Editor's Note: JR Minkel is in St. Louis this week for the annual "April meeting" of the American Physical Society. See his other blog posts on dark matter, the Higgs boson and the timeline for the Large Hadron Collider, and check back for frequent updates.

ST. LOUIS—Ten years ago, NASA researchers discovered that the Pioneer 10 and 11 spacecraft had fallen slightly behind course during their 35-year journeys to the outer reaches of the solar system. In what has become known as the Pioneer anomaly, which was the subject of one of the talks this weekend at the American Physical Society here in St. Louis, nobody knows for sure why it happened. It probably stemmed from leaking gas or heat.

But there's also the possibility, however remote, that gravity doesn't behave the way we expect. Until recently, researchers haven't had the data to distinguish the different possibilities. That changed in 2006, when NASA physicist Slava Turyshev, a co-discoverer of the anomaly, was visiting a colleague at the NASA Ames Research Center. The Moffett Field, Calif.–facility was about to throw out hundreds of magnetic disks containing the Pioneer telemetric data—temperature and power readings that the twin craft had sent back to NASA once every few minutes until they traveled out of range. (NASA finally lost contact with Pioneer 10 in 2003, after 31 years. It had lost contact with Pioneer 11 in 1995.)

Turyshev and his colleagues rescued the data, and Viktor Toth, a computer programmer in Ottawa, Ontario, volunteered to write brand new code that extracted the telemetry readings from the raw 1s and 0s encoded in the magnetic disks.

A group of some 50 researchers, including Turyshev, is now trying to match the data to a detailed computer model of the craft's inner workings. The model is designed to mimic the flow of heat and electricity produced by the craft's generators, which harnessed the heat from radioactive plutonium and turned a fraction of it into electricity to power the craft. The remaining heat [see note below] was lost to space or spread to other parts of the craft such as the antenna, which influenced each probe's overall momentum.

So far the model accounts for about 30 percent of the observed anomaly for Pioneer 10 at a single distance of 25 astronomical units (2.3 billion miles, or 3.7 billion kilometers) from the sun, Turyshev reported. The group still has to extend the model to other distances and to Pioneer 11. The full verdict may not be in for some time. "I'm trying to ensure we apply every relevant piece of information," Turyshev says. "It is likely that the thermal explanation will explain part of the anomaly," he says, but exactly how much is up for grabs.

He notes that the team is also expanding the original analysis that identified the anomaly from 11 years' worth of transmissions from Pioneer 10 and four years from Pioneer 11. Turyshev says the group now has access to 30 and 20 years of data, respectively, and will begin examining it in the coming weeks.

If the anomaly does turn out to have a straightforward explanation, researchers could use it to improve navigation for sensitive space experiments to test gravitational effects, notes programmer Toth. And if not? "If we actually had a means in the solar system here to measure deviations from Einstein's gravity, that would be phenomenal," he says.

Note: This article originally stated incorrectly that modeling had explained 30 percent of the Pioneer anomaly up to a distance of 10 astronomical units; and that heat from Pioneer 10 produced 160 watts of power. The average heat energy emitted by the craft was more than two kilowatts.