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3.8 Prospects for improved PPN parameter values

A number of advanced experiments or space missions are under development or have been proposed which could lead to significant improvements in values of the PPN parameters, of J2 of the Sun, and of G˙∕G.

LLR at the Apache Point Observatory (APOLLO project) could improve bounds on the Nordvedt parameter to the level 3 × 10–5 and on ˙ G∕G to better than 10–13 yr–1 [296].

The proposed 2012 ESA Bepi-Columbo Mercury orbiter, in a two-year experiment, with 6 cm range capability, could yield improvements in γ to 3 × 10–5, in β to 3 × 10–4, in α1 to 10–5, in G˙∕G to 10–13 yr–1, and in J2 to 3 × 10–8. An eight-year mission could yield further improvements by factors of 2 – 5 in β, α 1, and J2, and a further factor 15 in ˙ G ∕G [18717].

GAIA is a high-precision astrometric orbiting telescope (a successor to Hipparcos), which could measure light-deflection and γ to the 10–6 level [101]. It is planned for launch by ESA in the 2011 time frame.

LATOR (Laser Astrometric Test of Relativity) is a concept for a NASA mission in which two microsatellites orbit the Sun on Earth-like orbits near superior conjunction, so that their lines of sight are close to the Sun. Using optical tracking and an optical interferometer on the International Space Station, it may be possible to measure the deflection of light with sufficient accuracy to bound γ to a part in 108 and J2 to a part in 108, and to measure the solar frame-dragging effect to one percent [259260].

Nordtvedt [202] has argued that “grand fits” of large solar system ranging data sets, including radar ranging to Mercury, Mars, and satellites, and laser ranging to the Moon, could yield substantially improved measurements of PPN parameters. A recent contribution in that direction is [212].

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