Neutral atmosphere drag at the altitude of lares and Ajisai

Precise orbit determinations of the first 3.7 years (6 April 2012 - 25 December 2015) of LARES, a small laser-ranged spherical satellite with the lowest area-to-mass ratio among artificial objects, placed into a nearly circular orbit with an altitude of about 1454 km and with an inclination of 69.5°, revealed an average semi-major axis decay rate of approximately 1 meter per year. It corresponded to a non-conservative net force acting nearly opposite to the velocity vector of the satellite and with a mean along-track acceleration of â^'1.444 Ã-- 10 â^'11 m/s 2 . In spite of the smallness of this acceleration, the extremely good orbit determinations available made possible the use of LARES as a powerful probe for investigating the neutral atmosphere behavior at an altitude for which the dominant atomic species are helium and hydrogen, and accurate satellite measurements are scarce. By means of an ad hoc software code, the neutral drag perturbation acting on LARES was then investigated with six thermospheric density models (JR-71, MSIS-86, MSISE-90, NRLMSISE-00, GOST-2004 and JB2008), allowing the quantitative estimation of the acceleration components, and detailing both the secular and main periodic contributions. Adding to this, the analysis was further extended by studying the orbital decay of another passive spherical satellite, Ajisai, just 40 km higher than LARES, but at an inclination of 50° and with an area-to-mass ratio about 20 times greater. Based on the available data and results, it was concluded that nearly 99% of the observed secular semi-major axis decay of LARES was due to neutral atmosphere drag. This was fully consistent with the predictions, uncertainties and range of applicability of the thermospheric density models used. The comparative analysis of the neutral atmosphere drag acting on Ajisai and LARES also revealed an average drag acceleration higher than expected in the latter case, using the same models, specifically JR-71, NRLMSISE-00 and GOST-2004. Several mechanisms were investigated in order to account for these differences, leaving latitudinal atmospheric density biases as the most probable cause. However, further investigations will be needed to assess whether these results are applicable to JB2008 as well, or, more generally, to atmospheric density models employing a wider set of solar and geomagnetic activity indices. A significant role of charged particle drag at the altitudes considered was anyhow excluded.