Astronomers have had a solid handle on the orbits of our solar system's planets since at least the 17th century, when Johannes Kepler formulated his laws of planetary motion. But the fine details of those orbits are not always obvious, especially for the more distant outer planets, owing to the great distances separating Earth from those worlds and the limited number of spacecraft that have flown in their vicinities.
As recently as 10 or 15 years ago, the locations of the giant planets Jupiter and Saturn were known only to within hundreds of kilometers. Given that these distances from Earth are hundreds of millions or even billions of kilometers, this degree of precision has been more than sufficient for any astronomer to locate the planets in the sky. But better orbital measurements are a boon for interplanetary spacecraft, which often use the gravity of individual planets for course-correcting slingshot maneuvers that have a small margin of error. And precision astronomy tasks such as pulsar timing require an accurate chart of the planets in order to calibrate for an Earthbound observer's motion through a dynamic solar system.
Astronomers are now using the Cassini spacecraft, which took up residence around Saturn in 2004, to refine their knowledge of the ringed planet's position and motion. Cassini's everyday radio transmissions act as a beacon as the spacecraft goes about its mission; by pinpointing its location in the sky using an array of powerful radio telescopes and combining that information with other telescope data, researchers can also get an exceptionally accurate bead on Saturn's position as it moves along its orbit around the sun. Thanks to Cassini, astronomers can now foretell the position of Saturn decades down the road to within a few kilometers. A group of researchers from the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and the National Radio Astronomy Observatory published a report from this ongoing Cassini campaign in the February issue of The Astronomical Journal.
A numerical description of planetary positions and motions, known as an ephemeris, is "one of those fundamental tools of astronomy that you want to incrementally, continually improve over time," says JPL astronomer and lead study author Dayton Jones. JPL releases a dynamical ephemeris, or DE, every so often, upgrading the system with new information much the way software developers update a computer's operating system to add new features as well as increase stability.
The orbits of Earth's moon and the inner planets—Mercury, Venus, Earth and Mars—are well established, thanks to radar or laser-ranging readings of their surfaces, along with transmissions from the many orbiters and landers that have explored Earth's neighborhood. "We have much less precise data on the outer planets," Jones says. "With Cassini we have the first opportunity to follow the motion of Saturn very precisely over a large portion of its orbital period."
To do that, Jones and his colleagues used the Very Long Baseline Array (VLBA), a network of 10 radio telescopes stretching from Hawaii to the U.S. Virgin Islands, to pinpoint Cassini's location in the sky on eight occasions between 2006 and 2009. (Another group did the same in 2004.) Complementary data from the radio dishes of the NASA Deep Space Network, which track numerous spacecraft in flight, describe where Cassini is in relation to Saturn. Combining the two measurements gives the best available position for Saturn in the sky—the VLBA data are about 30 times better than the next-best measurements of Saturn's orbit, according to study co-author William Folkner, also of JPL.
The combined calculations are achieved in part by locking onto Cassini with the VLBA and comparing the spacecraft's position—and hence Saturn's—with a reference frame of background radio sources such as quasars. "The VLBA measurement accuracy is so good that we are currently limited by the accuracy of some of the radio source positions, and are near being limited by the uncertainty in the Earth's orbit as much as by the Saturn orbit," Folkner says.
Now that Saturn's position is coming into clearer focus, Jupiter looms large as the next target for orbital refinement. The Jovian mass dominates the outer solar system, so having a better understanding of its motion would have many trickle-down benefits. Luckily for the keepers of the ephemerides NASA is readying a Jupiter orbiter called Juno that will act as a similar, if shorter-lived, radio beacon. Juno is scheduled in August to begin a five-year journey across the solar system, after which it will spend a year making a few dozen laps around solar system's largest world to study its structure and try to better constrain its formation.