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.
|The Pioneer Anomaly
Mystery Force Baffles NASA
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
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
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
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.
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
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
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.
attempts to use data from current missions such as Cassini have not
yielded any conclusive results.
|A number of proposed explanations for the Pioneer anomaly have been analyzed and discussed. Unfortunately the anomaly has no universally accepted explanation.
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
Hence, this would result in approximation and statistical
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.
were made over this period, including changes in the receiving
instruments, reception sites, data recording systems and recording
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
Third, gas leaks, including helium from the spacecrafts'
radioisotope thermoelectric generators (RTGs) have been viewed as
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
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.
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
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.
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
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
Given the low accelerations placed on the spacecraft while in the
outer solar system, MOND may be in effect, modifying the normal
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.
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.
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.
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.
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.
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
- 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