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Orbital Debris Program Office

Evolutionary Models

LEGEND - NASA's Three-Dimensional Orbital Debris Evolutionary Model

LEGEND is a full-scale, three-dimensional, debris evolutionary model that is the NASA Orbital Debris Program Office’s primary model for study of the long-term debris environment. It covers the near-Earth space between 200 and 50,000 km altitude, including low Earth orbit (LEO), medium Earth orbit (MEO), and geosynchronous orbit (GEO) regions. The model provides debris characteristics (number, type, size distribution, spatial density distribution, velocity distribution, flux, etc.) as functions of time, altitude, longitude, and latitude. In addition, LEGEND includes both historical simulation and future projection components. Populations included in the model are active and spent satellites, rocket bodies, breakup fragments, mission-related debris, and Sodium-Potassium (NaK) droplets, making it possible for the minimum size (diameter) threshold in the model to be as small as 1 mm.

The main function of the LEGEND historical component is to reproduce the historical debris environment (1957 to the present) to validate the techniques used for the future projection component of the model. The model uses a recently updated historical satellite launch database (DBS database), two efficient state-of-the-art propagators (PROP3D and GEOPROP), and the NASA Standard Breakup Model. Both the overall program structure and the input and output options are optimized to allow reasonable execution time and manageable file storage and memory space. The program is capable of providing multi-dimensional descriptions of the debris environment, while additional modules have been developed to better analyze and visualize the model outputs.

Chart showing number of objects > 10 cm in LEO
LEGEND Three-Dimensional Orbital Debris Evolutionary Model

The main function of the LEGEND future projection component is to provide an understanding of how the orbital debris environment evolves in the future. It is also a reliable tool to examine how various mitigation practices may help protect the environment. A key element in the LEGEND future projection component is a three-dimensional evaluation model that provides a fast and accurate way to estimate future on-orbit collisions from LEO to GEO. Since no assumptions regarding the right ascensions of the ascending node and arguments of perigee of objects involved are required, this probability model captures the collision characteristics in real three-dimensional physical space. It is a critical component of a true three-dimensional debris evolutionary model.

The typical projection period in LEGEND is 100 years. Due to uncertainties involved in the process (e.g., future launch traffic, solar activity, explosions, collisions), conclusions are usually drawn based on averaged results from 100 Monte Carlo simulations.

The main program and subroutines of LEGEND are written in standard FORTRAN 90 language. Additional analysis tools are written in FORTRAN 90 and MATLAB.