Global average mass density at 400 km (near the altitude of the ISS) . The density data were derived from the orbits of thousands of objects.
Global average mass density at 400 km (near the altitude of the ISS) . The density data were derived from the orbits of thousands of objects.

Objectives
Develop methods for extracting thermospheric density from orbital drag and understand the response of the thermosphere to solar and lower atmospheric influences.
The thermosphere (90-800 km) is the operating environment of spacecraft in low Earth orbit (LEO); the drag it exerts on LEO objects is the largest source of uncertainty for orbit determination and prediction. The thermosphere expands in response to heating by solar UV irradiance and magnetospheric energy and contracts when these energy inputs are low. Thermospheric density also depends strongly on dynamics, chemical composition, and cooling efficiency at altitudes of 90-150 km.

Approach
Data and Models:

  • Routine DoD orbit data from U.S. Space Command
  • NRLMSISE-00 empirical model of thermospheric temperature and composition
  • NRLSSI specification of solar spectral UV irradiance

Method:

  • Changes in orbital period are proportional to integrated mass density along orbit. Many objects are combined to specify spatial and temporal dependence of density
  • Statistical analysis of data to determine response of the thermosphere to key drivers

Deliverable/Value/Accomplishment

  • Delivered 45-year database of thermospheric global-average density derived from thousands of objects
  • Result: Density is decreasing by 2-5% per decade in response to increased CO2 cooling (72 citations; media coverage)
  • Result: Density during unusually quiet 2008 solar minimum was the lowest on record, and 36% lower than previous minimum. Only a 16% change is expected by current theory (20 citations in first year; media coverage)
  • These basic research results demonstrate need for improved density specification in orbit prediction and in orbital debris remediation planning