Return of the Pioneer Anomaly

One of the many pieces of evidence put forward for the existence of Planet X over the last few decades is the so-called ‘Pioneer anomaly’.  The two Pioneer spacecraft were sent on an incredible voyage across the solar system, visiting a number of planets as they went.  They not only imaged these planets, but used the gravity of the planets to accelerate onwards, deeper into the solar system.  This gravity assist is often used to allow spacecrafts to pick up speed.  As the Pioneer probes travelled across the outer planetary zone and on towards the Heliopause beyond in the 1990s, it became apparent that the craft were not moving away from the solar system quite as quickly as the theoretical trajectory projections demanded.  Something was essentially slowing them down.  Additionally, similar effects were noted for the Galileo and Ulysses probes.

Many ideas were put forward, including either gravitational or physical interaction with clouds of interplanetary dust in the Kuiper Belt, or even the added gravitational tug of an undiscovered Planet X body.  One of the lead researchers into the Pioneer anomaly at the Jet Propulsion Laboratory  was John Anderson (1), who, interestingly, also had a longstanding interest in the possible existence of a Planet X body (2).  At one point, puzzled physicists began to wonder whether this marginal but definitive anomaly might require new laws of physics (3).  In the end, it was agreed by technical experts that the anomalous deceleration was a result of radiation pressure caused by non-uniform heat loss from the probes (4,5). Flights of fancy about missing planets and new physics were promptly put to bed.

Despite this, the anomaly seems to persist in the increasingly accurate navigation and telemetry data returning from various spacecraft performing flybys past the Earth (6).  Similarly, the Juno spacecraft, now orbiting fairly closely around Jupiter, is reported to be slightly misplaced from its expected position (7).  This has been determined by looking at the Doppler shift of ranging data from the probe as it circumnavigated the poles of the great gas giant.  Quixotically, Juno did not exhibit the same anomalous behaviour during a previous flyby of Earth.  This suggests that this is not, then, the result of an internal machination of the probe itself, as described for the Pioneer probes.  Instead, there does appear to be an unexplained external effect worth exploring:

Another mystery is that while in some cases the anomaly was clear, in others it was on the threshold of detectability or simply absent – as was the case with Juno‘s flyby of Earth in October of 2013. The absence of any convincing explanation has led to a number of explanations, ranging from the influence or dark matter and tidal effects to extensions of General Relativity and the existence of new physics. However, none of these have produced a substantive explanation that could account for flyby anomalies.” (8) 

I was intrigued by two possibilities.  The first is that the gravitational tug experienced by these spacecraft is ever so slightly affected by a non-uniform cross-section of the planet.  In other words, the Earth might provide a slightly variation in gravitational attraction, local to a spacecraft flying over it, depending upon whether the interior mass of our planet is slightly offset – or, at least, has areas of high interior density which are not uniformly distributed.  While I could see this being a possibility for a rocky planet like Earth, with its diversity of internal composition and layers and overall non-spherical ‘geoid’ shaping, it’s more difficult to see how that might be the case for a gas giant, like Jupiter, whose interior would surely be more uniformly distributed.  Indeed, the team studying the trajectories of Juno’s polar orbits of Jupiter accounted for Jupiter’s multipolar fields, which are the result of the planet’s oblate shape.

A look through research literature on the subject of how planetary gravitational fields vary with planetary structure and shape quickly descends into a world of mathematical pain.  Surely, somewhere in the discussion of zonal gravity coefficients and spherical harmonics lie the solution to these flyby issues?  I wrote to Dr Acedo to enquire further, and received this helpful response to my question:

(1) Could these anomalous accelerations be due to a non-uniform planetary interior creating slight gravitational differences affecting the craft during a flyby?

“Of course, they could arise from the longitudinal non-uniformities in the gravitational field of the planets. These are characterized by the so-called tesseral harmonics and I showed that a rotating planet with longitudinal non-uniformities can transfer energy to the spacecraft in a previous paper (9).  However, they are not enough (in the case of the Earth, at least) to explain away the anomalies. For Jupiter, It is still an open question and we have to await for a full analysis of the Juno’s orbital data at JPL because the conclusions in the paper are based on preliminary fits and our knowledge of Jupiter’s gravitational field is much more incomplete than that of the Earth.” (10)

My second thought is that probes moving through trajectories that are not along the plane of the ecliptic might be more susceptible to slight gravitational perturbations created by unknown planets which themselves lie substantially off the ecliptic.  So, for instance, the proposed Planet Nine body (11) is thought to be inclined from the ecliptic by thirty degrees or so.  This might drive anomalies in the solar system’s overall angular momentum vectoring away from the general plane of the planets.  For this to even be a possibility, it would be crucial to know whether these effects are more likely to happen with polar flybys than with equatorial ones – in other words, is there an effect felt perpendicular to the ecliptic?  Again, I asked Dr Acebo as to whether this was the case:

(2) Do these anomalies show up more with polar flybys … or, at least, flybys which are not sticking to the ecliptic plane?

