- The unexpected slowing of distant spacecraft
that has baffled scientists for decades may have a simple explanation:
It could be caused by heat, say a physicist and an astronomer.
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- Gravity appears to be working as everyone
always thought, much to physicists' relief.
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- 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 traveled away from the sun.
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- Physicists wondered if this meant they
would have to rewrite the equations of gravity. But now two scientists
have suggested an alternative solution, according to the most recent New
Scientist.
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- The spacecraft are powered by plutonium-based
radioisotope thermoelectric generators (RTGs). 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 louvered fins that open
when they get hot and radiate the heat away, Edward Murphy, an astronomer
at Johns Hopkins University in Baltimore, tells New Scientist.
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- 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.
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- "It's pretty close, and within observational
errors," he says.
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- Jonathan Katz of Washington University
in St. Louis 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.
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- "The radiation can bounce off the
back of the antenna and push the spacecraft towards Earth," he says.
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- 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.
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- "You can't get the force you need,"
he says.
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