GRAVITY appears to be working as everyone always thought, much to physicists鈥�
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鈥檚 Jet Propulsion Laboratory near Los
Angeles announced that the spacecraft鈥擯ioneer 10, Pioneer 11, Ulysses and
perhaps Galileo鈥攚ere 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鈥檚 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.
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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. 鈥淚t鈥檚 pretty close, and within observational errors,鈥� he says.
Jonathan Katz of Washington University in St Louis, Missouri, also blames
heat鈥攊n this case, the heat wasted because of the RTGs鈥� 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. 鈥淭he radiation can bounce off the back of the antenna and push the
spacecraft towards Earth,鈥� 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. 鈥淵ou can鈥檛
get the force you need,鈥� he says.