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Orbital Debris

Orbital debris generally refers to material that is on orbit as the result of space initiatives, but is no longer serving any function. Orbital debris can return to Earth via controlled or planned deorbiting or via uncontrolled deorbiting. Using interceptors from a space-based platform would create orbital debris, from successfully intercepting a threat missile and causing it to break up or from the break up of an unsuccessful interceptor or the space platform. Space-based weapons platforms would contribute to orbital debris while in orbit and upon deorbiting, potentially hitting other satellites in their paths.

The U.S. Air Force Space Command, located inside Cheyenne Mountain AFS, Colorado, tracks objects larger than 10 centimeters (4 inches) in diameter orbiting Earth. Space surveillance conducted by U.S. Space Command includes reentry assessment to predict when and where an object would reenter the Earth's atmosphere. U.S. Space Command does not, however, make surface impact predictions. NASA estimates that there are over 9,000 objects larger than 10 centimeters (4 inches) in diameter in space. The estimated population of particles between 1 and 10 centimeters (0.4 and 4 inches) in diameter is greater than 100,000, and the number of smaller particles probably exceeds tens of millions.

Debris as small as a fleck of paint approximately 0.02 centimeter (0.008 inches) in diameter traveling at a velocity of three to six kilometers per second (two to four miles per second) has been documented to create a 0.5 centimeter (0.2 inch) indention in the windshield of the Space Shuttle. In LEO, an aluminum sphere 0.13 centimeter (0.05 inch) in diameter has damage potential similar to that of a .22-caliber long rifle bullet. An aluminum sphere one centimeter (0.4 inch) in diameter is comparable to a 181-kilogram (400-pound) safe traveling at 97 kilometers per hour (60 miles per hour). A fragment 10 centimeters (3.9 inches) long is roughly comparable to 25 sticks of dynamite. In general, debris smaller than 0.1 centimeter (0.04 inch) in size does not pose a hazard to spacecraft functionality. Debris from 0.1 centimeter (0.04 inch) to one centimeter (0.4 inch) in size may or may not penetrate a spacecraft, depending on material and whether shielding is used. However, penetration through a critical component, such as the flight computer or propellant tank, can result in loss of the spacecraft. Debris fragments between one and 10 centimeters (0.4 and 3.9 inches) in size will penetrate and damage most spacecraft. Astronauts or cosmonauts engaging in extravehicular activities could be vulnerable to the impact of small debris. On average, debris 1 millimeter (0. 04 inch) is capable of perforating current U.S. space suits.

The addition of orbital materials from the operation of space-based weapons would contribute to the accumulation of orbital debris in LEO. Unless reboosted, satellites in orbits at altitudes of 200 to 399 kilometers (124 to 248 miles) reenter the atmosphere within a few months. At orbital altitudes of 399 to 900 kilometers (248 to 559 miles), orbital lifetimes can exceed a year or more depending on the mass and area of the satellite. Above 900-kilometer (559-mile) altitudes, orbital lifetimes can be 500 years or more.

Debris in orbit gradually loses altitude. When orbiting objects enter dense regions of the atmosphere, friction between the object and atmosphere generates heat. This heat can melt or vaporize all or portions of the object resulting in minimal amounts of debris reaching the surface of the Earth. During reentry, the deceleration of the debris creates loads on the structure that can exceed ten times the acceleration of gravity. These loads combine with the high temperature to cause the debris to break apart.

Some debris can survive reentry heating. This occurs if the debris component's melting temperature is high, or if its shape enables it to lose heat fast enough to keep the temperature below the melting point. In general, components made of aluminum and other materials with low melting temperatures do not survive reentry, while components made of materials with high melting temperatures, such as stainless steel, titanium, and glass, often do survive. Large pieces with moderate melting temperatures can also survive reentry, radiating heat over their large surface areas. Pieces that survive reentry tend to be large and in some cases heavy, posing a potential hazard to people and property within the bounds of the object's reentry debris footprint.

When possible, debris impact areas are carefully selected to include deep ocean areas or designated locations on military ranges. However, the majority of orbital debris burns on reentry and thus does not reach the Earth. It is unlikely that the impact of debris associated with an uncontrolled reentry would pose a significant threat to the environment on Earth.

Debris that survives reentry would impact within debris or impact footprints, i.e., the areas on the land or water surfaces that would contain all of the debris pieces. Debris is more likely to terminate in water than on land because water covers 75 percent of the Earth's surface.

It is possible to estimate the size of the impact footprint, but very difficult to predict precisely where the footprint would be on the Earth's surface or where specific pieces of debris would land. After initial and subsequent breakups, surviving pieces of the reentering object would hit down in the debris or impact footprint area. The size of the debris footprint is determined by estimating the breakup altitude of the orbiting object; then by estimating the mass and aerodynamic properties of surviving debris. Heavy debris would generally travel farther downrange within the debris footprint; lighter material would generally fall near the point of intercept. Footprint lengths can vary from 185 to 2,000 kilometers (115 to 1,243 miles), depending on the characteristics and complexity of the object.

The footprint width is generally determined by the impact of wind on the falling debris objects, with heavy objects less affected than lighter ones. The breakup process also may affect the width of the footprint. For example, if the object should explode during reentry, fragments would be spread out across the footprint. A footprint width of 20 to 40 kilometers (12 to 25 miles) is typical, with the most pronounced effects near the part of the footprint closest to the point of intercept.

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