GRAVITY appears to be working as everyone always thought, much to physicists' relief. The unexpected slowing of distant spacecraft reported last month may have a simple explanation. It could be caused by heat, say a physicist and an astronomer.
In September, John Anderson of NASA's Jet Propulsion Laboratory near Los Angeles announced that the spacecraft-Pioneer 10, Pioneer 11, Ulysses and perhaps Galileo-were slowing down faster than expected as they travelled away from the Sun. Physicists wondered if this meant they would have to rewrite the equations of gravity (This Week, 12 September, p 4). But now two scientists have suggested an alternative solution.
The spacecraft have plutonium-based radioisotope thermoelectric generators (RTGs) to power them. Resistance in the spacecraft's circuits turns some of the electrical power produced by the RTGs into heat. To get rid of it, the spacecraft are fitted with louvred fins that open when they get hot and radiate the heat away, according to Edward Murphy, an astronomer at Johns Hopkins University in Baltimore, Maryland.
The radiators face away from the Sun, so most radiation is emitted in this direction. Murphy says the departing photons give the spacecraft a small push in the opposite direction, towards the Sun, slowing them down. He believes the amount of radiation leaving the spacecraft could easily account for the observed push. "It's pretty close, and within observational errors," he says.
Jonathan Katz of Washington University in St Louis, Missouri, also blames heat-in this case, the heat wasted because of the RTGs' inefficiency at turning thermal energy into electricity. He points out that the satellites have large antennas that point to the Earth, and that the RTGs sit just off to the side. "The radiation can bounce off the back of the antenna and push the spacecraft towards Earth," he says.
Both Katz and Murphy have submitted their calculations to Physical Review Letters. But Anderson, who had last month ruled out a heat effect as the cause of the deceleration, is still unconvinced by the new arguments. "You can't get the force you need," he says.
Author: Charles Seife
New Scientist issue 17th October 1998
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