Pioneer anomaly

Unsolved problems in physics
Why do the Pioneer spacecraft seem to accelerate towards the sun?

The Pioneer anomaly or Pioneer effect is the observed deviation from predicted accelerations of the Pioneer 10 and Pioneer 11 spacecraft after they passed about 20 astronomical units (3×109 km; 2×109 mi) on their trajectories out of the Solar System.

Both Pioneer spacecraft are escaping the Solar System, but are slowing under the influence of the Sun's gravity. Upon very close examination of navigational data, the spacecraft were found to be slowing slightly more than expected. The effect is an extremely small but unexplained acceleration towards the Sun, of 8.74±1.33×10−10 m/s2. The two spacecraft were launched in 1972 and 1973 and the anomalous acceleration was first noticed as early as 1980, but not seriously investigated until 1994.[1] The last communication with either spacecraft was in 2003, but analysis of recorded data continues.

Most recent developments point towards the mundane cause of thermal radiation pressure forces inherent in the spacecraft.[2][3]

Contents

Description

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. 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. Measurements of the spacecraft's speed using the Doppler effect demonstrated the same thing: the observed redshift was less than expected, which meant that the Pioneers had slowed 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 an approximately constant sunward acceleration of 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.

Possible causes

Despite many proposed solutions, there is not yet a universally accepted explanation for the cause of the Pioneer anomaly. Proposed explanations fall into two classes — "mundane causes" or "new physics". Mundane causes include conventional effects that were overlooked or mis-modeled in the initial analysis, such as measurement error, thrust from gas leakage, or uneven heat radiation. The "new physics" explanations propose revision of our understanding of gravitational physics. Most recent developments point towards the mundane cause of thermal radiation pressure forces inherent in the spacecraft.[2][3][4][5]

If the Pioneer anomaly is a gravitational effect due to some long-range modifications of the known laws of gravity, it does not affect the orbital motions of the major natural bodies in the same way (in particular those moving in the regions in which the Pioneer anomaly manifested itself in its presently known form). Hence a gravitational explanation would need to violate the equivalence principle, which states that all objects are affected the same way by gravity. It is therefore argued[6][7][8][9][10][11][12][13][14][15] that increasingly accurate measurements and modelling of the motions of the outer planets and their satellites undermine the possibility that the Pioneer anomaly is a phenomenon of gravitational origin. Others believe that our knowledge of the motions of the outer planets and dwarf planet Pluto, is still insufficient to disprove the gravitational nature of the Pioneer anomaly.[16] The same authors ruled out the existence of a gravitational Pioneer-type extra-acceleration in the outskirts of the Solar System by using a sample of Trans-Neptunian objects.[17][18]

The magnitude of the Pioneer effect a_p is numerically quite close to the product of the speed of light c and the Hubble constant H_0, hinting at a cosmological connection, but the significance of this, if any, is unknown. In fact the latest Jet Propulsion Laboratory review (2010) undertaken by Turyshev and Toth[19] claims to rule out the cosmological connection by considering rather conventional sources. The equation a_p=H_0\cdot c is the exact solution given from theoretical analysis by one of the proponents of new physics.[20] On the other hand, other scientists disproved the possibility that the Pioneer anomaly can be due to cosmology.[21][22]

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[23] and by direct measurement.[24] This does not necessarily interfere with paths new physics can take with drag effects from planetary secular accelerations of possible cosmological origin.

Indications from other missions

The Pioneers were uniquely suited to discover the effect because they have been flying for long periods of time without additional course corrections. Most deep-space probes launched after the Pioneers either stopped at one of the planets, or used thrusting throughout their mission.

The Voyagers flew a mission profile similar to the Pioneers, but were not spin stabilized. Instead, they required frequent firings of their thrusters for attitude control to stay aligned with Earth. Spacecraft like 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; accelerations as big as 10−9 m/s2 would be undetectable.[19]

Newer spacecraft have used spin stabilization for some or all of their mission, including both Galileo and Ulysses. These 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.

