[escepticos] deceleracion en sondas de la NASA posiblemente explicada

[arteaga@...>]

	A lo mejor os acordais de que hace algo asi como un mes os 
mande un articulo diciendo que varias sondas de la NASA que han
viajado muy lejos del Sol estaban decelerando mas rapido de lo 
predicho. Esta deceleracion era muy, muy pequen~a, pero detectable y
consistente. 

	Bien, pues es posible que es misterio haya sido resuelto, y
que se deba simplemente a la presion de la radiacion termica.

	Edward Murphy sugiere que la causa del problema es que los
radiadores de estas sondas estan colocados apuntando en la direccion
contraria al Sol, por razones obvias. De forma que se desprende del
calor de las naves mandando fotones infrarojos hacia delante,
consiguiendo de esta forma el frenado.

	Jonathan Katz le echa las culpas a los RTG de las sondas, 
los generadores radiactivos. Estas cosas estan siempre calientes 
y se mantienen apartadas de la nave, detras de la antena. Katz 
sugiere que los fotones desprendidos hacia delante frenan la nave, 
y los desprendidos hacia atras rebotan en la antena y tambien frenan 
la nave.

	Sin estar claro el mecanismo exacto, parece que la
deceleracion observada es consistente con la presion de radiacion 
que se puede obtener con la energia generada en las naves. Pero 
John Anderson, "el que dio la voz de alarma", sigue sin estar
convencido. Ya veremos.

	Santi


 From: Andrew Yee <ayee en nova.astro.utoronto.ca>
 Newsgroups: sci.space.news
 [1] Mystery Force Is Traced To Satellites' Waste Heat (Forwarded)
 Date: Wed, 14 Oct 1998 14:48:48 -0400

New Scientist

UK Contact: Claire Bowles, claire.bowles en rbi.co.uk, 44-171-331-2751
US Contact: Barbara Thurlow, newscidc en 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.

Author: Charles Seife
New Scientist issue 17th October 1998

PLEASE MENTION NEW SCIENTIST IF YOU USE THIS ARTICLE - THANK YOU.


---
Andrew Yee
ayee en nova.astro.utoronto.ca