Pioneer anomaly

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As Pioneer anomaly a slight deviation of the 1972 and 1973 launched identical is NASA - probes Pioneer 10 and Pioneer 11 from their predicted trajectories called. The cause is considered to be anisotropic heat radiation from the probes. Up until around 2012 a whole range of explanations was discussed: from effects as simple as a thrust from escaping gas to a previously unknown physical effect because it was not clear whether thermal radiation could explain the observed effect.

A similar unexplained phenomenon is the so-called fly-by anomaly . Scientists speculated that both anomalies might have a common cause. In the meantime, however, it seems certain that the fly-by anomaly must have another reason.

Orbits of the Pioneer and Voyager probes; Elements of the heliosphere are also shown

general description

The effect was noticeable around 1980 when the Pioneer 10 spacecraft had crossed Uranus' orbit and was about 20  astronomical units from Earth. It was observed that the acceleration of the probe towards the sun (i.e. its deceleration) is a P = (8.7 ± 1.3) · 10 −10  m / s² greater than calculated with the known influences, including about 100,000 -fold greater gravity of the sun. Over a period of 15 years, a P resulted in a discrepancy of about 0.4 m / s in speed and about 100,000 kilometers (0.0007 astronomical units) in distance. The uncertainty of the direction of the effect includes the following directions: towards the sun, towards the earth, parallel to the axis of rotation of the probe or to its direction of movement.

At this time the known acceleration due to the radiation pressure of the sun , which was taken into account in the calculations,  had dropped to about 4 · 10 −10 m / s². Only then was the inexplicable acceleration measurable, which was previously lost in the variable radiation pressure. The deviation from the calculated values ​​was noticeable during the measurements of the Doppler effect on the radio signals sent back by the probes (for determining the speed) and confirmed by the measurements of the transit times of the signals (for determining the distance).

However, the anomaly was initially not taken seriously and interpreted as a random error. It was not examined more closely until 1994, when the effect did not go away. The trajectory values ​​of the probes Pioneer 10 and Pioneer 11 - which had moved away from each other in almost the opposite direction and whose data showed the same effect up to a maximum of three percent difference - were systematically analyzed for possible causes without a complete explanatory model being found could be.

Detailed description

Speed ​​measurement of the probes

The navigation of the pioneer probes was carried out with the help of the antennas of the Deep Space Network (DSN), an amalgamation of several radio telescope systems of the Jet Propulsion Laboratory (JPL). Today the DSN consists of large radio telescope systems in Goldstone / USA, Madrid / Spain and Canberra / Australia. In the past, there were also plants in Woomera / Australia and Johannesburg / South Africa. These are each complex of numerous antennas. The antennas were initially mostly 26 meters in diameter, later often 34 or 64 meters, sometimes up to 70 meters. The speed measurement of the pioneer probes, which is of central importance for the Pioneer anomaly, was carried out via the two-way Doppler shift of radio waves. Radio waves of known frequencies (S-band, approx. 2.11 GHz) were sent to the satellite from the ground stations (uplink). The satellite receives the signal with a Doppler shift:

Where c is the speed of light, the speed of the probe, the transmission frequency of the signal on earth and the frequency of the signal received by the spacecraft. (Indices: E = Earth, R = remote) The probe responds immediately with an 8-watt transmitter (antenna diameter: 137 cm) and a transponder with a frequency multiplied by the fixed (and exact) factor 240/221:

This is necessary because the radio signals are coherent waves and thus falsifications due to interference of the waves traveling back and forth are avoided. On the way back, the signal (downlink) is Doppler-shifted identically a second time. The received signal is thus twice Doppler and shifted by a factor of 240/221.

The relative shift is thus given by

In some sources, the constant frequency shift caused by the electronics is neglected for illustration, which leads to the simpler form:

Independently of this, the distance to the probe can also be determined by the transit time of the signal. However, this could only be used with the pioneer probes at the beginning of the mission.

Since the speed calculated above is the relative speed of the probe to the earth, but the orbit is calculated in barycentric coordinates, the speed of the antennas on earth must be determined extremely precisely. Among other things, one takes into account:

Calculation of the path and analysis of the data

When calculating the orbit, the gravitational influence of the sun, the planets, the earth's moon and the largest asteroids were taken into account; In addition, the solar radiation pressure and many other influences on the trajectory of the probes. The maneuvers have unknown strong effects on the size, but they are easy to recognize in the measurement data. The theoretical trajectory was adapted to the measured values ​​with a number of free parameters - including the maneuvers and the initial conditions as well as the size of the anomaly - in order to check and determine the anomaly.

The calculations were checked by independent persons using five different program packages, which means that calculation or software errors can be excluded.

description

The blue shift increases constantly. The measured value of the inexplicable rate of change of the blue shift is (5.99 ± 0.01) · 10 −9  Hz / s, this corresponds to an acceleration of (8.74 ± 1.33) · 10 −10  m / s². The acceleration points in the direction of the sun, although the exact direction could not be determined - so it could also be that it points in the direction of the earth, velocity vector or its own axis of rotation.

