Atmospheric drag measurements around 1500 km during solar cycle 24

The semi-empirical atmospheric density models widely used by the space community were mainly developed taking into account satellite drag measurements and other observations, either in situ and ground based, acquired at relatively low altitudes, mostly below 500-600 km, and in general below 1000 km. The launch of the Italian geodetic satellite LARES, in 2012, at the altitude of about 1450 km and with an inclination of 70 degrees, offered however the rare possibility of probing the atmosphere at such height. This spherical satellite, fully covered with corner-cube laser retro-reflectors, has the highest area-to-mass ratio of any artificial object launched so far, being therefore not well suited for detecting small non-gravitational forces, like atmospheric drag. However, the very high accuracy of its orbit determinations, made possible by the laser tracking technique, more than compensated its unfavorable area-to-mass ratio, and the signature of atmospheric drag was extremely evident in the measured semi-major axis decay. Such decay, observed since 2012, was therefore used to infer the neutral atmosphere drag at the height of LARES during a 7-year span of solar cycle 24, covering the solar maximum, the declining phase and the beginning of the minimum. These measurements were compared with the predictions of six semi-empirical density models (JR-71, MSIS-86, MSISE-90, NRLMSISE-00, GOST-2004, and JB2008), employed well outside of their typical application ranges. In general, their predictions resulted quite satisfactory, with uncertainties not so far from those already known at lower altitudes. This study was also supplemented by the simultaneous analysis of another spherical geodetic satellite, the Japanese Ajisai, just 50 km higher, but with an area-to-mass ratio nearly 20 times greater than that of LARES and a smaller inclination of 50 degrees. An attempt was also made to estimate the physical drag coefficients of both satellites, in order to derive the mean density biases of the models. None of them could be considered unconditionally the best, the specific outcome depending on solar activity and on the regions of the atmosphere crossed by the satellites. Moreover, during solar maximum conditions, an additional density bias, probably linked to the different high latitudes overflown by the satellites, was detected.