A Mission to Test the Pioneer Anomaly

Analysis of the radio tracking data from the Pioneer 10/11 spacecraft has consistently indicated the presence of an anomalous small Doppler frequency drift. The drift can be interpreted as being due to a constant acceleration of aP = (8.74 ± 1.33) × 10 −8 cm/s directed towards the Sun. Although it is suspected that there is a systematic origin to the effect, none has been found. The nature of this anomaly has become of growing interest in the fields of relativistic cosmology, astroand gravitational physics as well as in the areas of spacecraft design and high-precision navigation. We present a concept for a designated deep-space mission to test the discovered anomaly. A number of critical requirements and design considerations for such a mission are outlined and addressed. PACS: 04.80.-y, 95.10.Eg, 95.55.Pe Email addresses: [email protected], [email protected], [email protected] 1 The Pioneer Missions and the Anomaly The Pioneer 10/11 missions, launched on 2 March 1972 (Pioneer 10) and 4 Dec 1973 (Pioneer 11), were the first to explore the outer solar system [1]. After Jupiter and (for Pioneer 11) Saturn encounters, the two spacecraft followed escape hyperbolic orbits near the plane of the ecliptic to opposite sides of the solar system. Pioneer 10 eventually became the first man-made object to leave the solar system. Pioneer 10’s radio signal is weakening. Despite this, the Deep Space Network (DSN) is still able to deliver navigational data from distances ∼80 AU. Indeed, on the 30th anniversary of its launch, 2 March 2002, the Madrid DSN station received a return radio signal from Pioneer 10 at 22:47 CET. This was 22 h 06 m after the uplink signal was sent from the Goldstone, CA, DSN station. By 1980, when Pioneer 10 was at a distance of ∼ 20 AU from the Sun, the acceleration contribution from solar-radiation-pressure on Pioneer 10 (directed away from the Sun) decreased to < 5 × 10 cm/s. At that point the navigational data began to clearly indicate the presence of an anomaly in the Doppler navigational data, which was later interpreted as a constant acceleration, aP , directed toward the Sun [2]. Recently, we published a detailed study of the Pioneer anomaly, which used the existing Pioneer 10/11 Doppler data from 1987.0 to 1998.5. We specifically addressed all possible sources for a systematic origin for the detected anomaly. Our conclusion was that, even after all known systematics are accounted for, there remains an anomalous acceleration signal of aP = (8.74± 1.33)× 10 −5 cm/s directed towards the Sun [3]. We emphasize “known” because we must admit that the most likely cause of the effect is some as yet not understood systematic generated by the spacecraft themselves, perhaps caused by excessive heat or propulsion gas leaks. But neither we nor others with spacecraft or navigational expertise have been able to find it [3]. Further, due to its different mission and spacecraft designs, as well as its proximity to the Sun, the use of the Cassini spacecraft to test for the anomaly proved to be impractical.