Long Reach

May the Force Be With You? Mysterious Effect May Influence Spacecraft Trajectories
By Leonard David, Space.com, November 26, 2000

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. Scientists hope to confirm the unusual effect as the Stardust spacecraft whips by Earth this coming January. Analysis by radio scientists of the post-Earth flyby trajectories of three spacecraft have shown each craft to have picked up an unexpected increase in speed: The Galileo spacecraft in December 1990; the Near Earth Asteroid Rendezvous (NEAR) probe in January 1998; and the Saturn-bound Cassini spacecraft in August 1999. The Galileo spacecraft slipped by Earth a second time in December 1992. But the vehicle dipped too close to Earth making the measurement of any "flyby effect" unusable.

"This problem has been with us for about 10 years, and we haven't 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. "We re looking forward to the Stardust flyby. That would be our fourth measurement of this anomalous effect," Anderson told SPACE.com. Using JPL s 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 of sound due to the relative motion between the emitting source, Stardust s radio transmitter, and ground receiving equipment. Stardust is expected to show a bump up in velocity as it flies by, Anderson said. "We can t 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 we've seen in the other three, it should be clearly measurable," Anderson said. "That s why we re 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, Stardust's 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 (6,000-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. It s quite intriguing," Brownlee said. "It is possible, I guess, it s some new factor that hasn't been taken into account. But the most interesting possibility is it s 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 swingby could be fully analyzed by his four-person team at month s 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 Earth s magnetic field. This electromagnetic charge then interacts with the Earth's gravity, creating the anomalous motion in the spacecraft as it cruises by Earth, he said. Anderson also said the effect could be some outcome of string theory prediction. String theory is a supposition that space is imbued with electromagnetic, weak and strong nuclear interactions, as well as 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 shouldn't pursue this. There might be something to this. It s very hard to question our results. We just don't know what it is," he said.