<http://news.bbc.co.uk/hi/english/sci/tech/newsid_1332000/1332368.stm>
BBC News Online: Sci/Tech
Tuesday, 15 May, 2001, 15:46 GMT 16:46 UK
Mystery force tugs distant probes
By BBC News Online science editor Dr David Whitehouse
An unexplained force is pulling on distant spacecraft. Researchers have
come to this conclusion after a thorough analysis of the deep-space probes'
trajectories.
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"We've been working on this problem for several years, and we have
accounted for everything we could think of"
--Dr John Anderson, NASA
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It could be just a tiny unnoticed effect in the spacecraft themselves, but
scientists warn it could also be the first hint that modifications need to
be made to our understanding of the force of gravity.
"It is almost as if the probes are not behaving according to the known law
of gravity," said Dr John Anderson, of the American space agency's (NASA)
Jet Propulsion Laboratory, and lead scientist on the study.
He said: "We've been working on this problem for several years, and we have
accounted for everything we could think of."
Great detail
The unexplained force appears to be acting on four deep-space probes
scattered around the Solar System.
Pioneer 10 was launched towards the outer planets in 1972. It is now well
beyond Jupiter but still in radio contact with Earth.
By studying the Doppler shift (the "stretching") of the radio signals from
the probe, scientists have been able to calculate how fast the craft is
travelling. Since 1980, its trajectory has been mapped in very great detail.
The puzzle is that Pioneer 10 is slowing more quickly than it should.
It was initially suggested that this might be due to the force from a tiny
gas leak or that it was being pulled off course by the gravity of an unseen
Solar System object.
Unseen body
The mystery deepened further when an analysis of the trajectory being
followed by its sister spacecraft, Pioneer 11, launched in 1973, showed
that it too was being subjected to the same mysterious effect.
But Pioneer 11 is on the opposite side of the Solar System from Pioneer 10,
about 22 billion km (about 14 billion miles) away. This means the effect
cannot be the gravitational effect of some unseen body.
Add to all this hints that the same unexplained effect might have been
acting on the Galileo spacecraft on its journey to Jupiter, and the Ulysses
spaceprobe that is circling the Sun, and you have a Solar System-wide puzzle.
In a report soon to be published in a major astronomical journal, Dr
Anderson and colleagues have carried out an impressive study of the state
of the Pioneer spacecraft and all the tiny forces to which they could be
subjected.
Planets unaffected
"Our analysis strongly suggests that it is difficult to understand how any
of these mechanisms can explain the magnitude of the observed behaviour
of the Pioneer anomaly," the team says.
It has been suggested that the spacecraft tracking data have shown a
deviation in the force of gravity that is apparent only across vast
distances. It has also been pointed out that the strength of the effect
seems to be related to two of the Universe's physical constants: the speed
of light and the speed of the expansion of the Universe.
But others have dismissed this as being too fanciful, arguing that if the
Pioneer anomaly was really indicative of a change in our understanding of
gravity, then it would be apparent in the orbits of the planets around the
Sun - which it is not.
The effect is as yet unexplained and with all four affected probes never to
return to Earth for analysis, it may well remain that way.
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