http://arxiv.org/abs/1602.05949
General relativity is supported by great experimental evidence. Yet there is a lot of interest in precisely setting its limits with on going and future experiments. A question to answer is about the validity of the Strong Equivalence Principle. Ground experiments and Lunar Laser Ranging have provided the best upper limit on the Nordtvedt parameter $\sigma[\eta]=4.4\times 10^{-4}$. With the future planetary mission BepiColombo, this parameter will be further improved by at least an order of magnitude. In this paper we envisage yet another possible testing environment with spacecraft ranging towards the nearby Sun-Earth collinear Lagrangian points. Neglecting errors in planetary masses and ephemerides, we forecast $\sigma[\eta]=6.4\text{-}2.0\times10^{-4}$ (5 yr integration time) via ranging towards $L_1$ in realistic and optimistic scenarios depending on current and future range capabilities and knowledge of the Earth’s ephemerides. A combined measurement, $L_1$+$L_2$, gives instead $4.8\text{-}1.7\times10^{-4}$. In the optimistic scenario a single measurement of one year would be enough to reach $\approx3\times10^{-4}$. All figures are comparable with Lunar Laser Ranging, but below BepiColombo. Performances could be much improved if data were integrated over time and over the number of satellites flying around either of the two Lagrangian points, possibly reanalysing data of past missions. We point out that some systematics (gravitational perturbations of other planets or figure effects) are much more in control compared to other experiments. We do not advocate a specific mission to constrain the Strong Equivalence Principle, but we do suggest analysing ranging data of past and future spacecrafts flying around $L_1$/$L_2$. This spacecraft ranging would be a new and complementary probe to constrain the Strong Equivalence Principle in space.
G. Congedo and F. Marchi
Fri, 19 Feb 16
18/50
Comments: 8 pages, 2 figures, 2 tables
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