Max Planck Institute of Aeronomy
December 12, 1997
The Pioneer 10 data are obtained from a "two-way radio link" experiment (PI: John D. Anderson, JPL): a signal is transmitted from Earth (uplink), transponded by the spacecraft and hours later received back at Earth (downlink). With a local oscillator (a highly stable hydrogen maser) it is possible to determine the Doppler shift to an accuracy better than mHz. After removing systematic changes in the position and velocity of Pioneer 10 a main period of 25.3 days was found by the Max-Planck scientist K. Scherer, his colleagues H. Fichtner from the University of Bonn and J. Anderson and E. Lau from JPL. The data were recorded when the spacecraft was moving from a distance of 40 to 60 AU (6 to 9 billion km) from the Sun.
The modulation of the Pioneer 10 radio signal is caused by an electron density fluctuation of a particle stream (solar wind) from the Sun which is imprinting its (siderial) rotation period on the variations.
The pulsar is also observed via radio-signals at similar frequencies. A 25.3 day periodicity in this signal was originally interpreted as the reflex motion of the pulsar due to an orbiting (moon-sized) planet. According to that interpretation, the pulsar as well as the planet would describe ellipses around their common center of mass, because of the mutual gravitational attraction, The semimajor axis of the pulsar's ellipse would be less than 1 km. Therefore, the distance to the pulsar and, correspondingly, the time of arrival (TOA) of the radio waves would vary periodically. This way one could explain the microsecond variations in the TOA occuring with a period of 25.3 days.
However, the research team demonstrated, that, annalogous to the modulation of the Pioneer 10 signal, the 25.3 day variation in the electron density of the solar wind also affects the pulsar's radio waves and can cause their TOA variation in the range of microseconds. Therefore, the scientists concluded that this (innermost) planet around the pulsar PSR 1257+12 is simulated by solar rotation.