“Yes, polar flybys seems more prone to show large anomalies than equatorial orbits but this is a very qualitative classification. For example, the largest anomalies were found for the NEAR flyby on 1998 whose orbit was close to be contained in a plane perpendicular to the equator so the orbit orientation seems to play a role in the magnitude and sign of the anomalous velocity change. Altitude over the Earth surface is also an important factor.” (10)

That seems to keep the possibility alive, at least, although it’s far from conclusive evidence of a distant gravitational tug.  But how massive would a distant planet need to be to provide even this slight deviation?  Too large, according to Dr Acedo, in further correspondence with me:

“…the effect of a hypothetical ‘Planet Nine’ in the interior Solar System is not easy to measure and it has no significant contribution to the flyby anomaly. In the Astrophys. Space Sci. paper I considered the effect of the tidal forces exerted by Jupiter on a flyby of the Earth and dismissed it because its contribution to the anomalous velocity change is not comparable with the observations (6), the same can be said of ‘Planet Nine’. I also remember a paper by Lorenzo Iorio in which he studied the perturbations of a new planet in connection with the lingering anomaly of the Moon’s orbit varying eccentricity (12). But to explain this he concluded that it would require a Jupiter-like planet at 200 AU that it is much larger than the estimated size of ‘Planet Nine’. Summarizing, anomalies in the inner solar system (if they are real and not the result of measurement and modelling errors) cannot be accounted by new trans-neptunian planets.” (13)

These probes which have exited the solar system offer so much beyond the glorious array of images they sent back during their main missions.  In the case of Pioneer, questions were raised (and seemingly continue to be raised) about fundamental physics.  The tenacity of the Voyagers can be seen through their ability to respond to new thruster demands decades after these systems went into hibernation, potentially extending their missions even further (14).  Finally, after its flyby of Pluto and Charon, the New Horizons probe is set to approach the Kuiper Belt Object known as 2014 MU69 late next year, making its flyby on 1st January 2019.  Originally thought to be about 20 miles across, recent attempts to analyse the size of 2014 MU69 as it passed in front of a background star proved difficult.  Although detected by several groups of astronomers during this transit, 2014 MU69 appears to either much smaller than originally thought, or could be a binary object, or even a swarm of smaller objects (53).  These exiles from our solar system continue to surprise.

Written by Andy Lloyd,  5th December 2017


1) John Anderson et al “Study of the anomalous acceleration of Pioneer 10 and 11” April 2001, Physical Review D 65(8)

2)  John Noble Wilford ‘Looking for Planet X:  Old clues, new theory’ 1st July 1987

3) Robert Matthews “Mysterious force holds back Nasa probe in deep space” 10th February 2002

4)  Edward Murphy ‘Prosaic Explanation for the Anomalous Accelerations Seen in Distant Spacecraft’ Phys. Rev. Lett., 30th August 1999, 83, 1890

5)  B. Rievers & C. Lämmerzahl “High precision thermal modelling of complex systems with application to the flyby and Pioneer anomaly” 5th May 2011, Annalen der Physik. 523 (6): 439.

6)  Luis Acedo ‘Anomalous accelerations in spacecraft flybys of the Earth’ November 2017, Astrophysics and Space Science 362(12)

7)  L. Acedo, P. Piqueras and J. Morano “A possible flyby anomaly for Juno at Jupiter” Universitat Politecnica de Valencia, 27th November 2017

8) RAS-WEB “Juno Isn’t Exactly Where it’s Supposed To Be. The Flyby Anomaly is Back, But Why Does it Happen?” The Renfrewshire Astronomical Society, with thanks to Lee

9)  L. Acedo “On the effect of ocean tides and tesseral harmonics on spacecraft flybys of the Earth” MNRAS, Vol 463, 2, p.2119-2124,

10)  Correspondence from Luis Acedo, 11th December 2017

11)  K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” 20th January 2016, The Astronomical Journal, Volume 151, Number 2,

12)  Lorenzo Iorio “On the anomalous secular increase of the eccentricity of the orbit of the Moon” Monthly Notices of the Royal Astronomical Society, 1st February 2011, 415: pp1266-1275,

13)  Correspondence from Luis Acedo, 12th December 2017

14)  Leah Crane “NASA fires Voyager 1’s engines for the first time in 37 years” 5th December 2017 with thanks to Monika

15)  Jay Bennett “New Horizons Spacecraft On Its Way to Mysterious Kuiper Belt Object” 7th July 2017

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