Search for a Solution of the Pioneer Anomaly

In 1972 and 1973 the Pioneer 10 and 11 missions were launched. They were the first to explore the outer solar system and achieved stunning breakthroughs in deepspace exploration. But beginning in about 1980 an unmodeled force of ∼ 8× 10−8 cm/s2, directed approximately towards the Sun, appeared in the tracking data. It later was unambiguously verified as being in the data and not an artifact. The cause remains unknown (although radiant heat remains a likely origin). With time more and more effort has gone into understanding this anomaly (and also possibly related effects). We review the situation and describe ongoing programs to resolve the issue. PACS 95.10.Eg, (Orbit determination and improvement), 95.55.Pe, (Lunar, planetary, and deep-space probes), 96.12.De (Orbital and rotational dynamics) 1 Email: [email protected] 2 Email:[email protected] ar X iv :0 70 9. 38 66 v1 [ gr -q c] 2 5 Se p 20 07 1 The Pioneer Missions In the 1960’s the use of planetary flybys for gravity assists of spacecraft became of wide interest. That is when the Jet Propulsion Laboratory (JPL) first started thinking about what became the “Grand Tours” of the 1970’s and 1980’s (the Voyager missions). The concept was to use flybys of the major planets to both modify the direction of a spacecraft and also to add to its heliocentric velocity. At the time many found it surprising that energy could be transferred to a spacecraft from the orbital-motion angular-momentum of a planet about the Sun. This was despite the fact it had been known since the works of Lagrange, Jacobi, and Tisserand on the three-body problem [1, 2] that the energies of comets could be affected by passing near Jupiter. Even in the simplest, circular restricted 3-body problem [2], it is not that the energy of each object is conserved, only that the total energy of the entire system is conserved. Flybys can both give kinetic energy to a spacecraft (to boost its orbital velocity) and also can take kinetic energy from it (to slow it down for an inner body encounter). The first missions to fly to deep space were the Pioneers. By using flybys, heliocentric velocities were obtained that were unfeasible at the time by using only chemical fuels. Pioneer 10 was launched on 2 March 1972 local time, aboard an Atlas/Centaur/TE364-4 launch vehicle (see Figure 1). It was the first craft launched into deep space and was the first to reach an outer giant planet, Jupiter, on 4 Dec. 1973 [3, 4]. Later it was the first spacecraft to leave the “solar system” (past the orbit of Pluto, as it was then defined). The Pioneer project eventually extended over decades. It was managed at NASA/Ames Research Center under the hands of four successive project managers; the legendary Charlie Hall, Richard Fimmel, Fred Worth, and the current Larry Lasher. While in its Earth-Jupiter cruise, Pioneer 10 was still bound to the solar system. By 9 January 1973 Pioneer 10 was at a distance of 3.40 AU (Astronomical Units), beyond the asteroid belt. This in itself was a happy surprise. The craft had not been destroyed passing through the asteroid belt, which had been greatly feared at the time. With the Jupiter flyby, Pioneer 10 reached escape velocity from the solar system. Pioneer 10 has an asymptotic escape velocity from the Sun of 11.322 km/s (2.388 AU/yr). It is headed in the general direction of the star Aldebaran (scheduled to reach that region in about 2 million years). This direction is opposite the relative motion of the solar system in the local interstellar dust cloud and opposite to the direction towards the galactic center. Pioneer 11 followed soon after Pioneer10, with a launch on 6 April 1973. It, too, cruised to Jupiter on an approximate heliocentric ellipse. This time during the EarthJupiter cruise, it was determined that a carefully executed flyby of Jupiter could put the craft on a trajectory to encounter Saturn in 1979. So, on 2 Dec. 1974, when Pioneer 11 reached Jupiter, it underwent a Jupiter gravity assist that sent it back inside the solar system to catch up with Saturn on the far side. It was then still on an ellipse, but a more 3 An Astronomical Unit is the mean Sun-Earth distance, about 150,000,000 km.