Planetary and Lunar Ephemerides: testing alternate gravitational theories

The planetary and lunar ephemerides have been improved over the past century by many orders of magnitude. It is expected that they will improve again into the fairly near future. The improvements come from more accurate observational data, from increased modelling of the gravitational equations of motion, and from the ability to numerically integrate the equations of motion. Present-day ephemerides have the accuracy to be able to test a number of proposed possible modifications to the presently accepted laws of gravitation. In the future, with further improved accuracies, the ephemerides will be able to test even more subtle modifications. Experiments show that the Voyager range residuals at Uranus and Neptune, reported by Anderson et al. (1995) [1], are fully consistent with other observational data and with present-day ephemerides. The modification to the laws of gravitation which is usually quoted to explain the Pioneer Anomaly, that of a constant acceleration of 8.74 x 10(-10) m/sec(2) directed toward the sun, added to each planet's acceleration, has been shown to be completely inconsistent with the planetary ephemerides. A suggested alternative form, where the acceleration is proportional to the planet's radial velocity, is marginally acceptable, but only when applied to just the planets past Saturn. A further form, where the acceleration is proportional to the planet's radial velocity squared, is acceptable to the planets from Saturn and outward. In none of the cases that were tested, can the force modification be applied to Jupiter: the changes in the solution parameters used to adjust the ephemerides become totally unacceptable.