The Pioneer Anomaly
Mystery Force Baffles NASA

The Pioneer Anomaly
Mystery Force Baffles NASA

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.

Pioneer 10 and 11 were sent on missions to Jupiter and Jupiter/Saturn respectively. The spacecraft were spin-stabilised in order to keep their main antennas pointed towards Earth using gyroscopic forces. Although the spacecraft included thrusters, these were left unused after their primary encounters, leaving them on a long "cruise" phase through the outer solar system.

During this period, both spacecraft were repeatedly contacted to obtain various measurements on their physical environment, providing valuable information long after their initial missions were complete. Since the spacecraft were flying without additional stabilization thrusts during their "cruise", it is possible to characterize the density of the solar medium by its effect on the spacecraft's motion.

In the outer solar system this effect would be easily calculable, based on ground-based measurements of the deep space environment.

However, when these effects were taken into account, along with all other known effects, the calculated position of the Pioneers did not agree with measurements based on timing the return of the radio signals being sent back from the spacecraft.

These consistently showed that both spacecraft were closer to the inner solar system than they should be, by thousands of kilometres—small compared to their distance from the Sun, but still statistically significant.

This apparent discrepancy grew over time as the measurements were repeated, suggesting that whatever was causing the anomaly was still acting on the spacecraft.

As the anomaly was growing, it appeared that the spacecraft were moving more slowly than expected. Direct measurements of the spacecraft's speed using the Doppler effect demonstrated the same thing: the Pioneers were slowing down more than expected.

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 acceleration of (8.74 ± 1.33) × 10−10 m/s2, 540 ± 82.3 miles/year2, 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 ap is numerically quite close to the product of the speed of light c and the Hubble constant H0, hinting at a cosmological Jet Propulsion Laboratory review (2010) undertaken by Turyshev and Toth rules out the cosmological connection from considering rather conventional sources, a hesitant line of action in a new physics’ context.

is the exact solution given from theoretical analysis by one of the proponents of new physics.

Gravitationally bound objects such as the solar system, or even the galaxy, are not supposed to partake of the expansion of the universe—this is known both from conventional theory and by direct measurement, although it does not necessarily interfere with the paths new physics can take.
The Pioneer plaques are a pair of gold-anodized aluminium plaques which were placed on board the 1972 Pioneer 10 and 1973 Pioneer 11 spacecraft, featuring a pictorial message, in case either Pioneer 10 or 11 are intercepted by extraterrestrial life.

The plaques show the nude figures of a human male and female along with several symbols that are designed to provide information about the origin of the spacecraft.

The Pioneer spacecraft were the first human-built objects to leave the solar system. The plaques were attached to the spacecraft's antenna support struts in a position that would shield them from erosion by stellar dust.

The Voyager Golden Record, a much more complex and detailed message using (then) state-of-the-art media, was attached to the Voyager spacecraft launched in 1977.

NASA's Pioneer Space Probes

The Pioneer 10
(also known as Pioneer F) spacecraft is a 258-kilogram (569 lb) robotic space probe launched by NASA on March 2nd, 1972 to study Jupiter and eventually interstellar space. While the spacecraft is believed to still operate properly, communication has been lost since January 23rd, 2003.

Prior to this, the spacecraft was in extended mission to study the dynamics of the solar system boundary including, interplanetary magnetic fields, solar wind, cosmic rays, and the heliosphere.

The primary mission ended in December of 1973, encountering the Jovian system. It was the first probe to traverse the asteroid belt and the first to encounter Jupiter.

The Pioneer 11 (also known as Pioneer G) spacecraft is a 259-kilogram (569 lb) robotic space probe launched by NASA on April 6th, 1973 to study Jupiter, Saturn and eventually interstellar space. While the spacecraft is believed to still operate properly, communication has been lost since November 30th, 1995.

Prior to this, the spacecraft was in extended mission to study the dynamics of the solar system boundary including, interplanetary magnetic fields, solar wind, cosmic rays, and the heliosphere. The primary mission ended in September of 1973 after encountering the Jovian system and the Saturnian System. It was the first probe to encounter Saturn.

A number of proposed explanations for the Pioneer anomaly have been analyzed and discussed. Unfortunately the anomaly has no universally accepted explanation.

These proposed explanations are categorized under the following: observational errors, an unaccounted for real deceleration, and explanations that would essentially be New Physics.

  • Observational errors - The possibility of observational errors, which include measurement and computational errors, has been advanced as a reason for interpreting the data as an anomaly.

    Hence, this would result in approximation and statistical errors.

    However, further analysis has determined that significant errors are not likely because seven independent analyses have shown the existence of the anomaly (Pioneer anomaly) as of March 2010.
  • Recording errors - 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 1st, 2006 that 30 years of Pioneer data had been saved. It announced on 28 March 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.
  • The deceleration model - It has been viewed as possible that a real deceleration is not accounted for in the current model for several reasons.

    First, it is possible that deceleration is caused by gravitational forces from unidentified sources such as the Kuiper belt or dark matter. However, this acceleration does not show up in the orbits of the outer planets, so any generic gravitational answer would need to violate the equivalence principle.

    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 phenomenon based on range from the Sun.

    Second, the cause could be drag from the interplanetary medium, including dust, solar wind and cosmic rays. However, the measured densities are too small to cause the effect.

    Third, gas leaks, including helium from the spacecrafts' radioisotope thermoelectric generators (RTGs) have been viewed as possible causes.

