A Measurement of Planetary Relativistic Deflection

The radio interferometric observations described here constitute the first measurement of the deflection of electromagnetic radiation by a planetary gravitational field. They also demonstrate a few hundred microarcsecond (µarcsec) angular astrometric technique, which can be applied over many-degree fields of sky. The ray path of the radio source P0201+113 passed within 200 arcsec of Jupiter (~10 Jovian radii) on 21 March 1988. Its angular position was measured 10 times over 4 hr on that date, with a similar measurement set on 2 April 1988 to detect the differential angular deflection of the ray path. According to general relativity, the expected gravitational bend of the ray path averaged over the March experiment duration was approximately 300 µarcsec, projected onto the two California-Australia baselines over which it was measured. Measurement accuracies of the order of 160 µarcsec were obtained for each of the ten differential measurements. The χ^2^ per degree of freedom of the data for the hypothesis of general relativity was 0.6, suggesting that the modeled dominant errors due to system noise and tropospheric fluctuations fully accounted for the scatter in the measured angular deflections. The χ^2^ per degree of freedom for the hypothesis of no gravitational deflection by Jupiter was 4.1, which rejects the no-deflection hypothesis with greater than 99.999% confidence. The system noise contributed about 70 µarcsec per combined-baseline differential measurement and tropospheric fluctuations contributed about 145 µarcsec. Unmodeled errors were assessed which could potentially increase the 160 µarcsec error by about 8%.