Several different spacecraft have exhibited unexplained changes in speed during gravity assists when flying by Earth. First there was Galileo in 1990 and 1992, NEAR, which flew by Earth in January 1998, and then Cassini in August of 1999. Rosetta — the ESA spacecraft that recently flew by an asteroid â€“ swung by the home planet in March 2005, followed by MESSENGER in August of the same year. All these probes showed an expected change in speed during the flyby. The largest anomaly was recorded for NEAR, whose velocity changed 13 millimeters per second more than it should have. Earlier this year, a group of JPL researchers that had been working on the problem for years basically threw up their hands, saying they hoped other physicists could come up with a solution. They had concluded the anomaly was too large to be explained by known effects related to Einstein’s general theory of relativity. But a new paper proposes that Special Relativity may explain everything.
The speed of the spacecraft is measured by the Doppler shift in radio signals from the spacecraft to the antennas of the Deep Space Network. In a very short and concise paper, (reading it is like watching Will Hunting solve the MIT professor’s equation), Jean Paul Mbelek from CEA-Saclay in France says that the relative motion of the spacecraft and the spinning Earth have not been properly accounted for. When a well known but overlooked effect of Special Relativity is taken into account, where the transverse Doppler effect of the Earthâ€™s spin and the velocity of the craft are factored in, there is no flyby anomaly. “Thus, GR (General Relativity) does not need to be questioned and the flyby anomaly is merely due to an incomplete analysis using conventional physics,” says Mbelek.
Other explanations had proposed dark matter or “Unruh radiation” could be the answer. But Mbelek says we just haven’t been doing the physics right. He concludes that spacecraft “flybys of heavenly bodies may be viewed as a new test of Special Relativity which has proven to be successful near the Earth.” He proposes a follow-up of tracking the spacecraft trajectories beyond just the probes’ closest approach to Earth to test this hypothesis further.