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.
鈥淲e鈥檝e been working on this problem for several years, and we accounted for
everything we could think of,鈥� says John Anderson, a planetary scientist at
NASA鈥檚 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鈥攈ow 鈥渟tretched out鈥�
they are鈥擭ASA scientists have been able to work out how fast the probe is
travelling. Pioneer 10 seems to be slowing more quickly than it should.
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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鈥檚 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鈥檚 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鈥擯ioneer 10鈥檚
gauges show no unexpected loss of fuel. Aerodynamic drag from the interstellar
medium also couldn鈥檛 be involved, as there just isn鈥檛 enough material to account
for the effect. Thermal radiation from the spacecraft鈥檚 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鈥檛 be responsible, either. 鈥淲e ruled
out other sources of gravitation,鈥� 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鈥檚 poles, had an even larger anomalous pull towards the Sun. Data
from Galileo, now orbiting among Jupiter鈥檚 moons, appear to show the same
effect.
Other researchers aren鈥檛 ready to abandon cherished ideas about gravity on
the basis of the data gathered by Anderson鈥檚 team. 鈥淭hey鈥檙e extremely good at
what they do,鈥� says Clifford Will, a physicist at Washington University in St
Louis. 鈥淏ut I think there鈥檚 some kind of systematic effect that has corrupted
the data.鈥� John Ries, a planetary scientist at the University of Texas at
Austin, says he can鈥檛 believe a new gravitational force is involved, because
that should affect the motions of the planets.
Anderson and his colleagues are similarly cautious. 鈥淚t鈥檚 likely that it鈥檚
some systematic error,鈥� 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. 鈥淭here鈥檚 a small probability that it鈥檚 very important,鈥� says Anderson.