LLR at the Apache Point Observatory (APOLLO project) could improve bounds on the Nordvedt
parameter to the level and on
to better than
[296].
The proposed 2012 ESA Bepi-Columbo Mercury orbiter, in a two-year experiment, with range
capability, could yield improvements in
to
, in
to
, in
to
, in
to
, and in
to
. An eight-year mission could yield
further improvements by factors of 2 - 5 in
,
, and
, and a further factor 15 in
[187, 17].
GAIA is a high-precision astrometric orbiting telescope (a successor to Hipparcos), which could measure
light-deflection and to the
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 to a part
in
and
to a part in
, and to measure the solar frame-dragging effect to one
percent [259, 260].
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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