Credit: NASA
The best hope for the future lies with the New Horizons probe, which is presently headed for Pluto. "It wasn't designed for precision tracking," Anderson says. "[But] it may yet return some good data because it's on a good trajectory, and it's a spinner."
Interestingly, however, several closer spacecraft have shown anomalous behaviours. The first of these was Galileo, which whipped by Earth on 8 December 1990, en route to a 1990 rendezvous with Jupiter.
When the spacecraft emerged from its speed-boosting fly-by of Earth, it was quickly clear that it had gained more energy from the manoeuvre than expected. Since then, similar anomalies have been seen in some other Earth fly-bys.
"We get a little boost that we didn't expect," says Peter Antreasian, a spacecraft navigator at JPL. "I was working on the NEAR project in 1998 [Near Earth Asteroid Rendezvous probe], where we saw a sizable increase in energy, equivalent to 13.5 millimetres per second."
In an article published in March 2008, in Physical Review Letters, Anderson and four colleagues examined all such fly-bys for similar effects.
Sometimes it was there; sometimes not. Galileo, for example, made a second pass in 1992, but no unexpected deviation occurred. But in that case, the satellite's trajectory took it much closer to Earth.
"We were watching for [the effect]," Antreasian says, "but we were at a lot lower altitude, and you start to get drag effects from the atmosphere. The uncertainty in that masks out this anomalous delta-v."
Similarly, the Cassini data was useless because the spacecraft was firing its thrusters at the time of the fly-by. Of the others – NEAR, Rosetta and Messenger – only Messenger failed to show a fly-by anomaly.
What's happening is anybody's guess. Despite some sensational media reporting, the only thing the two have in common, Turyshev believes, is that both involve spacecraft, and both are called anomalies. "Nobody has established that the two are connected," he says.
Anderson's not so sure. He's wants to know if the new, extended trajectory data on the Pioneer missions will show that the Pioneer anomaly is somehow "turned on" by the probe's encounters with Jupiter and (in the case of Pioneer 11) Saturn. If it is, the two might be linked.
"We can see how it's affected by going by a planet," he says. Turyshev, on the other hand, finds this unlikely; any evidence of a 'turn on' effect is an artefact of the way in which the data was initially recorded, he says.
