Research Topic: Anomalous Acceleration?
By Ian Mander BSc, 11 September 1999. Link added 19 March 2006. Direction of acceleration corrected 28 October 2008 (D'oh!).
Question: What's the deal with Pioneer 10 and 11 not accelerating as expected away from the solar system?
Answer: Measurements of the position of the furthest-out space probes (such as Pioneer 10 and 11) showed that they were accelerating toward the sun slightly faster than they should have been, bringing speculations of all sorts of extra, hitherto unknown deep space accelerative forces. These have since been attributed to heat energy from the spacecraft being radiated and providing a very small extra boost. The discoverers of the extra acceleration claim this explanation is not enough.
See Pioneer anomoly on Wikipedia for more information.
Anomalous Acceleration Update
From American Institute of Physics Bulletin of Physics News Number 447 September 9, 1999.
Last year a team of scientists published an assessment of the longterm trajectories of certain spacecraft, including Pioneer 10 and 11, showing that even after all known sources of gravity (sun, planets, comets, etc.) and other forces were taken into account, an extra acceleration seemed to be present (Anderson et al., Physical Review Letters, 5 Oct 98; Update 391).
Now a series of letters concerning this assessment appears in the same journal (30 August 1999). John Murphy of Johns Hopkins (301-953-6214) argues that the explanation lies in the asymmetric radiation given off by the crafts' electronics as waste heat. Jonathan Katz of Washington University (314-935-6202) implicates the recoil of radiation (from radioactive thermal generators, or RTG, powering the craft) off the rear of the high-gain antenna. The authors of the original paper (contact John Anderson, JPL, 818-354-3956) basically assert that these two particular explanations fall short of accounting for the anomalous acceleration by roughly a factor of five or more. (All PRL papers are available to journalists from AIP Public Information.)