An Evaluation of Jacchia 71 and MSIS90 Atmosphere Models with NASA ODERACS Decay Data

The Space Shuttle Discovery (STS-60) deployed six Orbital Debris Radar Calibration Spheres (ODERACS 1) on February 9, 1994. A year later on February 3, 1995, three more spheres and three small dipoles (ODERACS 2) were placed in orbit from Discovery (STS-63). The orbital lifetimes of these metal spheres ranged from 8 to 13 months. The continuous NORAD 2-line orbital elements and the known dimension and mass of each sphere provided a good opportunity for the evaluation of atmospheric density models. This paper presents the results of the evaluation of two commonly used atmospheric density models, Jacchia 71 and MSIS90, based on decay histories of these spheres. The NORAD 2-line elements were processed by a semi-analytical computer program LIFETIME, which differentially adjusts the ballistic coefficient used with one of the two atmosphere models, so that the computed orbit agrees with the observed orbit. The daily measurements of solar flux, F10.7, and geomagnetic index, Ap, were used by LIFETIME for atmosphere density prediction. Two sets of drag coefficient, Cd, were deduced from the ballistic coefficients for four altitude bins covering an altitude range from 100 km to 350 km. Physical drag coefficients for spheres were compared with the observed drag coefficients deduced from the NASA data. Results of comparison strongly suggest that the Jacchia 71 density model is 9 to 22% too high in predicting density at the above mentioned altitude range at a time of low solar activity. The results of this study further support the estimated 15% error in Earth atmosphere density models.