The anomaly appears to be relatively constant, but a slow decrease over time cannot be ruled out as too little data have been analyzed so far. There are also smaller periodic fluctuations, the origin of which is also unclear. The acceleration was observed with both Pioneer probes and is only a maximum of 3% apart.

Data situation

The Pioneer probes were in contact with the earth for over three decades. The previous analyzes only looked at data from about 11.5 years from Pioneer 10 and 3.5 years from Pioneer 11. However, usable data was received from Pioneer by April 27, 2002 (the last weak signal was received on January 23, 2003) 10 and at least useful data from Pioneer 11 up to October 1990. So there are around 17.5 years of Pioneer 10 and 12.5 years of Pioneer 11 data that have not yet been evaluated; less than a quarter of the data has been used so far. The radio data from the Pioneer probes were not digitally recorded before 1987.

A new analysis of all data is currently in progress. The problem was that the data was poorly archived; In the meantime, however, it has been possible to collect and restore most of the data.

Other probes

In the early 1980s, Pioneer 10 was the most distant spacecraft. It is therefore not surprising that she was the first to notice the anomaly observed. Due to other, stronger accelerations in the vicinity of the sun (such as the solar wind mentioned ), the anomaly can only be measured at great distances. Therefore, no deviation can be found with geo-orbital satellites and lunar probes.

It was reported, however, that the same effect was also shown with the Jupiter probe Galileo, which had burned out in the meantime, and the European-American solar probe Ulysses , although the data for these were less precise and alone not too meaningful. One aspect why the Pioneer probes provide good data is their simple gyroscopic flight attitude stabilization ( spin stabilization ), which is easily predictable and calculable and can therefore easily be excluded as a possible source of error. Later long -range probes such as Galileo or the two Voyager probes were designed with 3-axis stabilization , with the result that the speed of the probe is more strongly influenced by the position control by the control nozzles and can be calculated less precisely.

Future missions

New missions have been designed to verify and more accurately identify the Pioneer anomaly. It has been planned to build an extremely symmetrical space probe which will determine the anomaly to 10 −10 to 10 −12 cm / s 2 and thereby rule out or check numerous possible causes. A separate probe would be ideal for this; however, it would also be possible to combine this with other space missions.

Alternatively, it has been suggested to launch a probe perpendicular to the ecliptic to test whether the anomaly still occurs.

Explanations

Anisotropic thermal radiation

The explanation of the anomaly, which is now widely accepted, is based on the spatially uneven heat radiation of the Pioneer probes in the amount of 2 to 5 kW, which comes from radioisotope batteries (RTGs). The radiation of the RTGs themselves is largely isotropic, but part of the radiation is reflected by the probe and the radiation from the electronics of the probe is mostly directed. These electronics are attached to the back of the parabolic antennas. Since the antennas installed in the outer area of ​​the probes are always directed towards the earth, they reflect the thermal radiation from the inner area of ​​the probes in the direction of flight and thus cause a small but continuous reduction in the orbital speeds of the probes. Assuming a heat source of 3 kW and a probe mass of 250 kg, one obtains:

Thrust = radiation power / speed of light: 10 µN
Deceleration = thrust / mass: 4 × 10 −8 m / s²

Assuming that only part of the radiation is focused in the direction of flight, the estimate is in the order of magnitude of the observed effect of 1 · 10 −9 m / s². With the values ​​mentioned, an anisotropy of 2.5% or a power of 75 W is sufficient, which is significantly below the power loss of the electronics of the probes.

In April 2011, a team of Portuguese researchers was able to use computer simulations to trace the effect completely back to uneven heat radiation, in particular the reflection of the heat radiation on the different components of the probe, based on new, detailed models for heat radiation. Also in April 2011, researchers from Bremen presented a method based on the finite element method with which the measured anomalous acceleration can be completely described as a thermal recoil effect . The results of the Bremen research group were confirmed in April 2012 by an analysis by the NASA Jet Propulsion Laboratory.

Previously suggested causes

Before investigating the thermal radiation of the probes, numerous other explanations were considered, such as:

  • Data and calculation errors of any kind
  • Effects of the probe itself like
    • Recoil from gas leaking from the fuel tanks
    • electromagnetic forces due to the electrical charge of the probe
    • uneven aging of the probe surface, which results in uneven heat radiation
    • Spatially uneven escape of helium (decay product) from the plutonium energy source
  • Effects of the solar system like
    • Gravitational forces of the Kuiper belt
    • Frictional resistance from interstellar matter
    • Micrometeorite impacts
    • inaccurate models of solar radiation and solar wind
  • Cosmological Effects
    • Expansion of the universe
  • fundamentally new effects like

In addition to the explanation given above, additional, but probably significantly smaller, effects from gas leaks cannot be completely ruled out. A group led by scientists from the Jet Propulsion Laboratory wrote in their detailed analysis: “Until more is known, we must admit that the most likely cause of the effect is an unknown systematic one. (We ourselves are divided on whether 'gas leaks' or 'heat' is this 'most likely cause'.) "

Possible external effects include gravitation effects due to an unusual mass distribution in the Kuiper belt or friction due to interstellar matter. The previous measurement data for areas outside the solar system indicate, however, that the density of the interstellar medium and the dust in the Kuiper Belt is several orders of magnitude too low to be able to explain the anomaly of the probes by this effect alone.

