Wed, 07 Mar 2007 07:21 CST
Scientists and engineers remain on course in their efforts to determine what caused the twin Pioneer spacecraft to apparently drift off course by hundreds of thousands of kilometres during their three-decade missions. Within a year, they expect to be able to decide whether this drift was caused by a fault on the spacecraft.
Launched 35 years ago on Friday, Pioneer 10 was the first spacecraft to reach the outer solar system and return pictures of Jupiter. It was followed by Pioneer 11, which launched on 5 April 1973 and also visited Saturn.
After these historic encounters, NASA kept track of the drifting spacecraft, finally losing contact with Pioneer 11 in 1995 and Pioneer 10 in 2003.
The so-called Pioneer anomaly showed up in the tracking data as a tiny deceleration for both spacecraft, even though they were heading in different directions. It was as if the Sun's gravity was pulling a little harder than Newton's laws predicted (see 13 things that do not make sense).
Escaping heatThe source of the deceleration has long been suspected to be heat escaping from the small nuclear generators on board, known as RTGs (Radioisotope Thermal Generators). Previous analyses that claimed to rule out this effect have been contested.
To supply a definitive answer, an international team of scientists are re-analysing the tracking data and the telemetry data (see Have we got gravity all wrong?).
The latter holds information about the spacecraft's internal behaviour, including the heat released by the RTGs. This can be compared to the tracking data to see whether the Pioneer Anomaly matches the changes in heat radiated throughout the spacecraft's lifetime.
Volunteer programmerThe second meeting of the self-styled Pioneer Explorer Collaboration took place in late February at the International Space Science Institute in Bern, Switzerland. "We now have hard numbers to work with," says Viktor Toth, a software developer from Canada whose hobbies are space exploration and early computer systems.
Armed with thousands of pages of Pioneer manuals, Toth worked in his own time to write programmes that could make sense of the telemetry from over 120 sensors carried by each spacecraft. "I essentially wrote new software to do what the old software used to," he says. He succeeded in fully recovering 40 gigabytes of telemetry data.
"We are so lucky to have Viktor, his contribution is enormous," says Slava Turyshev, who masterminds the investigation from NASA's Jet Propulsion Laboratory in Pasadena, California, US.
Painstaking processThe tracking data has proved more challenging. In July 2006, Turyshev thought the data was ready for analysis, but problems soon arose. "The tracking system evolved so many times, with new computers and updated receiving dishes, that we are having to look at each individual data file and work out how to translate it into a common format," says Turyshev.
The painstakingly reconstructed dataset should be ready for analysis around June 2007. It will supply the precise direction in which the anomaly acted and any changes in its strength over time.
If the direction is towards the Earth, it almost certainly indicates the anomaly was caused by faulty technology or an artefact of receiving the data at the ground stations. If, however, the direction is towards the Sun, new gravitational physics may be needed to explain the effect.