By John Fleck
Journal Staff Writer
A mysterious force apparently tugging on three NASA spacecraft could
mean Albert Einstein, Isaac Newton and their theories of gravity are
wrong. It's more likely, scientists say, that something's wrong with
the calculations of the spacecraft's orbits.
But after 18 years of trying unsuccessfully to find such an error, the
scientists involved have begun to suggest that our understanding of
gravity -- one of the most hallowed of scientific principles -- might
need a
bit of tinkering.
Scientists think they understand the basics of gravity well -- so
well, that when they launch a spacecraft, they should be able to
predict its path with great precision.
But the ordinary laws of gravity don't seem to apply quite the way
they should to three NASA craft now winging through the solar system.
It's as if a slight extra force, not accounted for by current
theories, is tugging the craft toward the sun.
The scientists, led by researchers at Los Alamos National Laboratory
and NASA's Jet Propulsion Laboratory, believe there must be a
conventional
explanation.
But unable to find it, they've begun to climb gingerly out onto a
scientific limb. Next month, they will publish a paper suggesting the
possibility that "new physics" might be at work.
The paper has already survived evaluation by two anonymous reviewers
for the prestigious journal Physical Review Letters, and it's being
rushed into
press unusually quickly.
But scientists tend to be a careful bunch when challenging
well-established theories, and this group is no exception. The
conversations are peppered
with caveats about the possibility of some "systematic effect" at
work-- a conventional explanation they have overlooked.
"If I had to bet the mortgage, I'd say it's a systematic effect we
haven't found out yet," said Los Alamos physicist Michael Nieto, one
of the paper's authors.
But after years of testing explanations with no success, Nieto said
he'd be willing to bet at least a quarter in a slot machine that "new
physics" are at work here.
Scientists can't begin to say what that new physics might be.
"We just couldn't think of anything else" to explain the phenomenon,
said John D. Anderson of the National Aeronautics and Space
Administration's
Jet Propulsion Laboratory, who led the study.
Our understanding of gravity dates to the 1600s, when Newton theorized
that all bodies, like the sun and Earth, attract one another.
The bigger the body, the stronger the force, which is why gravity
pulls us down on Earth, but not toward each other.
It's what holds Earth in orbit around the sun, and Newton offered a
mathematical account of its strength.
Newton's calculations held up for more than 200 years, offering
scientists a precise way of calculating the orbits of the planets
around the sun.
In the early 20th century, the theory was modified slightly by
Einstein's theory of general relativity, and has withstood repeated
tests since.
But the JPL scientist has been vexed for the last 18 years because the
orbits of some of his spacecraft didn't seem to fit the theories of
gravity.
He began noticing it in 1980 in data being collected by JPL on the
lab's Pioneer 10 spacecraft, then making a long, looping orbit out
through our solar system. It was not speeding away quite as fast as
expected. He thought it was just an error in the data, but the
problem got worse as similar anomalies showed up in data from two more
spacecraft, Pioneer
11 and Ulysses.
"I kept looking for things and trying to explain it and failing," he
said in a telephone interview this week.
The effect is so tiny, it's as if the tug on an apple falling from
Newton's tree was one-10 billionth stronger than it should have been,
Nieto said.
But if it's real, the effect is important enough to throw off the
theory of gravity in the same way that tiny discrepancies in the orbit
of the planet Mercury laid the groundwork for Einstein's theory of
general relativity.
Anderson and Nieto began working together on the problem five years ago.
he unexpected sunward tug on the craft doesn't seem to be explained by
gas leaking from spacecraft fuel tanks, they said, or heat from
radioactive batteries.
They've looked for errors in the clocks used when the data are
collected and in the software used to analyze it.
Publication opens the question to other scientists.
News of the study, percolating through the physics community, has
raised eyebrows, but no one has found any obvious flaws.
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