPSHOT nuclear reactor-powered test satellite). It has been suggested (Johnson, N., personal comm.) that while fragmentation debris are a measure of space traffic's effect upon the environment, anomalous debris may be a measure of the environment's effect upon resident space objects.

The approximate distribution of objects by type is depicted in Fig. 1; the reader should note that these objects are exclusively 10 cm (approximately) and larger in size, and are cataloged using ground-based sensors.

Fig. 1. Objects by type.

In addition to those debris objects produced as a satellite undergoes a fragmentation, debris have been identified as belonging to solid rocket motor (SRM) exhaust compounds (Al2O3) and paint pigments (surface degradation products). In the case of Aluminum Oxides, the Explorer 46 meteoroid survey satellite observed, with 95% confidence, a correlation between SRM firings and an increase in the incident, directional flux within 20 days of the firing.[7]

Such time-sequenced events may have been observed by the Long Duration Exposure Facility's Interplanetary Dust Experiment[8] and the SkiYMET meteor radars (Ref. X3) as well. Both exhaust products and paint pigments have been identified by scanning electron microscopy and elemental analysis in impact crater residue. Human biological wastes have also been identified by this technique[9], though these particulates should normally be confined to altitudes below about 400 km, the maximum altitude of most manned missions.

Degradation debris have also been measured on-orbit. Also referred to as local contamination, these debris tend to be most prevalent during the first weeks or months of operations; as such, they are similar to "out gassing" effects (Ref. X2).

Once classified by general type, a second objective method of characterizing the man-made population is by size and mass. The following table (after Ref. X4, with updated information) portrays the gross distribution of resident space objects in size and mass.

Size [cm]	Number of Objects	% Number	% Mass
0.1 - 1.0	35,000,000	99.67	0.035
1.0 - 10.0	110,000	0.31	0.035
> 10.0	8000	0.02	99.93
Total	35,118,000	100.0	1,400,000 kg

The categorization by size is not coincidentally broken out in decades of size; objects greater than approximately 10 cm (in low Earth orbit, or LEO) are observed by ground-based sensors, tracked and correlated, and cataloged by agencies performing the space surveillance mission worldwide. Those between 1 and 10 cm may be observed by special radars during statistical data collection campaigns, while those smaller are rarely observed. Rather, objects smaller than 1 mm are typically assessed by counting the number of impact features on surfaces exposed to, and returned from, space.

Yet another means of characterizing the environment is by the spatial density S, i.e. the number of equivalent objects per cubic kilometer. This quantity, derived in a manner analogous to that in the classical theory of gasses, is of great utility as it may be related to both the flux F and the expected collision rate C:

F = S*v [impacts/m2/year]

and

C = F*A = S*v*A [impacts/year],

Where v is the relative velocity between an object (the "target") and the impactor (the "projectile") and A is the area (cross-sectional or surface area) of the target object. The incident flux represents the number of particles striking a surface within a given time; the flux is usually expressed in units of [impacts/m2/yr], but may appear in other units. An excellent analogue for the flux is the amount of water falling on the windshield of a vehicle driving through a rainstorm. The final amount will depend upon the size of the raindrops, or the distribution in size, the velocity of the drops, and the velocity of the vehicle as it drives through the storm. The following figures (after Ref. X5) depict the spatial density of cataloged (> 10 cm in LEO, > approximately 1 m in Geosynchronous Earth orbit, or GEO) objects in LEO and deep space. The reader may mentally multiply the LEO figures by a factor of 300, and the GEO figure by a factor of 50, to obtain the flux at these altitudes.

Fig. 2. The spatial density of equivalent satellite objects in LEO. Altitude divided into 10 km wide altitude bins. Spatial density portrayed on a linear vertical axis to emphasize altitudes of high absolute concentration.

In Figure 2, perhaps the most prominent features are the "spikes" event just below 800 km altitude, and just above 1400 km altitude. These result from the relatively dense packing of specific spacecraft in the Iridium and Globalstar commercial communication satellite constellations, respectively.

Fig. 3. The spatial density of equivalent satellite objects in LEO. Altitude divided into 10 km wide altitude bins. Spatial density portrayed on a logarithmic vertical axis to emphasize distribution by type, altitude, and concentration. Concentration of anomalous debris around 1300 km altitude due to the SNAPSHOT satellite.

Fig. 4. The spatial density of equivalent objects in deep space (here, defined as altitudes above LEO and below GEO). Altitude in 100 km bins. Readily evident are the US and Russian navigation satellite constellations in middle Earth orbit.

Fig. 5. The spatial density of equivalent objects near GEO. Altitude in 100 km bins. "High" and "Low" boundaries define a nominal GEO operational region.

Because these figures only portray those objects capable of being cataloged (with certain exclusions for national security), it is important to recall that these are larger than approximately 10 cm in LEO and larger than 1 m in GEO. Whereas the LEO region is believed to be reasonably complete, this is not the case in deep space, and GEO in particular. Recent measurements (Ref. X6) indicate that a significant population of objects larger than 10 cm reside in the GEO belt. One reason for this may lie in a historical undercounting of objects (primarily operational debris) released in the GEO belt. For example, objects such as solar array retention straps have not been cataloged for many historical payloads.

Unrecognized fragmentations may also have contributed to the GEO local environment. Thus, the GEO environment portrayed in Fig. 5 may substantially be undercounting the actual spatial density/flux.

While these charts depict the distribution of cataloged objects, they are not directly translatable to either a "high quality" flux or a collision rate. In the case of a flux, this is because the relative velocity between two objects depends on the actual orbital properties of the pair of objects involved in any prospective collision. For objects whose orbital planes are randomly distributed with respect to each other and the remainder of the population, these are:

• the apogee (maximum altitude) and perigee (minimum altitude) of each object in the pair; and
• the inclination (the angle between the orbit plane and the Earth's equator) of each object.

Apogee/perigee altitudes determine the velocity, as a function of altitude, of each of the individual objects. For circular orbits, as are the majority in LEO, MEO, and GEO, the orbital velocities of both objects are roughly equal, and collisions on the front and sides surfaces of the "target" object are prevalent. However, if one object is in an elliptical orbit (i.e. a large difference in perigee and apogee altitudes), then (a) the elliptical orbit, at perigee, may be traveling up to 3 km/s faster than the other object, and (b) the object in the elliptical orbit may therefore "catch up" with the other object and strike it from "behind". This is observed on-orbit, as shuttles and other spacecraft flying at 28∞ inclinations commonly return with a multitude on craters on their rearward-oriented surfaces. The inclination is also an important determinant of the outcome of any collision, as certain inclination allow for "head on" collisions at up to 14-15 km/s. Conversely, the uniformly low inclinations found in GEO, along with the coordinated motion of the objects there, tends to lower the relative velocities possible.

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