Sunday, December 19, 2010

The mysterious tug home explained?

See the first post in this series here.

The plutonium inside the [spacecraft] Pioneer’s generators gave off 2,500 joules of thermal energy per second at the height of its powers. Some of that heat got converted into electricity and ran the instrumentation. The rest simply radiated into space. If for whatever mechanical reason the heat radiated out from the generators unevenly, the extra heat radiating in one direction would exert an unbalanced recoil force, causing the spacecraft to accelerate. ...

Duly noting that point, the JPL team spent the next few years investigating all heat-related evidence. They came back with their verdict in 2002. Heat: not guilty. For one thing, they said, as the plutonium inside the generators decayed, the heat they gave off decreased, and so if heat were its cause then the anomalous acceleration of the spacecraft ought to have lessened with time as well. But it didn't – it seemed constant. Secondly, the generators were positioned quite far from the body of the spacecraft on the ends of long poles. From that remote distance, they calculated that very little heat would hit the spacecraft and exert a recoil force – an order of magnitude too little to cause the observed effect. Third and fourth, there was the tentative evidence offered by Galileo and Ulysses, both of which employed quite different power systems from that of the Pioneers.

Their arguments persuaded hordes of physicists, who began vying with great gusto for the thrones of Einstein and his non-relativistic assistant Newton. ...

[Viktor Toth] didn't think the study completed by Anderson and his team in 2002, which dismissed thermal effects as the cause of the anomaly, was even close to thorough enough. ...

In search of a more complete data set to work with, Toth started corresponding with [Slava] Turyshev at JPL. They agreed that an analysis was needed of the Doppler data spanning a much longer period of time than the decade-long segment analyzed previously. ...

It was 2005. Toth drove down from his home in Ottawa and met Turyshev for the first time, at Ames Research Center, where all the old data was stored. When they arrived, they found large dumpsters parked outside the building's entrance. All 30-plus years of Pioneer Doppler data and corresponding logbooks were due to be thrown out in two weeks. Funding at Ames was skeletal at that time, and it couldn't afford to archive anything. ...

Using the telemetry data, the two scientists created an extremely elaborate “finite element” 3-D computer model of each Pioneer spacecraft, in which the thermal properties of 100,000 positions on their surfaces are independently tracked for the duration of the 30-year mission. ...

The results of the telemetry analysis? “The heat recoil force accounts for part of the acceleration,” said Turyshev. They wouldn’t tell me how significant a part. (Turyshev: “We’d like to publish that in the scientific literature.”) But according to Toth, “You can take it to the bank that whatever remains of the anomaly after accounting for that thermal acceleration, it will at most be much less than the canonical value of 8.74 x 10-10 m/s2...” ...

“Let me tell you this,” said Turyshev, as I begged him for details about the analysis. “Physics as we know it worked well.”
--Natalie Wolchover, Popular Science, on Einstein and Newton striking back. HT: Gizmodo

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