WASHINGTON -- Space probes using Earth to slingshot their way outward into the solar system appear to have received an extra boost by a mysterious force -- perhaps an unknown component of gravity.

"This problem has been with us for about 10 years, and we havent found a solution," said John Anderson, a senior research scientist and member of the Stardust science team at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

"Were looking forward to the Stardust flyby. That would be our fourth measurement of this anomalous effect," Anderson told SPACE.com.

An artist's rendering of Stardust with its dust collector deployed, using Aerogel to capture interstellar grains.

Using JPLs Deep Space Network of radio telescopes, the velocity of Stardust is measured by analyzing its Doppler shift -- in this case, a change in frequency or wavelength due to the relative motion between the emitting source, Stardusts radio transmitter, and ground receiving equipment.

Stardust is expected to show a bump-up in velocity as it flies by, Anderson said. "We cant find any source or any mechanism that would do that," he said.

"Cassini, NEAR, Galileo...they all show it. If it follows the pattern that weve seen in the other three, it should be clearly measurable," Anderson said. "Thats why were so anxious to get the Stardust data," he said.

The Stardust spacecraft will zoom past Earth on January 15, 2001, at the end of its first elongated orbit of the Sun, said Donald Brownlee, Stardusts principal investigator of the University of Washington, Seattle.

Launched in February 1999, Stardust is on a long-and-winding road to comet Wild 2. In 2004 the probe will snag cometary material, then return the samples to Earth in 2006.

~

Stardust is equipped with an X-band transponder (radio transmitter/receiver), allowing radio scientists on Earth to precisely track the spacecraft, Brownlee said.

As Stardust slips by Earth to attain a flight path change, it will pass 4,000 miles (6,437 kilometers) above Africa, Brownlee said. There is a prediction of where the spacecraft should be, one that takes into account the flyby effect, he said.

"Those past spacecraft, after the flyby...they are leaving with slightly more energy than expected. Each one had a consistent anomaly. Its quite intriguing," Brownlee said.

"It is possible, I guess, its some new factor that hasnt been taken into account. But the most interesting possibility is its a previously undiscovered component of gravity," Brownlee said.

Just what the effect might be remains a puzzle, Anderson said.

"It could be fundamental physics...it might not be. I view it as a mysterious anomaly. To be speculative, it could be revealing something new in physics," he said.

Anderson said he could not discount that some systematic navigation error, yet to be identified, has been uncovered.

"Either way, it is important to pin it down, hopefully after we get the Stardust flyby," Anderson said. Stardust radio data collected during the January swing-by could be fully analyzed by his four-person team at months end, or later in February, he said.

"If the force was with us, basically, this would be a phenomenal discovery," Brownlee told SPACE.com.

Anderson said colleagues have ventured guesses as to what might explain the effect, if it is a true phenomenon in the first place.

One possibility being aired, Anderson said, is that spacecraft become charged as they whisk through the Earths magnetic field. This electromagnetic charge then interacts with the Earths gravity, creating the anomalous motion in the spacecraft as it cruises by Earth, he said.

Artist's representation of Near Earth Asteroid Rendezvous spacecraft.

Anderson also said the effect could be some outcome of string theory prediction.

String theory is a supposition that space is imbued with weak and strong nuclear interactions, along with electromagnetic and gravity forces, that form curled-up dimensions, in addition to the observable dimensions of length, height and width.

But putting such speculation aside, Anderson said, obtaining matter-of-fact data in January is important.

"Assuming that we see it again on Stardust...we should be able to start seeing a pattern to this," Anderson said. "Nobody has suggested that we shouldnt pursue this. There might be something to this. Its very hard to question our results. We just dont know what it is," he said.