6.8 A mission to explore the Pioneer anomaly

The apparent inability to explain the anomalous behavior of the Pioneers with conventional physics has contributed to growing discussion about its origin. A number of researchers emphasized the need for a new experiment to explore the detected signal. As a result, [391Jump To The Next Citation Point] advocated for a program to study the Pioneer anomaly. This program effectively includes three phases:
Analysis of the entire set of existing Pioneer data, obtained from launch to the last useful data received from Pioneer 10 in April 2002. This data provides critical new information about the anomaly [379391Jump To The Next Citation Point]. If the anomaly is confirmed,
Development of an instrument, to be carried on another deep space mission, to provide an independent confirmation for the anomaly. If further confirmed,
Development of a dedicated deep-space experiment to explore the Pioneer anomaly with an accuracy for acceleration resolution at the level of 10–12 m/s2 in the extremely low frequency (or nearly DC) range.

Significant progress was accomplished concerning the first and second phases [260305379394]. Work on the third phase was also initiated [55106Jump To The Next Citation Point309379391Jump To The Next Citation Point]. Mission studies conducted in 2004 – 2005 [106Jump To The Next Citation Point] have identified two options: i) an experiment on a major mission to deep space capable of reaching acceleration sensitivity similar to that demonstrated by the Pioneers and ii) a dedicated mission to explore the Pioneer anomaly that offers full characterization of the anomaly. Below we discuss both of these proposals.

6.8.1 A Pioneer instrument package

A way to confirm independently the anomaly is to fly an instrumental package on a mission heading to the outer regions of the solar system. The primary goal is to provide an independent experimental confirmation of the anomaly. One can conceive of an instrument placed on a major mission to deep space. The instrument must be able to compensate for systematic effects to an accuracy below the level of 10–10 m/s2. Another concept is a simple autonomous probe that could be jettisoned from the main vehicle, such as the proposed Interstellar Probe27, presumably further out than at least the orbit of Jupiter or Saturn. The probe would then be navigated from the ground yielding a navigational accuracy below the level of 10–10 m/s2. The data collected could provide an independent experimental verification of the anomaly’s existence.

Dittus et al. [106Jump To The Next Citation Point] emphasized that the option of an instrument on a major mission to deep space would have a major impact on spacecraft and mission designs with limited improvement in measuring aP. Nevertheless, a highly-accurate accelerometer has been proposed as part of the Gravity Advanced Package, which is a fundamental physics experiment that is being considered by the ESA for the future Jupiter Ganymede Orbiter Mission28. At the same time, it is clear that to explore the anomaly one needs to travel beyond the orbit of Jupiter. Furthermore, an acceleration sensitivity at the level of ∼ 10–12 m/s2 would be preferable, which can be done only with a dedicated mission, as discussed in Section 6.8.2 below.

6.8.2 A dedicated mission concept

The available knowledge of the Pioneer anomaly lead to the following science objectives for a dedicated mission to explore the Pioneer anomaly: i) investigate the origin of the anomaly with an improvement by a factor of 1,000; ii) improve spatial, temporal, and directional resolution; iii) identify and measure all possible disturbing and competing effects; iv) test Newtonian gravity potential at large distances; v) discriminate amongst candidate theories explaining aP, and vi) study the deep-space environment in the outer solar system.

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Figure 6.1: A drawing for the measurement concept chosen of the Deep Space Gravity Probe (from [106Jump To The Next Citation Point], drawing courtesy of Alexandre D. Szames). The formation-flying approach relies on actively controlled spacecraft and a set of passive test-masses. The main objective is to accurately determine the heliocentric motion of the test-mass by utilizing the 2-step tracking needed for common-mode noise rejection purposes. The trajectory of the spacecraft will be determined using standard methods of radiometric tracking, while the motion of the test mass relative to the spacecraft will be established by laser ranging technology. The test mass is at an environmentally quiet distance from the craft, ≥ 250 m. With occasional maneuvers to maintain formation, the concept establishes a flexible craft to test mass formation.

A viable concept would utilize a spacecraft pair capable of flying in a flexible formation (see Figure 6.1View Image). The main craft would have a precision star-tracker and an accelerometer and would be capable of precise navigation, with disturbances, to a level less than ∼ 10–10 m/s2 in the low-frequency acceleration regime. Mounted on the front would be a container holding a probe – a spherical test mass covered with corner cubes. Once the configuration is on its solar system escape trajectory and will undergo no further navigation maneuvers, and is at a heliocentric distance of ∼ 5 – 20 AU, the test mass would be released from the primary craft. The probe will be passively laser-ranged from the primary craft with the latter having enough capabilities to maneuver with respect to the probe, if needed. The distance from the Earth to the primary would be determined with either standard radiometric methods operating at Ka-band or with optical communication. Note that any dynamical noise at the primary would be a common mode contribution to the Earth-primary and primary-probe distances. This design satisfies the primary objective, which would be accomplished by the two-staged accurate navigation of the probe with sensitivity down to the 10–12 m/s2 level in the DC of extremely low frequency bandwidth.

Since the small forces affecting the motion of a craft in four possible directions all have entirely different characteristics (i.e., sunward, earthward, along the velocity vector or along the spin-axis [260391Jump To The Next Citation Point]), it is clear that an antenna with a highly directional radiation pattern along with star sensors will create even better conditions for resolving the true direction of the anomaly when compared to standard navigation techniques. On a craft with these additional capabilities, all on-board systematics will become a common mode factor contributing to all the attitude sensors and antennas. The combination of all the attitude measurements will enable one to clearly separate the effects of the on-board systematics referenced to the direction towards the Sun.

To enable fast orbital transfer to distances greater than 20 AU, hyperbolic escape trajectories enabled by solar sail propulsion technology were considered as an attractive candidate. Among other options is a standard chemical rocket and nuclear electric propulsion, as was successfully demonstrated recently [260]. The proposed combination of a formation-flying system aided by solar sail propulsion for fast trajectory transfer, leads to a technology combination that will benefit many missions in the future [389399].

Two missions were recently proposed to explore the Pioneer anomaly in a dedicated space experiment. The Solar System Odyssey mission will use modern-day high-precision experimental techniques to test the laws of fundamental physics, which determine dynamics in the solar system [78]. The mission design is similar to the one proposed for the Deep Space Gravity Probe (DSGP) [106]. Also, the proposed SAGAS (Search for Anomalous Gravitation using Atomic Sensors) mission [424] aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a solar system escape trajectory. SAGAS has numerous science objectives in solar system exploration and fundamental physics, including an accurate test of the Pioneer anomaly.

The extraordinary nature of the Pioneer anomalous acceleration led to serious questions concerning the possible origin of the effect. Answering these questions requires further in-depth analysis. This is especially true before any serious discussion of a dedicated experiment can take place. In fact, prior to the development of any dedicated mission to investigate the Pioneer anomaly, it is absolutely essential to analyze the complete Pioneer Doppler data in order to rule out, as much as possible, any engineering cause. This study is on-going and will be reviewed in Section 7.

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