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. 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. The measured value is the sum of the uncertain thermal effects and the possible anomaly. Therefore, the Cassini cruise measurements neither conclusively confirm nor refute the existence of the anomaly.[25]

After Cassini arrived at Saturn, it shed a large fraction of its mass from the fuel used in the insertion burn and the release of the Huygens probe. This increases the acceleration caused by the radiation forces, since they are acting on less mass. This change in acceleration allows the radiation forces to be measured independently of any gravitational acceleration.[26] Comparing cruise and Saturn-orbit results shows that for Cassini, almost all the unmodelled acceleration was due to radiation forces, with only a small anomalous acceleration, much smaller than the Pioneer acceleration, and with opposite sign.[27]

Proposed explanations

A number of proposed explanations for acceleration towards the sun have been pursued. These are categorized under the following: an unaccounted for real deceleration, observational errors, and explanations that would essentially be New Physics. During March 2011, experts proposed new calculations which seem to confirm that heat is the cause of the spacecraft slowing down.[28]

The deceleration model

It has been viewed as possible that a real deceleration is not accounted for in the current model for several reasons.

Thermal radiation pressure

A real deceleration not accounted for in the model could result from asymmetrical thermal radiation pressure of the heat from the spacecraft (the effect cannot be from the radiation pressure of sunlight or the spacecraft's radio emissions as it is too small at this distance, and points in the wrong direction).

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

The asymmetrical radiation of heat remains a prime suspect, as presented at the second ISSI meeting in Bern, February 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.[29]

A research team from Portugal has proposed that previous modelling used to predict the directions of radiation pressures was incorrect. By using the Phong reflection model to model diffusive and specular reflections they believe that the observed and theoretical results no longer diverge. This proposed explanation finds most of the diverging thrust in the heat from the main equipment compartment reflecting off the back of the main antenna, which would tend to produce a thrust in the direction of the sun.[30] The Jet Propulsion Laboratory is currently attempting to confirm this explanation by studying their own thermal data.[31]

According to Slava Turyshev of JPL in a paper titled "Support for temporally varying behavior of the Pioneer anomaly from the extended Pioneer 10 and 11 data sets," to be published in Physical Review Letters in 2011, the anomaly has a temporally-decaying (not constant as previously thought) nature and points towards Earth. This strengthens the case for on-board generated recoil forces as the reason behind the anomaly.[32]

Gravity

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 (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 phenomenon based on range from the Sun.[14]

Drag

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.

Gas leaks

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

Observational or recording 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 Pioneer anomaly as of March 2010.[33]

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.

New physics

Because the "Pioneer anomaly" does not show up as an effect on the planets, Anderson et al. 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.[34]

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.[34]

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.[note 1][34]

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.[35] Given the low accelerations placed on the spacecraft while in the outer solar system, MOND may be in effect, modifying the normal gravitational equations. The Lunar Laser Ranging experiment combined with data of LAGEOS satellites refutes that simple gravity modification is the cause of the Pioneer anomaly.[36] The precession of the longitudes of perihelia of the solar planets[8] or the trajectories of long-period comets[37] 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.[38] 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.[39][40] A possible terrestrial test for evidence of a different model of modified inertia has also been proposed.[41]

Cosmological effects due to the rotation of the Universe

It is possible that the Universe has a peculiar kind of rotation which makes each observer measure this specific Pioneer deceleration when observing other points in outer space. It only affects hyperbolic orbits, which are those which extend towards infinity, and acquire cosmological character (Berman, 2007). A General Relativistic treatment was made by Berman and Gomide (2010; 2011) which also solves the other two anomalies, the fly-by and the spinning down of the Pioneers when they were not disturbed.[42][43][44]

Further research avenues

It is possible, but not proven, that this anomaly is linked to the flyby anomaly, which has been observed in other spacecraft.[45] 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)[46] 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:

Meetings and conferences about the anomaly

A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly.[52]

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 Bern, Switzerland to discuss the anomaly, and discuss possible means for resolving the source.[53]

See also

Notes

  1. ^ non-uniform time in relation to a constant acceleration is a summarized term derived from the source or sources used for this sub-section.

References

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Further reading

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 cause.')"

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:

Further elaboration on a dedicated mission plan (restricted access)

External links