Pioneer anomaly
Encyclopedia
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 AU 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 . 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. 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.
and Jupiter/Saturn respectively. The spacecraft were spin-stabilised
in order to keep their main antenna
s 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 kilometre
s—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 sun
ward acceleration of 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.
forces inherent in the spacecraft.
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 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. 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 object
s.
The magnitude of the Pioneer effect
is numerically quite close to the product of the speed of light
and the Hubble constant 
, 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 claims to rule out the cosmological connection by considering rather conventional sources. The equation
is the exact solution given from theoretical analysis by one of the proponents of new physics. On the other hand, other scientists disproved the possibility that the Pioneer anomaly can be due to cosmology.
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. This does not necessarily interfere with paths new physics can take with drag effects
from planetary secular accelerations
of possible cosmological origin.
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.
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.
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. 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.
During March 2011, experts proposed new calculations which seem to confirm that heat is the cause of the spacecraft slowing down.
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.
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 reflection
s 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. The Jet Propulsion Laboratory is currently attempting to confirm this explanation by studying their own thermal data.
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.
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.
from the interplanetary medium
, including dust
, solar wind
and cosmic rays. However, the measured densities are too small to cause the effect.
leaks, including helium
from the spacecrafts' radioisotope thermoelectric generator
s (RTGs) have been viewed as possible causes.
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 1 June 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.
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.non-uniform time in relation to a constant acceleration is a summarized term derived from the source or sources used for this sub-section.
(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. The Lunar Laser Ranging experiment
combined with data of LAGEOS
satellites refutes that simple gravity modification is the cause of the Pioneer anomaly. 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.
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.
, which has been observed in other spacecraft. 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:
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 Bern, Switzerland
to discuss the anomaly, and discuss possible means for resolving the source.
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:
Pioneer 10
Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the...
and Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
spacecraft after they passed about 20 AU 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. 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 . 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. 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
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
forces inherent in the spacecraft.
Description
Pioneer 10 and 11 were sent on missions to JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Jupiter/Saturn respectively. The spacecraft were spin-stabilised
Spin-stabilisation
Spin-stabilisation is the method of stabilizing a satellite or launch vehicle by means of spin. For most satellite applications this approach has been superseded by three-axis stabilisation. It is also used in non-satellite applications such as rifle and artillery.Despinning can be achieved by...
in order to keep their main antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s pointed towards Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
using gyroscopic forces
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
. 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
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
, 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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
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
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
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 kilometre
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
s—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
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
demonstrated the same thing: the observed redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
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 sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
ward acceleration of 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 pressureRadiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
forces inherent in the spacecraft.
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
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
, which states that all objects are affected the same way by gravity. It is therefore argued 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
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, is still insufficient to disprove the gravitational nature of the Pioneer anomaly. The same authors ruled out the existence of a gravitational Pioneer-type extra-acceleration in the outskirts of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
by using a sample of Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
s.
The magnitude of the Pioneer effect
is numerically quite close to the product of the speed of lightSpeed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
and the Hubble constant , hinting at a cosmologicalPhysical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
connection, but the significance of this, if any, is unknown. In fact the latest Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
review (2010) undertaken by Turyshev
Slava Turyshev
Slava G. Turyshev is a Russian physicist now working in the US at the NASA Jet Propulsion Laboratory . He is known for his investigations of the Pioneer Anomaly, affecting Pioneer 10 and Pioneer 11 spacecraft, and for his attempt to recover early data of the Pioneer spacecraft to shed light on such...
and Toth claims to rule out the cosmological connection by considering rather conventional sources. The equation
is the exact solution given from theoretical analysis by one of the proponents of new physics. On the other hand, other scientists disproved the possibility that the Pioneer anomaly can be due to cosmology.Gravitationally bound objects such as the solar system, or even the galaxy, are not supposed to partake of the expansion of the universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
—this is known both from conventional theory and by direct measurement. This does not necessarily interfere with paths new physics can take with drag effects
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
from planetary secular accelerations
Secular phenomena
In astronomy, secular phenomena are contrasted with phenomena observed to repeat periodically. In particular, astronomical ephemerides use secular to label the longest-lasting or non-oscillatory perturbations in the motion of planets, as opposed to periodic perturbations which exhibit repetition...
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
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
flew a mission profile similar to the Pioneers, but were not spin stabilized. Instead, they required frequent firings of their thrusters
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...
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.
Newer spacecraft have used spin stabilization for some or all of their mission, including both Galileo
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...
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.
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. 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.
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.
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 radiationThermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits 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
Radioisotope rocket
The radioisotope rocket is a type of rocket engine that uses the heat generated by the decay of radioactive elements to heat a working fluid, which is then exhausted through a rocket nozzle to produce thrust...
) 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
American Physical Society
The American Physical Society is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen scientific journals, including the world renowned Physical Review and Physical Review Letters, and organizes more than 20...
April 2008 meeting suggests that differential heating may account for as much as one third of the observed acceleration.
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
Phong reflection model
The Phong reflection model is an empirical model of the local illumination of points on a surface...
to model diffusive
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...
and specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...
s 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. The Jet Propulsion Laboratory is currently attempting to confirm this explanation by studying their own thermal data.
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.
Gravity
It is possible that deceleration is caused by gravitational forces from unidentified sources such as the Kuiper beltKuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
or dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
. 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
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
(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.
