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The Pioneer anomaly or Pioneer
effect is the observed deviation from predicted trajectories and velocities of various unmanned spacecraft visiting the outer solar system, most notably Pioneer 10 and Pioneer 11.
Both Pioneer spacecraft are escaping from the solar system, and
are slowing down under the influence of the Sun's gravity. Upon
very close examination, however, they are slowing down slightly
more than expected. The effect can be modeled as a slight
additional acceleration towards the Sun.
The anomaly has no universally accepted explanation. The
explanation may be mundane, such as measurement error, thrust from gas leakage or uneven
radiation of heat. However, it is also possible
that current physical theory does not correctly explain the
behaviour of the craft relative to the sun.
The effect is seen in radio Doppler and ranging data, yielding
information on the velocity and distance of the spacecraft. When
all known forces acting on the spacecraft are taken into
consideration, a very small but unexplained force remains. It
appears to cause a constant sunward
(8.74 ± 1.33) × 10−10 m/s2
for both spacecraft. If the positions of the spacecraft are
predicted one year in advance based on measured velocity and known
forces (mostly gravity), they are actually found to be some
400 km closer to the sun at the end of the year. The magnitude
of the Pioneer effect is numerically quite close to the product of
the speed of
light and the Hubble constant,
but the significance of this, if any, is unknown. Gravitationally
bound objects such as the solar system, or even the galaxy, do not
partake of the expansion of the universe —
this is known both from theory and by
Data from the Galileo and Ulysses spacecraft indicate a
similar effect, although for various reasons (such as their
relative proximity to the Sun) firm conclusions cannot be drawn
from these sources. These spacecraft are all partially or fully spin-stabilised.
The effect is much harder to measure accurately with craft that
use thrusters for attitude control. These spacecraft, such as
the Voyagers, acquire small and
unpredictable changes in speed as a side effect of the frequent
attitude control firings. This 'noise' makes it impractical to
measure small accelerations such as the Pioneer effect.
The Cassini mission has reaction wheels
as well as thrusters for attitude control, and during cruise could
rely for long periods on the reaction wheels alone, thus enabling
precision measurements. However, it also had radioisotope thermoelectric generators
(RTGs) mounted close to the spacecraft body, radiating kilowatts of
heat in hard-to-predict directions. The measured value of
unmodelled acceleration for Cassini is
(26.7 ± 1.1) × 10−10 m/s2,
roughly three times as large as the Pioneer acceleration.
Unfortunately, the measured value is the sum of the uncertain
thermal effects and the possible anomaly. Therefore the Cassini
measurements neither conclusively confirm nor refute the existence
of the anomaly.
- observational errors, including
measurement and computational errors, in deriving the acceleration.
- Approximation/statistical errors
- Significant errors in computation are not likely since (at
current count) 7 independent analyses have shown the effect.
- a real deceleration not accounted for in the
current model, such as:
- gravitational forces from unidentified
sources such as the Kuiper belt or dark matter. However, an acceleration does
not show up in the orbits of the outer planets, so any generic
gravitational answer would need to violate the equivalence principle  (see
modified inertia below). Likewise, the anomaly does not appear in
the orbits of Neptune's moons, challenging the possibility that the
Pioneer anomaly may be an unconventional gravitational
- drag from
the interplanetary medium, including
dust, solar wind and cosmic rays. However,
the measured densities are too small to cause the effect.
- gas leaks, including helium from the spacecrafts' radioisotope
thermoelectric generators (RTGs)
- radiation pressure of sunlight, the
transmissions, or thermal radiation pressure from the
RTGs (See Radioisotope rocket), or
asymmetrical radiation of the heat from the spacecraft electronics,
reflecting from the back of the spacecraft’s dish-like main
antenna, causing a recoil like sunlight striking a solar sail.
- The pressure of sunlight is too small at this distance, and
points into the wrong direction. The same applies to the
spacecraft's radio emissions.
- The others are prime suspects, as presented at the second ISSI
meeting in Berne, Feb 2007.
- A recent presentation at the APS April 2008 meeting
suggests that differential heating may account for as much of 1/3
rd of the observed acceleration.
- electromagnetic forces due to an
charge on the spacecraft
- New physics
- clock acceleration between coordinate or Ephemeris time
and International Atomic
- A modification of the law of gravity. The theory MOND (Modified Newtonian
Dynamics) proposes that the force of gravity deviates to a very
different force law at very low accelerations of order:
10−11 m/s2 from the traditional
- Modified inertia. MOND can also be
interpreted as a modification of inertia, perhaps due to an
interaction with vacuum energy and such a
trajectory-dependent theory could account for the different
accelerations apparently acting on the orbiting planets and the
Pioneer craft on their escape trajectories.
