System Design Assessment for a Mission to Explore the Pioneer Anomaly

Jonathan FittSchool of Physics and Astronomy, University of Birmingham, Edgbaston, UK

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Comparison of Test Mass Observable Between Grav and Non-Grav

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Pioneer 10 & 11

Pioneer 10 was launched in March 1972 and Pioneer 11 was launched in April 1973, both probes encountered Jupiter and then went on to leave the solar system in the late 1990s.

Contact was lost with Pioneer 10 in 2002 and Pioneer 11 in 1995.

Both craft were of a simple spin stabilised design making them ideal for celestial mechanics experiments having an acceleration sensitivity of ~10-10m/s2.

The Pioneer Anomaly

The Pioneer craft were tracked using radiometric techniques. The returned Doppler shift of the radio carrier wave was analysed for a period between 1987 and 1998 and compared to that predicted from orbital determination programs based on current gravitational theory.

The radiometric data compared very well to that predicted apart from a small constant Doppler frequency drift which has been represented as a Doppler residual. This residual has been interpreted as a constant acceleration acting on the Pioneer craft of (8.74±1.33)x10-10 m/s2 in a direction towards the sun.

This anomalous acceleration is known as the Pioneer Anomaly and at the moment there is no substantiated scientific explanation for the anomalous acceleration.

A New Mission

One proposal to explore the Pioneer Anomaly is to use a primary spacecraft with a free flying test mass in the shadow of it.

This design eliminates the possibility that any onboard systematic forces could be responsible for the Anomaly by making a range measurement from the Earth to the test mass via the primary, thus any movement in the primary is removed through vector addition of the Earth/primary range and the primary/test mass range.

Mass/Power Budget

The designed system mass/power budget. The system values which have been entered into the table are those which have come about from the work carried out in this project, they are necessary for making the required range measurements. The power usages and masses are design guidelines to illustrate the feasibility of the mission being run as part of the medium class explorer spacecraft series.

There is a proposed mass budget of 139 Kg for the structure and thermal components of the spacecraft, along with a power margin of 63.5 W, which must include power necessary for thermal control.

Earth/Test Mass Range Residual, including Environmental Effects and the PCS

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Range Measurements

Because the Pioneer Anomaly is potentially slowing the spacecraft down it is reducing their velocity. Therefore it is reasonable to assume that the Pioneer Anomaly can also be inferred from collecting range residuals; the spacecraft would be out of position from that predicted by orbital models.

A range model presented here to the right is based on the primary/test mass system proposed, for an observation time of 80 days. The curvature of the plot can be inferred as the Pioneer Anomaly and the project model shows that the acceleration accuracy is ~10-

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Doppler Velocity at S-band of Pioneer 10 between 1987 to 1998

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Original Results

Presented to the left is a model of the original results. The model is derived from published information about the Pioneer Anomaly.

On the far left is a published graph of the original data used to infer the Pioneer Anomaly. It is the gradient of the graph which can be interpreted as an anomalous acceleration in the direction towards the sun; ap.

Positioning Control System

In order to make the range measurement the primary craft must track the test mass so the range between the primary and the test mass is continually recorded. There will be environmental effects moving the primary away from its optimal position.

The project has devised a Positioning Control System (PCS) capable of negating these environmental effects. The fuel usage of the system was modelled for the duration of the mission. The PCS could potentially use any of the thruster systems highlighted in the bar chart to the left.

The project has initially chosen to use FEEP thrusters because of their low fuel consumption and high level of accuracy.

Operation of PCS

It is proposed that the PCS will operate in what is known as a ‘dumb’ mode. Every half a day the thrusters will fire to move the primary craft 0.96 m towards the sun, this, according to the model of environmental forces, should reset the primary/test mass range back to 1000 m.

The ‘dumb’ mode is in contrast to an ‘intelligent’ PCS system which would compare every primary/test mass range measurement to the optimal 1000 m value and if there was a difference operate the PCS until the primary/test mass is rest to optimal.

Motion of the Primary/Test Mass System

The equation to the left is an expression for the acceleration of the test mass relative to the primary. The unknown force is potentially providing what is known as the Pioneer Anomaly. The environmental force is primarily due to solar radiation pressure.

Taking this equation it follows that if the Pioneer Anomaly is gravitational and obeys the equivalence principle then the only forces contributing to relative test mass motion are those due to the environmental.

If the Pioneer Anomaly does not obey the equivalence principle and for example contributes to an inverse mass dependent acceleration then the relative motion of the test mass towards to the primary will become sizeable, affecting the performance of the system in exploring the anomalous acceleration.

A Strategy for Determining the Origin

By analysing just the vector addition of the Earth/primary range it would be inherently difficult to make a firm conclusion about the origin of the Pioneer Anomaly.

By analysing the primary/test mass range directly a conclusion on whether or not the Anomaly is gravitational can easily be obtained. The chart above shows how the primary/test mass range would appear if the Anomaly is gravitational (red line), and non-gravitational (blue line); a clear difference can be seen.

This analysis relies on the PCS system operating in a ‘dumb’ mode so as not to eliminate the relative test mass motion caused by a non-gravitational anomaly.

Mass/Power Budget

Subsystem Component Mass (kg) Power (w) Subsystem Component Mass (kg) Power (w) Total (W)TT&C Thermal 0Science Transponder 3 -12.9 0

Antenna 10 0TWTA 2.6 -51.6 0Laser 4.7 -16.8 0Laser reciever 2 -10.8 0Laser range unit 2 -10.8 Structure 0

Test mass 20.6 0Power 0

GPHS-RTG 55 290 Propulsion 00

Command/Data Handling 0Solid State Memory 1.64 -6.8 Fuel 0Solid State Memory 1.64 -6.8 0

Attitude Complete FEEP system 57 -110 00

Total Mass (Kg) 160.18 Total Power (W) 63.5

Anderson et al. Phys Rev. Lett. 81 2858-61

Two way Doppler residuals for Pioneer 10

Pioneer Collaboration, preprint arxiv: gr-qc/0506139