The Pioneer Anomaly and Its Implications

The Pioneer 10/11 spacecraft yielded the most precise navigation in deep space to date. However, their radio-metric tracking data has consistently indicated the presence of a small, anomalous, Doppler frequency drift. The drift is a blue-shift, uniformly changing with a rate of ∼ 6× 10 Hz/s and can be interpreted as a constant sunward acceleration of each particular spacecraft of aP = (8.74±1.33)×10 −10 m/s. The nature of this anomaly remains unexplained. Here we summarize our current knowledge of the discovered effect and review some of the mechanisms proposed for its explanation. Currently we are preparing for the analysis of the entire set of the available Pioneer 10/11 Doppler data which may shed a new light on the origin of the anomaly. We present a preliminary assessment of such an intriguing possibility. 1 The Pioneer Missions and the Anomaly The Pioneer 10/11 missions, launched on 2 Mar 1972 (Pioneer 10) and 5 Apr 1973 (Pioneer 11), respectively, were the first spacecraft to explore the outer solar system (Anderson et al., 2002a). Their objectives were to conduct, during the 1972-73 Jovian opportunities, exploratory investigation beyond the orbit of Mars of the interplanetary medium, the nature of the asteroid belt, and the environmental and atmospheric characteristics of Jupiter and Saturn (for Pioneer 11). After Jupiter and Saturn encounters, the craft followed escape hyperbolic orbits near the plane of the ecliptic to opposite sides of the solar system, continuing on their extended missions. Pioneer 10 eventually became the first man-made object to leave the solar system. The last coherent telemetry data point was obtained from Pioneer 10 on 27 Apr 2002 when the craft was 80 AU from the Sun. (Pioneer 11 sent its last useful data on 1 Oct 1990 while at ∼ 30 AU from the Sun.) The Pioneers were excellent for precise celestial mechanics experiments (Turyshev et al., 2005a). However, by 1980, when Pioneer 10 had already passed a distance of ∼ 20 AU from the Sun and the acceleration contribution from solar-radiation pressure on the craft (away from the Sun) had decreased to less than 4 × 10 m/s, the anomalous acceleration (towards the Sun) became evident in the data. The detailed study of this anomaly (Anderson et al., 2002a) led to a better understanding of its properties, as summarized in the next section. 1.1 Summary of the Pioneer Anomaly The analysis of the Pioneer 10 and 11 data (Anderson et al., 1998, 2002a) demonstrated the presence of an anomalous, Doppler frequency blue-shift drift, uniformly changing with a rate of ḟP ∼ 6× 10 −9 Hz/s (Turyshev et al., 2005a). To understand the phenomenology of the effect, consider fobs, the frequency of the re-transmitted signal observed by a DSN antenna, and fmodel, the predicted frequency of that signal. The observed, two-way (round-trip) anomalous effect can be expressed to first order in v/c as [fobs(t)− fmodel(t)]DSN = −2ḟP t, with fmodel being the modeled frequency change due to conventional forces influencing the spacecraft’s motion. After accounting for the gravitational and other large forces included in standard orbit determination programs this translates to [fobs(t)− fmodel(t)]DSN = −f0 2aP t c . (1) Here f0 is the reference frequency (Anderson et al., 2002a). Furthermore, after accounting for all known (not modeled) sources of systematic error (discussed in Anderson et al. (2002a,b)), conclusion reached was that there exists an anomalous sunward constant acceleration signal of aP = (8.74 ± 1.33) × 10 −10 m/s. (2) For the most detailed analysis of the Pioneer anomaly to date, Anderson et al. (2002a) used the following Pioneer 10/11 Doppler data: • Pioneer 10: The data set had 20,055 data points obtained between 3 Jan 1987 and 22 Jul 1998 and covering heliocentric distances ∼ 40 − 70.5 AU. • Pioneer 11: The data set had 10,616 data points obtained between 5 Jan 1987 to 1 Oct 1990 and covering heliocentric distances ∼ 22.42 − 31.7 AU. By now several studies of the Pioneer Doppler navigational data have demonstrated that the anomaly is unambiguously present in the Pioneer 10 and 11 data. These studies were performed with three independent (and different!) navigational computer programs (Anderson et al., 1998, 2002a; Markwardt, 2002). Namely: • the JPL’s Orbit Determination Program (ODP) developed in 1980-2005, • The Aerospace Corporation’s CHASPM code extended for deep space navigation (Anderson et al., 1998, 2002a), and finally • a code written in the Goddard Space Flight Center (Markwardt, 2002) that was used to analyze Pioneer 10 data for the period 1987-1994 obtained from the National Space Science Data Center (http://nssdc.gsfc.nasa.gov/). The recent analyses of the Pioneer 10 and 11 radio-metric data (Anderson et al., 1998, 2002a; Markwardt, 2002; Turyshev et al., 2005a) have established the following basic properties of the Pioneer anomaly: • Direction: Within the 10 dbm bandwidth of the high-gain antennae, aP behaves as a line-of-sight constant acceleration of the craft toward the Sun. • Distance: It is unclear how far out the anomaly goes, but the Pioneer 10 data supports its presence at distances up to ∼70 AU from the Sun. The Pioneer 11 data shows the presence of the anomaly as close in as ∼20 AU. • Constancy: Both temporal and spatial variations of the anomaly’s magnitude are of order 10% for each craft, while formal errors are significantly smaller. This information was used as guidance in investigating the applicability of proposals to explain the Pioneer anomaly using both conventional and ‘new’ physical mechanisms. In the next section we will briefly review these proposals.