In this paper we investigate the opportunities offered by the new Earth
gravity models from the dedicated CHAMP and, especially, GRACE missions to the
project of measuring the general relativistic Lense-Thirring effect with a new
Earth's artificial satellite. It turns out that it would be possible to abandon
the stringent, and expensive, requirements on the orbital geometry of the
originally prosed LARES mission (same semimajor axis a=12270 km of the existing
LAGEOS and inclination i=70 deg) by inserting the new spacecraft in a
relatively low, and cheaper, orbit (a=7500-8000 km, i\sim 70 deg) and suitably
combining its node Omega with those of LAGEOS and LAGEOS II in order to cancel
out the first even zonal harmonic coefficients of the multipolar expansion of
the terrestrial gravitational potential J_2, J_4 along with their temporal
variations. The total systematic error due to the mismodelling in the remaining
even zonal harmonics would amount to \sim 1% and would be insensitive to
departures of the inclination from the originally proposed value of many
degrees. No semisecular long-period perturbations would be introduced because
the period of the node, which is also the period of the solar K_1 tidal
perturbation, would amount to \sim 10^2 days. Since the coefficient of the node
of the new satellite would be smaller than 0.1 for such low altitudes, the
impact of the non-gravitational perturbations of it on the proposed combination
would be negligible. Then, a particular financial and technological effort for
suitably building the satellite in order to minimize the non-conservative
accelerations would be unnecessary.Comment: LaTex2e, 28 pages, 2 tables, 8 figures. To appear in New Astronom