http://www.nature.com/news/2004/041115/full/041115-16.html

The mission would need to have very accurate navigation, and instruments that could detect the tiny deceleration and potential causes such as leaking gas. And to get an answer in the next couple of decades would take a fast-travelling probe, one faster than the Cassini spacecraft currently orbiting Saturn. That planet is only half as far away as Uranus, but it still took Cassini seven years to reach its destination.

Instead of relying on conventional rocket fuel, the craft might use the faster propulsion systems being investigated by NASA and the European Space Agency (ESA), which involve nuclear power.

The first of these craft is likely to be NASA's Jupiter Icy Moons Orbiter, scheduled to launch around 2015. But the researchers say that none of the currently proposed missions could probe the Pioneer anomaly as they stand, because they will not have sufficiently accurate navigation or instruments.

So the researchers are arguing for a spacecraft that draws on the lessons learnt from the Pioneer missions, in which various accidents of design have made it possible to track the spacecraft's movement very closely.

"It could be developed in five years and flown early in the next decade," they say.