Credit: B. Kent, A. Angelich, NRAO/AUI/NSF
Same image as above, without labels
Scientists have used the National Science Foundation's Very Long
Baseline Array (VLBA) radio-telescope system and NASA's Cassini
spacecraft to measure the position of Saturn and its family of moons to
within about a mile -- at a range of nearly a billion miles. This feat
improves astronomers' knowledge of the dynamics of our Solar System and
also benefits interplanetary spacecraft navigation and research on
fundamental physics.
The researchers, from the National Radio
Astronomy Observatory (NRAO) and NASA's Jet Propulsion Laboratory (JPL),
used the continent-wide VLBA to pinpoint the position of Cassini as it
orbited Saturn over the past decade by receiving the signal from the
spacecraft's radio transmitter. Combined with information about
Cassini's orbit from NASA's Deep Space Network, the VLBA observations
allowed the scientists to make the most accurate determinations yet of
the position of the center of mass, called the barycenter, of Saturn and
its numerous moons.
The scientists presented the results of their work at the American Astronomical Society's meeting in Seattle, Washington.
The
measurement, some 50-100 times more precise than those provided by
ground-based optical telescopes, was possible because of the VLBA's
great resolving power, or ability to discern fine detail. With its 10
dish antennas spread from Hawaii to the Virgin Islands, the VLBA
operates as a single radio telescope with a virtual size nearly equal to
the Earth's diameter.
The result is a greatly improved ephemeris -- a table of predicted positions -- for the Saturnian system.
"An
accurate ephemeris is one of the basic tools of astronomy, and this
work is a great step toward tying together our understanding of the
orbits of the outer planets and those of the inner planets," said Dayton
Jones, of JPL, in Pasadena, California. "The orbits of the inner
planets are well tied together, but those of the outer planets,
including Saturn, have not been tied as well to each other or to those
of the inner planets," Jones said.
The improved positional
information will directly benefit scientists' ability to precisely
navigate interplanetary spacecraft. In addition, it will help refine
measurements of the masses of other Solar System objects. Also, the
positional precision will improve predictions of when Saturn or its
rings will pass in front of background stars, events that provide a
variety of research opportunities.
Other benefits will come to
studies of several aspects of fundamental physics. The new positional
information will help researchers improve their precision when timing
the radio pulses from pulsars -- spinning superdense neutron stars. Such
timing will help answer unsolved questions about particle physics and
the exact nature of the highly-compressed material inside a neutron
star. Ongoing projects that time the pulses from multiple pulsars spread
across our Milky Way Galaxy in an attempt to detect the effects of
passing gravitational waves also will benefit from the improved Saturn
ephemeris, which also improves the overall Solar System ephemeris.
VLBA
measurements of the position of Cassini 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 strongly-emitting quasars as Saturn passes near them on the sky.
The
position the scientists determined is that of the barycenter -- the
center of mass -- of Saturn and its moons. When two bodies are in orbit,
they both rotate about the barycenter. For example, the barycenter of
the Sun and Jupiter is just outside the surface of the Sun, and the
barycenter of the Earth and our Moon is about 1700 kilometers beneath
the Earth's surface. The barycenter of Saturn and its largest moon,
Titan, is about 30 kilometers from the center of Saturn. The barycenter
of Saturn and all its moons (some 62 at current count) is what follows
an elliptical orbit around the Sun.
In other studies, the VLBA
has been used to measure the positions of Mars-orbiting satellites, and
Voyager 1, the most distant man-made object, now some 12 billion miles
(19 billion kilometers) from Earth on a journey that began with its
launch in 1977.
In 2016, NASA's Juno spacecraft will begin
orbiting Jupiter. "We plan to use similar techniques on this spacecraft,
and improve the orbit for Jupiter as well," Jones said.
Jones
worked with William Folkner, Robert Jacobson, and Christopher Jacobs,
all of JPL, and Jon Romney, Vivek Dhawan, and Edward Fomalont, of the
NRAO.
The National Radio Astronomy Observatory is a facility of
the National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc. JPL, a division of the California
Institute of Technology, Pasadena, manages the Cassini mission and Deep
Space Network for NASA.
Media Contacts:
Dave Finley, NRAO
(575) 835-7302
Preston Dyches, JPL
(818) 354-5011
