The LAser Ranged Satellites Experiment (LARASE) has the goal to carry out a sequence of measurements of relativistic effects using specific Earth-orbiting satellites for verifying the predictions of General Relativity and of alternatives theories proposed for the description of the gravitational interaction. These measurements are achieved in the so-called weak-field and slow-motion (WFSM) limit of the theory, by means of passive "cannonball" satellites tracked very precisely with the Satellite Laser Ranging (SLR) technique through the International Laser Ranging Service (ILRS).
The three satellites used in the LARASE experiment were chosen for the very good quality of their orbit determination, for the smallness of non-gravitational perturbations and for the accuracy of the dynamical model. They are the twin LAser GEOdynamics Satellites (LAGEOS and LAGEOS II), launched, respectively, in 1976 and in 1992, and LARES (LAser RElativity Satellite). LARES was launched from Kourou, in French Guiana, on 13 February 2012, with the qualification flight of the VEGA launcher. With a radius of 18.2 cm and a mass of 386.8 kg, this completely passive sphere, made of tungsten alloy and uniformly hosting on its surface 92 corner cube laser retro-reflectors, is the densest object in orbit in the Solar System, with an area-to-mass ratio of 2.7 × 10E-4 sqm/kg. Placed into a nearly circular orbit at a mean geodetic altitude of about 1454 km and with an inclination of 69.5 degrees, LARES was conceived with the goal of measuring the Lense-Thirring effect induced by the rotating mass of the Earth and predicted by General Relativity with an accuracy of the order of 1%.
The scientific objectives of the LARES mission are therefore very ambitious. Certainly, the achievement of these objectives would be facilitated if, besides the high quality of the laser tracking of the satellite and its extremely low value for the area-to-mass ratio, it were possible to improve the dynamical models describing its motion, and that of the LAGEOS satellites, particularly concerning the subtle effects of non-gravitational perturbations. The very low area-to-mass ratio of LARES was chosen to reduce as much as possible the disturbing effects of non-gravitational perturbations. However, because of its height, much lower than the about 5800-5900 km of 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 with the GEODYN II (NASA/GSFC) code, it was found an average semi-major axis decay rate of 1 m per year, corresponding to a non-conservative net force acting nearly opposite to the velocity vector of the satellite and with a mean along-track deceleration of 1.4 × 10E-11 meters per second squared.
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 (JR-71, MSIS-86, MSISE-90, NRLMSISE-00 and GOST-2004). All of them provided consistent results, well within their acknowledged uncertainties. Moreover, when the same models (JR-71 and MSIS-86) were used within GEODYN II in a least-squares fit of the tracking data, the differences between the average drag coefficients estimated with SATRAP and GEODYN were of the order of 1% or less.
Unlike what happened for the two LAGEOS, where Yarkovsky thermal drag and charged particle drag were the leading causes, it was found that neutral atmosphere drag alone was able to explain most (about 98.6%) of the observed semi-major axis decay of LARES. The remaining 1.4%, corresponding to an average along-track deceleration of about 2 × 10E-13 meters per second squared (i.e. approximately 1/70 of neutral drag), was probably linked to thermal thrust effects. It was 50%, or less, of the value previously reported in the literature, but further and more detailed investigations, including the detection of the signature of the periodic terms, will be needed in order to characterize such smaller non-gravitational perturbation.
Authors: Carmen Pardini, Luciano Anselmo, David Lucchesi, Roberto Peron
Contact point: Carmen Pardini (carmen.pardini@isti.cnr.it)
Principal Investigator of the LARASE experiment: David Lucchesi (david.lucchesi@iaps.inaf.it)
Website of the LARASE experiment: larase.roma2.infn.it/index.php/larase-description
Acknowledgements: the work was in part supported by the Commissione Scientifica Nazionale II (CSNII) on Astroparticle Physics Experiments of the Istituto Nazionale di Fisica Nucleare (INFN).
Reference: Pardini C., Anselmo L., Lucchesi, D.M., Peron, R., On the Secular Decay of the LARES Semi-major Axis, Acta Astronautica, 140 (2017) 469-477, doi: 10.1016/j.actaastro.2017.09.012.
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