Space Pollution

In the most general sense, the term space pollution includes both the natural micrometeoroid and man-made orbital debris components of the space environment; however, as “pollution” is generally considered to indicate a despoiling of the natural environment, space pollution here refers to only man-made orbital debris. Orbital debris poses a threat to both manned and unmanned spacecraft as well as the earth’s inhabitants.

Environmental and Health Impacts

The effects of debris on other spacecraft range from surface abrasion due to repeated small-particle impact to a catastrophic fragmentation due to a collision with a large object. The relative velocities of orbital objects (10 kilometers per second [km/s] on average, but ranging from meters per second up to 15.5 km/s) allow even very small objects—such as a paint flake—to damage spacecraft components and surfaces.

For example, a 3-millimeter (mm) aluminum particle traveling at 10 km/s is equivalent in energy to a bowling ball traveling at 60 miles per hour (or 27 m/s). In this case, all the energy would be distributed in an area of the same size as the particle, causing cratering or penetration, depending on the thickness and material properties of the surface being impacted.

There has been one accidental collision between cataloged objects to date, but surfaces returned from space and examined in the laboratory confirm a regular bombardment by small particles. Space Shuttle vehicle components, including windows, are regularly replaced due to such damage acquired while in orbit. Debris also poses a hazard to the surface of the Earth.

High-melting-point materials such as titanium, steel, ceramics, or large or densely constructed objects can survive atmospheric reentry to strike the earth’s surface. Although there have been no recorded fatalities or severe injuries due to debris, reentering objects are regularly observed and occasionally found.

Debris is typically divided into three size ranges, based on the damage it may cause: less than 1 centimeter (cm), 1 to 10 cm, and larger than 10 cm. Objects less than 1 cm may be shielded against, but they still have the potential to damage most satellites. Debris in the 1 to 10 cm is not shielded against, cannot easily be observed, and could destroy a satellite.

Finally, collisions with objects larger than 10 cm can break up a satellite. Of these size ranges, only objects 10 cm and larger are regularly tracked and cataloged by surveillance networks in the United States and the former Soviet Union.

The other populations are estimated statistically through the analysis of returned surfaces (sizes less than 1 mm) or special measurement campaigns with sensitive radars (sizes larger than 3 mm). Estimates for the populations are approximately 30 million debris between 1 mm and 1 cm, over 100,000 debris between 1 and 10 cm, and 8,800 objects larger than 10 cm.

The number, nature, and location of objects greater than 10 cm in size are provided in the fragmentation debris table and in the image of space debris around Earth. Low Earth orbit (LEO) is defined as orbital altitudes below 2,000 km above the earth’s surface and is the subject of the image of space debris around Earth.

Middle Earth orbit (MEO) is the province of the Global Positioning System (GPS) and Russian navigation satellite systems and is located at approximately 20,000-km altitude, whereas the geosynchronous Earth orbit (GEO) “belt” is inhabited primarily by communications and Earth—observation payloads around 35,800 km. The majority of objects in these orbital regions are in circular or near-circular orbits about the earth.

In contrast, the elliptical orbit category includes rocket bodies left in their transfer (payload delivery) orbits to MEO and GEO as well as scientific, communications, and Earth-observation payloads. Of all objects listed in the fragmentation debris table, the vast majority are “debris”—only about 5 percent of objects in orbit represent operational payloads or spacecraft.

Also, of the approximately 28,000 objects that have been tracked, beginning with the launch of Sputnik 1 in October 1957, those not accounted for in the fragmentation debris table have either reentered the earth’s atmosphere or have escaped the earth’s influence (to land on Mars, for example). The distribution of debris smaller than 10 cm is predicated on the orbits of the parent objects and is assumed to be very similar to the distributions presented in the image of space debris around Earth.

Remediation Strategies

Remediation takes two courses: protection and mitigation. Protection seeks to shield spacecraft and utilize intelligent design practices to minimize the effects of debris impact. Mitigation attempts to prevent debris from being created. Active mitigation techniques include collision avoidance between tracked and maneuverable objects and the intentional reentry of objects over the oceans.

Passive techniques include venting residual fuels or pressurized vessels aboard rockets and spacecraft, retaining operational debris, and placing spacecraft into disposal orbits at the end of a mission. Space salvage or retrieval, while an option, is currently too expensive to employ on a regular basis.

The United States and international space agencies recognize the threat of debris and are cooperating to limit its environmental and health hazards. The Interagency Space Debris Coordination Committee (IADC), sponsored originally by the National Aeronautics and Space Administration (NASA), has grown to include all major space-faring nations.

The IADC charter includes the coordination and dissemination of remediation research, and strategies based on research results are being adopted by the worldwide space community.

Remediation strategies have resulted in a decline in the rate of debris growth in the 1990s although the overall population continues to grow. Continued work is necessary, however, to reduce the orbital debris hazard for future generations and continue the safe, economical utilization of space.