More recently, a model for modified inertia using Unruh radiation and a
Hubble-scale Casimir effect has been proposed to
explain the Pioneer anomaly,
and a possible test for evidence of modified inertia on Earth has
It has also been suggested that a modification of inertia can
explain the flyby
Investigation of the Pioneer
The effect is so small that it could be a statistical anomaly
caused by differences in the way data were collected over the
lifetime of the probes. Numerous changes were made over this
period, including changes in the receiving instruments, reception
sites, data recording systems and recording formats.
The Planetary Society announced
on June 1, 2006 that 30 years of Pioneer data had been saved. It
announced on March 28, 2007 that analysis of the data was underway.
On March 19, 2008, it announced that one source of acceleration,
uneven thermal radiation had been found to explain some of the
deviation, but not all.
It is possible, but not proven, that this anomaly is linked to
Although the circumstances are very different (planet flyby vs.
deep space cruise), the overall effect is similar - a small but
unexplained velocity change is observed on top of a much larger
conventional gravitational acceleration.
The Pioneer spacecraft are no longer providing new data, (the
last contact having been on 23 January 2003) and
Galileo was deliberately burned up in
Jupiter's atmosphere at the end of its mission. So far, attempts to
use data from current missions such as Cassini have not
yielded any conclusive results. There are several remaining options
for further research:
- Further analysis of the retrieved Pioneer data.
- The New
Horizons spacecraft to Pluto is spin-stabilised for much
of its cruise, and there is a possibility that it can be used to
investigate the anomaly. New Horizons may have the same
problem that precluded good data from the Cassini mission
- its RTG is mounted close to the spacecraft body, so thermal
radiation from it, bouncing off the spacecraft, may produce a
systematic thrust of a not-easily predicted magnitude, several
times as large as the Pioneer effect. Nevertheless efforts are
underway to study the non-gravimetric accelerations on the
spacecraft, in the hopes of having them well modeled for the long
cruise to Pluto after the Jupiter fly-by that occurred in February
2007. In particular, despite any large systematic bias from the
RTG, the 'onset' of the anomaly at or near the orbit of Saturn
might be observed.
- A dedicated mission has also been proposed. Such
a mission would probably need to surpass 200AU from the Sun in a hyperbolic escape orbit.
- Observations of asteroids around 20AU may provide insights if
the anomaly's cause is gravitational.
- In November 2009, ESA's Rosetta spacecraft measured its
velocity during flyby to within a few millimeters per
second in an effort to explain the flyby anomaly. 
However, no anomaly was found on this most recent flyby. 
conferences about the anomaly
A meeting was held at the University of Bremen in 2004 to
discuss the Pioneer anomaly.
The Pioneer Explorer Collaboration was formed to study the
Pioneer Anomaly and has hosted three meetings (2005, 2007, and
2008) at International Space
Science Institute in Berne, Switzerland to discuss the anomaly, and
discuss possible means for resolving the source.
- John D. Anderson, Philip A.
Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, Slava
G. Turyshev (1998). "Indication, from Pioneer
10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak,
Long-Range Acceleration". Phys. Rev.
Lett. 81: 2858–2861. doi:10.1103/PhysRevLett.81.2858. http://prola.aps.org/abstract/PRL/v81/i14/p2858_1.
- The original paper describing the anomaly
- A lengthy survey of several years of debate by the authors of
the original 1998 paper documenting the anomaly. The authors
conclude, "Until more is known, we must admit that the most likely
cause of this effect is an unknown systematic. (We ourselves are
divided as to whether 'gas leaks' or 'heat' is this 'most likely
Noerdlinger, P.D. and Petrosian, V.,
(1971). "The Effect of Cosmological
Expansion on Self-Gravitating Ensembles of Particles,". The
Astrophysical Journal, 168,: pp. 1,. http://adsabs.harvard.edu/full/1971ApJ...168....1N.
Williams, J.G. and Turyshev, S.G. and
Boggs, D.H., (2004,). "Progress in Lunar Laser
Ranging Tests of Relativistic Gravity," (PDF). Physical
Review Letters, (APS,) 93, (26,):
pp. 261101,. http://iau-comm4.jpl.nasa.gov/JGWSGTDHB.pdf.
John D. Anderson, Eunice L. Lau, Giacomo
Giampieri. "Improved Test of General
Relativity with Radio Doppler Data from the Cassini Spacecraft"
corresponding arXiv pre-print John D.
Anderson, Eunice L. Lau, Giacomo Giampieri. "Improved Test of General Relativity with Radio
Doppler Data from the Cassini Spacecraft". http://arxiv.org/abs/gr-qc/0308010v2.
Pioneer Anomaly Project
Update: A Letter From the Project Director by Slava G.