Drag
The cause could be dragDrag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
from the interplanetary medium
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...
, including dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
, solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
and cosmic rays. However, the measured densities are too small to cause the effect.
Gas leaks
GasGas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
leaks, including helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
from the spacecrafts' radioisotope thermoelectric generator
Radioisotope thermoelectric generator
A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
s (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.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 1 June 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.
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.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 10Pioneer 10
Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the...
and Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
.
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
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...
, 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.non-uniform time in relation to a constant acceleration is a summarized term derived from the source or sources used for this sub-section.
Definition of gravity modified
The theory MONDModified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
(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. The Lunar Laser Ranging experiment
Lunar laser ranging experiment
The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...
combined with data of LAGEOS
LAGEOS
LAGEOS, or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth...
satellites refutes that simple gravity modification is the cause of the Pioneer anomaly. 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
MONDModified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
can also be interpreted as a modification of inertia, perhaps due to an interaction with vacuum energy
Vacuum energy
Vacuum energy is an underlying background energy that exists in space even when the space is devoid of matter . The concept of vacuum energy has been deduced from the concept of virtual particles, which is itself derived from the energy-time uncertainty principle...
, 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
Unruh effect
The Unruh effect , was first described by Stephen Fulling in 1973, Paul Davies in 1975 and Bill Unruh in 1976. It is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none...
and a Hubble-scale Casimir effect
Casimir effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, like capacitors placed a few micrometers apart, without any external electromagnetic field...
, which, unlike MOND, has no adjustable parameters, has been proposed to explain the Pioneer anomaly and the flyby anomaly
Flyby anomaly
The flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. This anomaly has been observed as shifts in the S-Band and X-Band Doppler and ranging telemetry. Taken together it causes a significant unaccounted velocity increase of over 13 mm/s during...
. A possible terrestrial test for evidence of a different model of modified inertia has also been proposed.
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.Further research avenues
It is possible, but not proven, that this anomaly is linked to the flyby anomalyFlyby anomaly
The flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. This anomaly has been observed as shifts in the S-Band and X-Band Doppler and ranging telemetry. Taken together it causes a significant unaccounted velocity increase of over 13 mm/s during...
, which has been observed in other spacecraft. 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
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...
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
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...
have not yielded any conclusive results. There are several remaining options for further research:
- Further analysis of the retrieved Pioneer data.
- The New HorizonsNew HorizonsNew Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
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 AUAstronomical unitAn astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
from the Sun in a hyperbolic escape orbitHyperbolic trajectoryIn astrodynamics or celestial mechanics a hyperbolic trajectory is a Kepler orbit with the eccentricity greater than 1. Under standard assumptions a body traveling along this trajectory will coast to infinity, arriving there with hyperbolic excess velocity relative to the central body. Similarly to...
. - Observations of asteroidAsteroidAsteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s around 20 AU may provide insights if the anomaly's cause is gravitational.
Meetings and conferences about the anomaly
A meeting was held at the University of BremenUniversity of Bremen
The University of Bremen is a university of approximately 23,500 people from 126 countries that are studying, teaching, researching, and working in Bremen, Germany...
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
International Space Science Institute
The International Space Science Institute is an Institute of Advanced Studies based in Bern, Switzerland.The institute's work is interdisciplinary, focusing on the study of the solar system, and encompasses planetary sciences, astrophysics, cosmology, astrobiology, and the Earth sciences.A main...
in Bern, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
to discuss the anomaly, and discuss possible means for resolving the source.
See also
- Flyby anomalyFlyby anomalyThe flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. This anomaly has been observed as shifts in the S-Band and X-Band Doppler and ranging telemetry. Taken together it causes a significant unaccounted velocity increase of over 13 mm/s during...
- Galaxy rotation problem
- STVG
- Modified Newtonian dynamicsModified Newtonian dynamicsIn physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
- General relativityGeneral relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
- Unsolved problems in physicsUnsolved problems in physicsThis is a list of some of the major unsolved problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result...
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
- "Pioneer Anomaly Solved By 1970s Computer Graphics Technique" (March 2011)
- "Pioneering Gas Leak? The strange motions of two space probes have mundane explanations--probably" Scientific AmericanScientific AmericanScientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...
(December 1998) - "A Force to Reckon With: What applied the brakes on Pioneer 10 and 11?" Scientific AmericanScientific AmericanScientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...
(October 2005) - "Gravity theory dispenses with dark matter" - STVG (Scalar-tensor-vector gravityScalar-tensor-vector gravityScalar–tensor–vector gravity is a modified theory of gravity developed by John Moffat, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario...
) theory claims to predict Pioneer anomaly - Planetary Society data recovery effort enables study
- Shows number of publications about the Pioneer anomaly on arXiv.org, by year.
- Space.com: The Problem with Gravity: New Mission Would Probe Strange Puzzle
- "Wanted - Einstein Junior" the EconomistThe EconomistThe Economist is an English-language weekly news and international affairs publication owned by The Economist Newspaper Ltd. and edited in offices in the City of Westminster, London, England. Continuous publication began under founder James Wilson in September 1843...
(March 2008) - The Pioneer Anomaly, a 30-Year-Old Cosmic Mystery, May Be Resolved At Last - Popular Science (December 2010)