Detecting Jupiter's tug on radio waves.

The gravitational deflection of starlight when it grazes the sun's limb has become a standard test of Einstein's general theory of relativity. With the help of sophisticated instrumentation stretched to its limits, researchers have now confirmed that an object as small as Jupiter also has a discernible effect on the paths of radio waves from a distant source as they speed past the planet.

These observations "constitute the first measurement of the deflection of electromagnetic radiation by a planetary gravitational field," Robert N. Treuhaft and Stephen T. Lowe of the Jet Propulsion Laboratory in Pasadena, Calif., report in the November Astronomical Journal. The findings also demonstrate the usefulness of a new approach for locating and tracking distant objects -- including spacecraft on deep-space missions -- observed within a wide field of view.

"It's a nice result," says physicist Clifford M. Will of Washington University in St. Louis. "It illustrates that we can now measure radio waves accurately enough to see the deflection caused by a planet."

To make such a measurement possible, Jupiter in the course of its passage across the sky had to come nearly in front of a strong celestial source of radio waves, an event that happens only once or twice in a decade. Such a close encounter occurred on March 21, 1988. On that day, Treuhaft and Lowe measured the differences between the arrival times of radio waves at receiving stations at each end of two lines stretching between California and Australia. They repeated the measurements 13 days later, after Jupiter had moved farther away from the radio source's point in the sky.

Careful data analysis, which took nearly three years to complete, revealed a tiny deflection consistent with theoretical predictions of the gravitational effect Jupiter would have an electromagnetic radiation. This deviation of 300 microarcseconds corresponds to a shift of only about 1 kilometer in the radiation's path when measured at Jupiter's distance from Earth. "Our measurement agrees with general relativity," Treuhaft says.

Although this planetary measurement doesn't test gravitational theory as well as solar-deflection measurements, "it shows that we can now detect the deflection due to Jupiter's warping of space-time," Will says. "Modern VLBI [very long baseline interferometry] has reached the stage where we can really see the curvature of space-time essentially everywhere."

Further development of VLBI techniques will allow more accurate measurements of such deflections in the future.