ASTRA is working to understand thermospheric neutral density variations for Earth and Mars via modeling and data analysis. On both planets, thermospheric density is important because of its effects on satellites. Changes in the density of the neutral atmosphere create variable satellite drag, adversely affecting missions like maneuver planning, re-entry prediction, collision avoidance, risk analysis, and identification and tracking of objects in space using narrow field-of-view sensors.
Predicting the orbits of the shuttle, space station and the debris field in LEO requires accurate (to better than 5%) specification of neutral densities. The current capability of semi-empirical models is an orbit-averaged standard deviation of ~15%-30%, although the assimilation of orbital information from dozens of satellites has recently demonstrated that this can be improved to 5% (in the 300-500 km altitude range). Existing empirical density models do not adequately account for dynamic changes in neutral density, leading to errors in predicted satellite positions. The major discrepancies are at high latitudes, where extreme density variability is caused by the varying auroral inputs.
The US Strategic Command (USSTRATCOM) tracks about 8,500 man-made space objects orbiting Earth that are at least 10 centimeters in diameter. These space objects consist of active/inactive satellites, spent rocket bodies, or fragmentation. A major reason for tracking orbiting objects is for manned space flight safety and the protection of space assets. For example, as of January 2003, the space station had been reoriented six times to avoid collisions with orbiting debris. During shuttle missions, USSTRATCOM computes possible close approaches of other orbiting objects with the shuttle’s flight path, and it is not uncommon for the Shuttle to maneuver to avoid predicted conjunctions with space debris.