LOS ALAMOS, N.M., Sept. 24, 1998 — A team of planetary scientists and physicists has identified a tiny, unexplained sunward acceleration in the motions of the Pioneer 10, Pioneer 11 and Ulysses spacecraft.
Pioneer 10 was officially tracked until March 1997, when it was some six billion miles away (64 AU) from the sun. (Pioneer 10 is still transmitting, and occasional additional radio Doppler data is provided to the research team.)
Pioneer 11, due to a radio failure, last sent useful radio Doppler transmissions in October 1990, when it was less than three billion miles from the sun (about 32 AU, about Pluto’s perihelion). Ulysses has been tracked on its looping flight out of the ecliptic and around the sun’s poles.
Trusty and reliable Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.