|© iStockphoto.com / Karl Dolenc|
The Pioneer 10 and 11 space probes were launched in 1972 and 1973 respectively with missions to survey Jupiter and the outer solar system. At the end of their successful missions, both probes had trajectories which left them on hyperbolic courses to exit the solar system forever.
After their primary missions were completed, NASA continued to monitor the probes until they were no longer able to transmit signals. The last time Pioneer 11 was heard from was in November 1995, and Pioneer 10's signal has not been detected since January 2003.
Close examination of data regarding the paths of the spacecraft has shown that there is a very small acceleration towards the sun that cannot be accounted for after every known force is taken into account. A large number of possible effects have generally been ruled out including fuel leakage, the solar wind, and navigational errors.
The Pioneer probes are not the only probes that have experienced unexplained changes in acceleration. A number of more recent missions have also experienced small changes in velocity as they passed close to the Earth for gravitational-assist maneuvers:
- The Galileo mission to Jupiter
- Cassini-Huygens to Saturn
- The NEAR mission to the asteroid Eros
- The European Space Agency's Rosetta mission to a comet
- Ulysses mission to observe the Sun
The cause of the effect is still an open question and their is not enough data to resolve the question conclusively. A number of potential causes have been suggested, including:
- The effect of unseen dark matter
- New physics, including Unruh radiation
- Incorrect application of existing physics
- Additional dimensions to the Universe creating small forces that we don't understand
A mission to specifically to study the effect has been proposed, but has not been approved. Scientists will be especially interested in the third flyby of Earth by the Rosetta mission which will occur on November 13, 2009.