Turyshev, March 28, 2007
'(Preprint) Can the Pioneer anomaly be of
gravitational origin? A phenomenological answer' by Lorenzo
'Does the Neptunian system of satellites challenge a
gravitational origin for the Pioneer anomaly?' by Lorenzo
Pioneer spacecraft a step
closer to being boring, April 13th, 2008 David Harris
Antonio F. Ranada (10 January 2005). "The Pioneer anomaly as acceleration of the
clocks". http://arxiv.org/abs/gr-qc/0410084. Retrieved
- ^ Jacob D. Bekenstein. "The modified Newtonian dynamics-MOND-and its
implications for new physics". Contemporary Physics 47, 387
(2006). http://arxiv.org/abs/astro-ph/0701848. Retrieved
- ^ M.Milgrom. "The Modified Dynamics as a vacuum effect".
Phys. Lett. A253 (1999). http://arxiv.org/abs/astro-ph/9805346. Retrieved
- ^ M.E.McCulloch. "Modelling the Pioneer anomaly as modified
inertia". MNRAS 376, 338-342 (2007). http://arxiv.org/abs/astro-ph/0612599. Retrieved
- ^ A.Yu.Ignatiev. "Is violation of Newton's second law possible?".
Phys. Rev. lett. 98, 101101 (2007). http://arxiv.org/abs/gr-qc/0612159. Retrieved
- ^ M.E.McCulloch. "Modelling the flyby anomalies using a modification
of inertia.". MNRAS-letters, 389(1), L57-60. http://arxiv.org/abs/0806.4159.
"The Pioneer Anomaly: Changing
the Laws of Physics?". September 10, 2007. http://www.planetary.org/radio/show/00000253/.
Anomaly". The Planetary Society. May 19, 2008. http://www.planetary.org/programs/projects/pioneer_anomaly/update_20080519.html.
NASA Baffled by Unexplained
Force Acting on Space Probes
The Pioneer Missions. NASA.
Michael Martin Nieto. "New Horizons and the Onset of the Pioneer
"Pioneer anomaly put to the
test". September 1, 2004. http://physicsworld.com/cws/article/print/20123.
Stuart Clark (10 May 2005). "Lost asteroid clue to Pioneer
puzzle". New Scientist. http://www.newscientist.com/article.ns?id=mg18624984.700.
"Can Minor Planets be Used to Assess Gravity in the
Outer Solar System?". 2 Jan 2006. http://arxiv.org/abs/astro-ph/0504367.
"Conference on The Pioneer Anomaly - Observations,
Attempts at Explanation, Further Exploration". http://www.zarm.uni-bremen.de/Pioneer/.
- ^ "The Pioneer Explorer
Collaboration: Investigation of the Pioneer Anomaly at ISSI".
February 18, 2008. http://www.issi.unibe.ch/teams/Pioneer/. Retrieved January 10,
The ISSI meeting above has an excellent reference list divided
into sections such as primary references, attempts at explanation,
proposals for new physics, possible new missions, popular press,
and so on. A sampling of these are shown here:
- Scientific American, vol 279, #6, December 1998,
- (preprint) A Mission
to Test the Pioneer Anomaly, Anderson et al., Int.J.Mod.Phys. D11 (2002)
-  A Route to
Understanding of the Pioneer Anomaly, Slava G. Turyshev,
Michael Martin Nieto, and John D. Anderson (2004)
- (preprint) A Mission
to Explore the Pioneer Anomaly, Dittus et al. (2005)
- Finding the origin of the Pioneer anomaly, Nieto &
Turyshev (2004), Class. Quantum Grav.
- Further elaboration on a dedicated mission plan (restricted
- (preprint) Utilizing
Minor Planets to Assess the Gravitational Field in the Outer Solar
System, Page et al., 2005
- "Opening New Doors", Seattle
Conventional Forces can Explain the Anomalous Acceleration of
Pioneer 10, Scheffer, Phys.Rev. D67 (2003)
084021. One of several arguments that the "Pioneer Anomaly" can
be well explained by conventional physics.
- (preprint) Using Early
Data to Illuminate the Pioneer Anomaly, M. M. Nieto, J. D.
- Scientific American, vol 293, #4, October 2005,
- J. R. Brownstein and J. W.
Moffat (2006). "Gravitational solution to the
Pioneer 10/11 anomaly". Classical and Quantum Gravity
23: 3427–3436. doi:10.1088/0264-9381/23/10/013.
- Masreliez C. J., The Pioneer Anomaly - A
cosmological explanation. (2005) Ap&SS, v. 299, no. 1,
- John Anderson, "Is there something we don't know about
gravity?", Astronomy Magazine, vol
37, # 3, March 2009, pp. 22–27.