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EMBARGOED FOR RELEASE: 9 SEPTEMBER 1998 AT 2:00 p.m. EDT
Distant Spacecraft Seem To Be Showing No Respect For The Laws Of Physics
Gravity may not be working as advertised. Spacecraft hurtling through
the Solar System have been behaving so bizarrely that some scientists
wonder whether our theories of gravity are wrong.
"We've been working on this problem for several years, and we accounted
for everything we could think of," says John Anderson, a planetary
scientist at NASA's Jet Propulsion Laboratory in Pasadena.
In 1972, NASA launched Pioneer 10 in the direction of Jupiter. For a
quarter of a century, radio signals have been beamed to the spacecraft
and reflected back to Earth as it continued its odyssey to the outer
Solar System and beyond. By studying the red shift of the returning
radio waves -- how "stretched out" they are -- NASA scientists have
been able to work out how fast the probe is travelling. Pioneer 10
seems to be slowing more quickly than it should.
The signals bouncing back from Pioneer 10 are far from clean. The Earth
revolves around the Sun, stretching and compressing the radio waves
periodically. The probe also occasionally corrects its course so that
its antenna remains pointing towards the Earth. But scientists have a
good handle on these effects, and can cancel them out.
That is, they thought they could, until Anderson's team started analysing
Pioneer 10 data collected since 1987. They found a systematic anomaly,
as if Pioneer 10 were receiving an extra tug from the Sun's gravity. The
disagreement is 80 billionths of a centimetre per second squared, a tiny
rate of deceleration that would take more than 650 years to bring a car
travelling at 60 kilometres an hour to a halt. But to scientists used
to working with absolute precision it is a glaring discrepancy.
What could be to blame? A fuel leak was quickly ruled out - Pioneer
10's gauges show no unexpected loss of fuel. Aerodynamic drag from the
interstellar medium also couldn't be involved, as there just isn't
enough material to account for the effect. Thermal radiation from the
spacecraft's batteries would also be too puny, and would be emitted in
all directions rather than pushing the probe towards the Sun. An unknown
asteroid couldn't be responsible, either. "We ruled out other sources of
gravitation," says Anderson.
If just one spacecraft were being affected, the discrepancy would be
infuriating, but certainly not enough to start questioning current
theories of gravity. But Pioneer 11, launched in 1973 towards the other
end of the Solar System (see Diagram), is also slowing at about the
same rate. The Ulysses probe, launched in 1990 towards Jupiter, before
swinging into an orbit that took it over the Sun's poles, had an even
larger anomalous pull towards the Sun. Data from Galileo, now orbiting
among Jupiter's moons, appear to show the same effect.
Other researchers aren't ready to abandon cherished ideas about
gravity on the basis of the data gathered by Anderson's team. "They're
extremely good at what they do," says Clifford Will, a physicist at
Washington University in St Louis. "But I think there's some kind of
systematic effect that has corrupted the data." John Ries, a planetary
scientist at the University of Texas at Austin, says he can't believe
a new gravitational force is involved, because that should affect the
motions of the planets.
Anderson and his colleagues are similarly cautious. "It's likely that
it's some systematic error," says Michael Nieto of the Los Alamos
National Laboratory in New Mexico, a member of the team. But until
someone can identify an error in the data, outlined in a paper to be
published in Physical Review Letters , the possibility that the team
has broken new ground in physics remains. "There's a small probability
that it's very important," says Anderson.
Author: Charles Seife
NEW SCIENTIST MAGAZINE - ISSUE 12 SEPT 1998, PAGE 4
PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS ARTICLE