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There's a mystery at the edge of our solar system: Two spacecraft, Pioneers 10 and 11, which were launched to Jupiter and Saturn more than 30 years ago, are hurtling towards the edge of our solar system - but at a slower than expected rate. Called the Pioneer Anomaly, the effect of this slowing is small, but measurable, and so far unexplained.

This riddle has sparked an array of possible explanations, ranging from dark matter to spacecraft equipment to - most provocative of all - a new physics.

More data is what scientists need to solve the mystery, and more data is what they now possess, thanks to Planetary Society members....The Planetary Society issued an appeal to its worldwide membership and raised the funding needed to recover and validate this trove of information.

....Scientists and engineers led by Slava G. Turyshev at JPL were able to recover much of the more-than-30-year navigational histories of both spacecraft, including data from their Jupiter and Saturn encounters in the 1970s. The data are now being collected, arranged, validated, and written to modern media and will be provided to teams of scientists to analyze.

Success extended beyond the recovery of the velocity data. Information about the spacecraft themselves, as well as science data, was contained in what are called Master Data Records, discovered in storage at NASA Ames Research Center - which operated the Pioneer spacecraft.

...Planetary Society college intern Merek Chertkow is beginning to analyze this information, as are various professional scientists.

The Pioneer Anomaly was discovered when John D. Anderson and colleagues at JPL realized that the trajectories of the two spacecraft were deviating from the known laws of motion.

After about 30 years of travel, the Pioneer Anomaly has resulted in the spacecraft being about 240,000 miles - the Earth-Moon distance - closer to the Sun than expected. That seemingly trivial amount - the Pioneer spacecraft are traveling at 30,000 miles per hour – has intrigued scientists, because no known factor explained the slowdown.

What could be affecting their speed? Many hypotheses have been suggested: the interplanetary plasma and solar wind thermal recoil force due to heat from the spacecraft's nuclear power sources mysterious dark matter in the galaxy a manifestation of new physics.

http://www.spacedaily.com/reports/TPS_Enables_Study_Of_Mysterious_Pioneer_Anomaly.html

Is this the only case where the anomaly has been observed? I was thinking of a couple of things, but they're pretty elementary and probably thought of before. Still, is this the only place where the pinoeer anomaly has been dected? Because then we know if its specific to these two spacecrafts and their particular route. WHat if there is some property of our solar sytem which we have thus been living unaware of because our instruments are not made to dectect such a thing? It may be specific to our solar system, or it may have something to do with the fabric of the universe itself and the laws governing it (as you and the article mention).

observational errors, including measurement and computational errors, in deriving the acceleration

Approximation/statistical errors

a real deceleration:

by gravitational forces from unidentified sources such as the Kuiper belt or dark matter

drag from the interplanetary medium, including dust, solar wind and cosmic rays

gas leaks, including helium produced by radioactive decay escaping from the spacecrafts' radioisotope thermoelectric generators (RTGs)

radiation pressure of sunlight, the spacecraft's radio transmissions, or thermal radiation pressure from the RTGs

electromagnetic forces due to an electric charge on the spacecraft
new physics

clock acceleration (gr-qc/0410084) between coordinate or Ephemeris time and International Atomic Time.

A modification of the law of gravity. The theory known as MOND (MOdified Newtonian Dynamics), states that at very low accelerations of order: 10−10 m/s2 the force of gravity deviates from the traditional Newtonian value. However, since the accelerations of the Pioneer craft are still much larger than this, MOND cannot explain the Pioneer Anomaly.

A modified theory of gravity (gr-qc/0511026) that incorporates quantum effects can explain a trio of puzzling astronomical observations – including the wayward motion of the Pioneer spacecraft in our solar system and Galaxy rotation curves without dark matter (as reported in the New Scientist Online).