archive: SETI FW: [ASTRO] Mystery Force Traced To Satellites' Waste Heat

SETI FW: [ASTRO] Mystery Force Traced To Satellites' Waste Heat

Larry Klaes ( lklaes@zoomtel.com )
Thu, 15 Oct 1998 09:47:19 -0400

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From: Ron Baalke
Sent: Wednesday, October 14, 1998 3:33 PM
To: astro@lists.mindspring.com
Subject: [ASTRO] Mystery Force Traced To Satellites' Waste Heat

New Scientist

UK Contact: Claire Bowles, claire.bowles@rbi.co.uk, 44-171-331-2751
US Contact: Barbara Thurlow, newscidc@idt.net, 202-452-1178

EMBARGOED FOR RELEASE: 14 OCTOBER 1998 at 2:00 p.m. EDT

Mystery Force Is Traced To Satellites' Waste Heat

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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