Pioneer 10

1
Two way Doppler residuals for Pioneer 10
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.
Anderson et al. Phys Rev. Lett. 81 2858-61
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
(8.741.33)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.
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.
Pioneer Collaboration, preprint arxiv
gr-qc/0506139
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 10-15m/s2.
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.
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.
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.
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.
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.