The LARES mission revisited: an alternative scenario

In the original LARES mission, the general relativistic Lense-Thirring effect was to be detected using as an observable the sum of the residuals of the nodes of the existing passive geodetic laser-ranged LAGEOS satellite and of its proposed twin LARES. The proposed nominal orbital configuration of the latter would reduce the systematic error due to the mismodelling in the even zonal harmonics of the geopotential, which is the main source of error (to 0.3%) according to the most recent Earth gravity model EGM96. This observable turns out to be sensitive to possible departures of the LARES orbital parameters from their nominal values due to the orbital injection errors. By adopting a suitable combination of the orbital residuals of the nodes of LAGEOS, LAGEOS II and LARES and the perigees of LAGEOS II and LARES, it should be possible to reduce the error due to the geopotential by one order of magnitude, according to the EGM96 model. Moreover, the sensitivity to the orbital injection errors should be greatly reduced. According to a preliminary estimate of the error budget, the total error of the experiment should be reduced to less than 1%. In the near future, when the new data on, the terrestrial gravitational field from CHAMP and GRACE missions become available, a further increase in the accuracy should be obtained. The proposal to place LARES in a polar 2000 km altitude orbit and consider only its nodal rate would present the drawback that even small departures from the polar geometry would yield notable errors due to the mismodelled even zonal harmonics of the geopotential, according to the EGM96 model.