Credit: NASA
IN 1969, JOHN D. ANDERSON, an astronomer with NASA's Jet Propulsion Laboratory (JPL) in California, proposed a simple experiment.
When the Pioneer 10 and 11 spacecraft launched (1972 and 1973, respectively) flew by Jupiter (in 1973 and 1974), why not use the frequency shifts in their radio signals to map the giant planet's gravity field?
The theory was simple: as the radio signals fought their way out of Jupiter's gravity, they would lose energy, slipping slightly toward lower frequencies.
Careful measurements of that shift would translate into ultra-precise measures of the planet's gravity, which is something planetary scientists could use in order to make deductions about Jupiter's interior.
There was only one glitch: when the data was compiled, it turned out that the radio signals were slowly shifting frequency even while the two spacecraft were nowhere near Jupiter.
The most obvious explanation was that the spacecraft were changing speed. That would produce a Doppler effect in their signals, comparable to a police siren changing pitch as the car flashes past.
Such an effect, in fact, was expected as the probes slowed, on their way toward interstellar space. But they were slowing by too much. "I was having to account for a small, unmodelled force in order to fit the data," Anderson says.
As far as scientists listening from Earth knew, aliens could have lassoed the probes with some kind of force field and tugged them ever so slightly back toward the Sun. Not that anyone believes extraterrestrials were involved. The problem is that nobody's really sure exactly what is involved.
"It's one of those serendipitous things where sometimes the good science is in the surprises," says Anderson, who spearheaded the team that published their discovery in the Physical Review Letters in 1998.
The effect isn't large: equivalent to about .00000001 times the strength of gravity on Earth. That's enough to change the spacecraft's velocity by a whopping millimetre per second every fortnight, give or take a bit. Not much, but over the course of years it adds up.
Theories for the 'Pioneer anomaly', as it has come to be known, range from the mundane to the sublime.
At the mundane end: something on the spacecraft sprang a leak, producing an exhaust jet that slowed it down. At the exotic end: there's a fundamental flaw in our understanding of physics, possibly involving a previously unknown type of force.
But don't get too excited. "The thinking is that it's probably not new physics," Anderson says. "Those things don't come along very often."
