Preliminary orbits with over-determined systems of Keplerian conservation laws

Abstract

We consider different choices of Keplerian conservation laws for the computation of preliminary orbits with two very short arcs (VSAs) of astrometric observations. In total we have 7 equations in 4 unknowns. Adding two auxiliary variables we can embed the full set of conservation laws into a polynomial system of 9 equations. This complete system generically has no solutions. However, combining these equations, in [10] the authors found an over-determined polynomial system that is consistent, and leads by variable elimination to a univariate polynomial $\mathsf{p}_9$ of degree 9 in one radial distance. In [9] the authors showed that this corresponds to taking a subsystem with 7 equations of the complete system. In this paper we consider all the other possibilities and we find two additional over-determined cases which are consistent and lead to a univariate polynomial $\mathsf{p}_{18}$ of degree 18 in the same variable as $\mathsf{p}_9$. In the other over-determined cases the corresponding system is inconsistent. We also present a method to compute an approximate gcd of $\mathsf{p}_9$ and $\mathsf{p}_{18}$, that can allow us to find preliminary orbits that approximately satisfy inconsistent systems of conservation laws, or to discard incompatible pairs of VSAs. We show this through some numerical tests with real asteroid data.