A team of researchers using NASA’s Cassini spacecraft and a ground-based radio-telescope system has accurately determined the center of mass, or barycenter, of Saturn and its moons to within about 4 km. The measurement is about fifty times more precise than the other existing measurements.
This image was obtained on 10 September 2007 by NASA’s Cassini spacecraft, at a distance of approximately 3.3 million km from Saturn. Rhea – Saturn’s second largest moon – is visible against the bluish backdrop of the planet’s northern hemisphere. Mimas – Saturn’s seventh largest moon – appears as a speck against the ring shadows on planet’s western limb. Image credit: NASA / JPL / Space Science Institute.
The team used the Very Long Baseline Array (VLBA) to pinpoint the position of Cassini as it orbited Saturn over the past decade. They then combined these data with information about the spacecraft’s orbit from NASA’s Deep Space Network.
The combined observations allowed the team to make the most accurate determinations yet of the position of the barycenter of Saturn and its numerous moons.
“This work is a great step toward tying together our understanding of the orbits of the outer planets of our Solar System and those of the inner planets,” said team leader Dr Dayton Jones of NASA’s Jet Propulsion Laboratory in Pasadena, California.
The new measurement was made possible by two factors: Cassini’s long-term presence in the Saturn system and the VLBA’s ability to discern extremely fine detail.
The result is a greatly improved table of predicted positions of objects in the Saturn system, known as an ephemeris.
The improved positional information will help enhance precise navigation of interplanetary spacecraft and help refine measurements of the masses of Solar System objects. It will also improve predictions of when Saturn or its rings will pass in front of background stars – events that provide a variety of research opportunities for astronomers.
VLBA measurements of Cassini’s position have even helped scientists who seek to make ever-more-stringent tests of Albert Einstein’s theory of general relativity by observing small changes in the apparent positions of actively feeding black holes, or quasars, as Saturn appears to pass in front of them on the sky.
Cassini’s navigation team began using new positional information provided by the ongoing study in 2013.
The new ephemeris has enabled them to design better maneuvers for the spacecraft, leading to mission-enhancing savings in propellant.
Previously, the navigators performed their own estimates of the positions of Saturn and its satellites using data gleaned by tracking Cassini’s radio signal during its communications with Earth.
The results were presented January 8 at the 225th Meeting of the American Astronomical Society in Seattle.
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