* Astronomy

NASA engineers report Pioneer 10's radioisotope power source has decayed, and it may not have enough power to send additional transmissions to Earth. NASA's Deep Space Network (DSN) did not detect a signal during the last contact attempt Feb. 7, 2003. The previous three contacts, including the Jan. 22 signal, were very faint with no telemetry received. The last time a Pioneer 10 contact returned telemetry data was April 27, 2002. NASA has no additional contact attempts planned for Pioneer 10.

"Pioneer 10 was a pioneer in the true sense of the word. After it passed Mars on its long journey into deep space, it was venturing into places where nothing built by humanity had ever gone before. It ranks among the most historic as well as the most scientifically rich exploration missions ever undertaken.," - Dr. Colleen Hartman, director of NASA's Solar System Exploration Division, NASA Headquarters, Washington.

The spacecraft was the first to make direct observations and obtain close-up images of Jupiter. Pioneer also charted the gas giant's intense radiation belts, located the planet's magnetic field, and established Jupiter is predominantly a liquid planet. In 1983, Pioneer 10 became the first human-made object to pass the orbit of Pluto, the most distant planet from the Sun.

Following its encounter with Jupiter, Pioneer 10 explored the outer regions of the solar system, studying energetic particles from the Sun (solar wind), and cosmic rays entering our portion of the Milky Way. The spacecraft continued to make valuable scientific investigations in the outer regions of the Solar System until its science mission ended March 31, 1997.

that the Pioneer 10 and 11 space probes, launched more than 30 years ago, appear to be in the grip of a mysterious force that is holding them back as they sweep out of the solar system.
As a result, scientists pressed a European Space Agency (Esa) meeting, called Cosmic Visions, in Paris for a new mission that would follow the Pioneers and pinpoint the cause of their erratic movements. The strange behaviour of the Pioneers was discovered by John Anderson and Slava Turyshev of the Jet Propulsion Laboratory in Pasadena and Michael Martin Neito of the Los Alamos National Laboratory. They noted there were persistent anomalies in their trajectories. Every time they looked the Pioneers were in the wrong place. The effect was not large, but it was significant. One proposal put forward is that Newton's idea that the force of gravity weakens as distance increases may be incorrect over very large spaces, and may drop off over very long distances.
The acceleration of the Pioneer spacecraft could be linked to another anomaly involving apparent gravitation anomalies that were recorded during solar eclipses. According to Chris Duif at the Delft University of Technology, the 'Allais effect' is real, and unexplained.
More detailed information can be found in his paper here. (pdf)
Update:
The Pioneer 10 and 11 spacecraft have had their a small, anomalous, Doppler frequency drift, interpreted as an acceleration, calculated as (8.74 +/- 1.33) x 10^-8 cm/s² towards the Sun
The new study investigated the possibility that this anomaly could be due to a drag on the spacecraft from their passing through a interplanetary medium, but found that the radiometric data would require an unexpectedly high mass density, -19 g/cm³, of interplanetary dust for this mechanism to work. Further, the magnitude of the density would have to be nearly constant at distances ~ 20-70 AU. Therefore, the researchers found such an explanation to be very unlikely.
A mission to investigate the anomaly would be able to place a better limit on the density, or perhaps even to measure it.

Incidentally, the solar storms of October and November 2003 produced massive coronal mass ejections (CMEs) are about to overtake Voyager 1.
The Voyager 1 spacecraft is nearly 9 billion miles (14.5 billion km) from the sun, at the edge of our solar system.
A solar blast wave is heading for the spacecraft, and "it could arrive at any moment,"
Hopefully, a receiver on board will record a 2 to 3 kHz radio burst .
Voyager 2 measured (indirectly at 600 km/s) the speed of the shock, as well as its composition, temperature and magnetism.