Lost asteroid clue to Pioneer puzzle


FAR-FLUNG asteroids could help reveal the nature of the mysterious force that has nudged NASA's 33-year-old Pioneer 10 spacecraft about 400,000 kilometres off course.


The so-called Pioneer anomaly could be accounted for by a force pulling the probe towards the sun with a strength of just one ten-billionth of the gravity at Earth's surface. But no one has managed to explain the nature of this force, and many suspect that it is just a systematic error in the data or a fault of the spacecraft design. Others have suggested sending another spacecraft to study the effect, at a cost of hundreds of millions of dollars.

But there might be a cheaper way to find an explanation. Gary Page of George Mason University in Fairfax, Virginia, and his colleagues have identified 15 asteroids that might also be subjected to the mysterious force. The asteroids' orbits all stretch far into the outer solar system. This is crucial because the Pioneer anomaly only shows up beyond about twice the distance from the sun to Saturn.

Of the 15 candidates, the best is 1995SN55. This 370-kilometre-wide space rock has spent the past 54 years in the anomaly zone, so it should have experienced the largest perturbation. And tantalisingly, it is not where predictions say it should be. "It could be lost because of the Pioneer effect," says Page. "Asteroids are just big and dumb and go where gravity tells them."

Michael Martin Nieto of the Los Alamos National Laboratory in New Mexico thinks the asteroid study is a great idea, but cautions that it is unlikely to reveal the whole truth. "It will only test whether gravity is the cause," he says. "The anomaly could also be caused by a subtle effect in inertia or even time." So a spacecraft mission may still be needed.

Page and his colleagues are applying for telescope time to track the distant asteroids precisely (www.arxiv.org/astro-ph/0504367).

From issue 2498 of New Scientist magazine, 07 May 2005, page 17

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