    Fourth, a real deceleration not accounted for in the current model could result from radiation pressure of sunlight, the spacecraft's radio transmissions, or asymmetrical thermal radiation of the heat from the spacecraft.

    Possibilities include the asymmetrical radiation of heat from the RTGs (See Radioisotope rocket) or the spacecraft electronics.

    Additionally, even if the RTGs themselves radiate symmetrically, some of their radiation will reflect from the back of the spacecraft’s dish-like main antenna, causing a recoil like sunlight striking a solar sail.

    However, the radiation pressure of sunlight is too small at this distance, and points in the wrong direction. The same applies to the spacecraft's radio emissions.

    The asymmetrical radiation of heat remains a prime suspect, as presented at the second ISSI meeting in Bern, Feb 2007.

    A presentation at the APS April 2008 meeting suggests that differential heating may account for as much as one third of the observed acceleration.

    Finally, a real deceleration not accounted for in the current model could be from electromagnetic forces due to an electric charge on the spacecraft.
  • New physics - Because the "Pioneer anomaly" does not show up as an effect on the planets, it was speculated that this would be interesting if this was new physics.

    Later, with the doppler shifted signal confirmed, the team again speculated that one explanation may lie with new physics, if not some unknown systemic explanation.

    Hence, new physics would take into consideration at least one of three possibilities: clock acceleration, a modification of the law of gravity, or modified inertia.
  • Clock acceleration - Clock acceleration is an alternate explanation to anomalous acceleration of the spacecraft towards the Sun. This theory takes notice of an expanding universe, which creates an increasing background 'gravitational potential'. The increased gravitational potential then accelerates cosmological time.

    It is proposed that this particular effect causes the observed deviation from predicted trajectories and velocities of Pioneer 10 and Pioneer 11. From their data, Anderson's team deduced a steady frequency drift of 1.5 Hz over 8 years.

    This could be mapped on to a clock acceleration theory, which means all clocks would be changing in relation to a constant acceleration.

    In other words, that there would be a nonuniformity of time. Moreover, for such a distortion related to time, Anderson's team reviewed several models in which time distortion as a phenomenon is considered.

    They arrived at the "clock acceleration" model after completion of the review.

    Although the best model adds a quadratic term to defined International Atomic Time, the team encountered problems with this theory. This then led to non-uniform time in relation to a constant acceleration, as the most likely theory.
  • Definition of gravity modified - The theory MOND (Modified Newtonian Dynamics) proposes that the force of gravity deviates from the traditional Newtonian value to a very different force law at very low accelerations on the order of 10−10 m/s2.

    Given the low accelerations placed on the spacecraft while in the outer solar system, MOND may be in effect, modifying the normal gravitational equations.

    However, notice that Lunar Laser Ranging experiment combined with data of LAGEOS satellites refutes that simple gravity modification is the cause of the Pioneer anomaly.

    Also note that the precession of the longitudes of perihelia of the solar planets or the trajectories of long-period comets have not been reported to experience an anomalous gravitational field toward the Sun of the magnitude capable of describing the Pioneer anomaly.
  • Definition of inertia modified - 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.

    A model of inertia using Unruh radiation and a Hubble-scale Casimir effect, which, unlike MOND, has no adjustable parameters, has been proposed to explain the Pioneer anomaly and the flyby anomaly.

    A possible terrestrial test for evidence of a different model of modified inertia has also been proposed.

It is possible, but not proven, that this anomaly is linked to the flyby anomaly.

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 January 23rd, 2003) and Galileo was deliberately burned up in Jupiter's atmosphere at the end of its mission.

Pioneer 10 was built with six thrusters (first pair used to maintain a constant spin-rate of 4.8-rpm, second pair used for thrust, third pair used for attitude control), a star sensor able to reference Canopus, and two sun sensors.

Combined, these systems would allow the space probe to maintain pointing of the high-gain antenna toward Earth autonomously.

The probe carried a scientific payload of 11 instruments that would provide data for 13 experiments during the mission.

The space probe included a redundant system of receivers, one attached to the high-gain antenna, the other to an omni antenna and medium-gain antenna.

Each receiver is 8 watts and transmits data across the S-band using 2110 MHz for the uplink from Earth and 2292 MHz for the downlink to Earth with the Deep Space Network tracking the signal.

Prior to transmitting data, the probe was built with error correction utilizing a convolutional encoder to avoid sending corrupted data.

Pioneer 10's weak signal continued to be tracked by the Deep Space Network.

After 1997 the probe was used in the training of flight controllers on how to acquire radio signals from space.

There was an Advanced Concepts study applying chaos theory to extract coherent data.

The last successful reception of telemetry was on April 27th, 2002; subsequent signals were barely strong enough to detect.

Loss of contact was probably due to a combination of increasing distance and the spacecraft's steadily weakening power source, rather than structural failure of the craft.

The last, very weak signal from Pioneer 10 was received on January 23rd, 2003, when it was 12 billion kilometers from Earth.

A contact attempt on February 7th, 2003 was unsuccessful.

One final attempt was made on the evening of March 4th, 2006, the last time the antenna would be correctly aligned with Earth.

No response was received from Pioneer 10.

Pioneer 10 is heading in the direction of the star Aldebaran in the constellation Taurus at roughly 2.6 AU per year.

If Aldebaran had zero relative velocity, it would take Pioneer 10 about 2 million years to reach it.

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 200 AU from the Sun in a hyperbolic escape orbit.
  • Observations of asteroids around 20 AU 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.