  The laser-ranged satellite LARES is expected to provide new refined
measurements of relativistic physics, as well as significant contributions to
space geodesy and geophysics. The very low area-to-mass ratio of this passive
and dense satellite was chosen to reduce as much as possible the disturbing
effects of non-gravitational perturbations. However, because of its height,
about 1450 km compared with about 5800-5900 km for the two LAGEOS satellites,
LARES is exposed to a much stronger drag due to neutral atmosphere. From a
precise orbit determination, analyzing the laser ranging normal points of LARES
over a time span of about 3.7 years, it was found an average semi-major axis
decay rate of -0.999 m per year, corresponding to a non-conservative net force
with a mean along-track acceleration of -1.444 x 10^-11 m/s^2. By means of a
modified version of the SATRAP (ISTI/CNR) code, the neutral drag perturbation
acting on LARES was evaluated over the same time span, taking into account the
real evolution of solar and geomagnetic activities, with five thermospheric
density models. All of them were able to model most of the observed semi-major
axis decay, with differences among the average drag coefficients smaller than
10%. A further independent check carried out analyzing the orbital decay of a
passive spherical satellite (Ajisai) just 40 km higher than LARES, it was then
concluded that some of the currently best models developed for neutral
atmosphere, within their uncertainties and range of applicability, were able to
account for most (about 98.6%) of the observed semi-major axis decay of LARES.
Finally, after modeling the neutral atmosphere drag, a residual semi-major axis
decay, corresponding to an average along-track acceleration of about -2 x
10^-13 m/s^2 (i.e. about 1/72 of neutral drag), was detected as well.
