The Final Days of Mir

Reentry Debris Footprint

Reentry Breakup As Mir descends along its final reentry trajectory, reached either by natural orbit decay for an uncontrolled reentry or deorbiting burns for a controlled reentry, the density of the surrounding atmosphere will steadily increase. The aerodynamic drag force and heating applied to the reentering station will increase as density increases. When the station reaches an altitude of approximately 54 to 59 nmi (100 to 110 km), light, flimsy external elements of Mir, its solar panels and antennas, will break off the main body of the station and begin to melt. Most of this low melting point material will never reach the ground but will disintegrate under reentry heating, posing very little danger to people and property. As Mir descends still lower into Earth's atmosphere, an extremely strong shock wave will form around the station. Traveling at approximately Mach 29, the air density and temperature increases will be extremely high and the station will experience very high heating and drag. At an altitude between 43 and 49 nmi (80 and 90 km), catastrophic breakup of Mir will occur. This is the main breakup event. Due to the aerothermal heating, critical structural components will have melted by this point, causing the station's main body to break apart into its component modules. The breakup process will continue as the altitude decreases. Mir's modules will melt and fragment into still smaller and smaller pieces. At an altitude between 22 and 27 nm, the breakup process will cease. At this point, drag will have decreased the velocities of the debris fragments to such an extent that aerodynamic heating and loads will no longer be sufficient to cause melting and fragmentation. The resulting debris fragments will continue to fall, dispersing into a debris footprint by the breakup process Due to Mir's complicated geometry and structure, the actual dynamics and breakup sequence are impossible to simulate precisely. In addition, the pressurized modules and fuel tanks of the station could very well explode under reentry heating, causing catastrophic breakup of the main body to occur at altitudes greater than 49 nmi. The uncertainty of when and what debris fragments will be formed from breakup, in addition to the associated uncertainty of the shapes and corresponding aerodynamic characteristics of the resulting fragments, leads to corresponding uncertainties in the length and width of the reentry debris footprint for the station. In Mir's case, the total mass of debris fragments expected to survive reentry to impact the earth's surface is estimated to be between 20 and 25 tons. These fragments will generally be made of materials with high melting temperatures, including steel, titanium, high-temperature alloys, illuminators, and optical equipment lenses. Thin-walled fragments, often made of aluminum, which has a low melting temperature, will disintegrate under reentry heating well before they reach the ground. Two recent reentries of Delta 2 second stages, one on January 22, 1997 over North America, and the other on April 27, 2000 west of South Africa, provided excellent examples of reentry debris. In both cases, the 530 lb stainless steel propellant tank, one of four 67 lb titanium pressure gas spheres, and the 97 lb asbestos/fiberglass thrust chamber of the stage were recovered. For more on these reentries, as well as photographs, click here. Debris Footprint Footprint size, both length and width, is strongly influenced by breakup altitude: the higher the catastrophic breakup of the reentering spacecraft, the longer the fall times of the resulting debris fragments. The fragments will then have more time to move both downrange and cross range to the ground trace of the final orbit, resulting in a larger footprint. After breakup, the cloud of debris will be composed of pieces of various sizes, shapes, and weights which are traveling at different velocities. The length of the footprint depends on the sizes, shapes, and properties of the fragments. A light and large debris fragment, such as a solar panel, will decelerate quickly, while a heavy and small object like a battery will continue to move downrange at a high velocity. As a result, the solar panel will travel a shorter downrange distance before impact than the battery. According to the Russian Aviation and Space Agency (RASA), Mir's reentry debris footprint should be about 3240 nmi (6000 km) long and 108 nm (200 km) wide. If RASA's deorbit plan is completely successful, the center of Mir's footprint will be located at about 47 degrees south latitude and 140 degrees west longitude in the south Pacific ocean. An estimation of the debris footprint for Mir in the event of an uncontrolled reentry over the United States is shown here.

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