Some scientists also saw the anomaly as an indication of a "new physics" which cannot be explained by the established standard theories. One approach to this is the modified Newtonian dynamics, as this would also produce an anomalous acceleration of the probes in the order of magnitude observed.

The magnitude of the acceleration also corresponds to the product of the Hubble constant H 0 and the speed of light c of - depending on the measurement method - approximately (7 ± 0.6) · 10 −10 m / s² within the scope of the measurement accuracy . However, due to the more plausible explanation with the asymmetrical heat radiation set out above, this number also appears to be rather coincidental.

media

The Pioneer anomaly has been covered by numerous media outlets over time. In March 2005, the science magazine New Scientist listed the Pioneer anomaly as one of 13 scientific puzzles.

The January 2006 issue of Physik Journal also dealt with the anomaly. A measurement error is also ruled out there with great probability.

Individual evidence

  1. ↑ a b Christopher Seidler: Braked space probes. Nasa unravels the mysterious "Pioneer" effect on Spiegel Online on July 18, 2012
  2. ↑ a b Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, Jordan Ellis: Support for the Thermal Origin of the Pioneer Anomaly . Physical Review Letters 108, 241101 (2012), arxiv : 1204.2507 .
  3. ↑ Sergei Kopeikin: Celestial ephemerides in an expanding universe . Phys. Rev. D 86, 064004 (2012), arxiv : 1207.3873 .
  4. ↑ Guido Meyer: Mysterious force throws space probes off track , Spiegel Online, March 12, 2008.
  5. ↑ Ute Kehse: Pioneer anomaly solved, Rosetta puzzle remains exciting . In: Wissenschaft.de. May 31, 2011, accessed September 9, 2019 .
  6. ^ A b John D. Anderson et al .: Study of the anomalous acceleration of Pioneer 10 and 11 , Physical Review D 65, 2002, 082004 pp. 1-50, arxiv : gr-qc / 0104064 (English).
  7. ↑ a b Slava G. Turyshev, Viktor T. Toth: The Pioneer anomaly , Living Reviews in Relativity 13, 2010, 4, September 1, 2010 (English)
  8. ↑ John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, Slava G. Turyshev: Study of the anomalous acceleration of Pioneer 10 and 11 . In: Phys. Rev. D . tape 65 , 2002, arxiv : gr-qc / 0104064v5 .
  9. ^ Spiegel: Nasa unravels mysterious "Pioneer" effect: Pioneer anomaly: puzzle solved , accessed on April 8, 2011
  10. ↑ B. Rievers and C. Lämmerzahl: High precision thermal modeling of complex systems with application to the flyby and Pioneer anomaly . Annalen der Physik, 523: 439–449, June 2011, quoted by Sergei Kopeikin: Celestial ephemerides in an expanding universe . arxiv : 1104.3985 (English)
  11. ↑ C. Johan Masreliez: A cosmological explanation to the Pioneer anomaly , Astrophysics and Space Science 299, 2005, pp. 83-108, bibcode : 2005Ap & SS.299 ... 83M (English)
  12. ^ A b Hansjörg Dittus, Claus Lämmerzahl: Die Pioneer-Anomalie ( PDF file, 594 kB, free registration required), Physik Journal 5, January 2006, pp. 25–31
  13. ↑ Michael Brooks: 13 things that do not make sense , New Scientist 2491, March 19, 2005, p. 30 (English)

literature

  • Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, Jordan Ellis: Support for the Thermal Origin of the Pioneer Anomaly . Physical Review Letters 108, 241101 (2012), arxiv : 1204.2507
  • Michael Martin Nieto, Slava G. Turyshev: Finding the origin of the Pioneer anomaly , Classical and Quantum Gravity 21, September 7, 2004, pp. 4005-4024; arxiv : gr-qc / 0308017 (English)
  • Michael Martin Nieto, John D. Anderson: Search for a solution of the Pioneer anomaly , Contemporary Physics 48, January 2007, pp. 41-54; arxiv : 0709.3866 (English)
  • Oliver Preuss, Hansjörg Dittus and Claus Lämmerzahl: Surprises on the doorstep , Stars and Space 46 No. 4, April 2007, pp. 26–36
  • Slava G. Turyshev: The Pioneer anomaly: effect, new data and new investigation (slides, PDF file, 7.2 MB), American Physical Society Meeting, St. Louis, Missouri, April 13, 2008
  • Claus Lämmerzahl: The Pioneer anomaly or Do we really understand the physics within the solar system? (Slides, PDF file, 7.5 MB), lecture Utrecht, May 26, 2008 (English)
  • Slava G. Turyshev, Viktor T. Toth: The Pioneer anomaly , Living Reviews in Relativity 13, 2010, 4, September 1, 2010 (English)

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