This artist's concept shows NASA's Voyager spacecraft against a backdrop of stars.
This artist's concept shows NASA's Voyager spacecraft against a backdrop of stars. › larger image

A tracking station in Japan has been added to the network of giant antennas trained on Voyager 2 during its flyby of Neptune on August 24-25 to help the spacecraft mission's radio science experiment.

Scientists say they will be able to "see" twice as deeply into the atmosphere of the giant gas planet, thanks to the participation of the 64-meter (210-foot) antenna at the Usuda Deep Space Center.

The collaboration was arranged through an agreement signed in 1988 between NASA and Japan's Institute of Space and Aeronautical Studies (ISAS), which operates the Usuda center.

Under the agreement, physicist Dr. Nobuki Kawashima of ISAS will join the Voyager Radio Science Team.

"Using Usuda will allow us to extract information on deeper parts of Neptune's atmosphere," said Dr. Len Tyler of Stanford University, principal investigator for the Voyager radio experiment. "Also, the quality of the data we will have for any given point will be twice as good."

During these observations, scientists will listen not to the information carried by Voyager's radio signal but rather to the signal itself -- especially its strength and frequency -- as the spacecraft sails over Neptune's north pole and dips behind the planet.

Barely perceptible changes in the radio signal convey signatures of the structure, composition and temperature of the seas of gases that constitute Neptune's atmosphere, as well as of the distant planet's gravity field.

As the spacecraft disappears behind planet or moon from the Earth's point of view, its radio signal is refracted, or bent, during passage through the planet's or moon's atmosphere.

Tiny changes in the signal's frequency and strength give scientists portrait of the atmosphere's structure, composition, temperature and location of clouds, as well as information on small-scale atmospheric dynamics.

Tyler likened the refraction effect to how sunset appears on Earth, as the Sun seems to linger at the horizon before it disappears.

"You would think that the Sun would appear to keep moving. But because its light is being refracted, you can continue to see it as it seems to stand still for few moments," he explained.

In addition to studying atmospheres of planets and moons, scientists have used the Voyagers' radio systems during the mission to investigate rings surrounding planets. As the spacecraft flies behind ring system, changes in the radio signal provide information not only on the rings' overall dimensions but also on the particles that constitute the rings.

Also, the radio experiment is able to detect minute changes in Voyager's velocity as it curves past each planet, offering detailed look at the planet's gravity field.

Some of the most significant findings from the radio experiment during the 12-year Voyager Mission have included revelations on the nitrogen atmosphere surrounding and the surface pressure at Saturn's largest moon, Titan; measurements on wave-like structures within Saturn's sprawling rings and the sizes of particles that constitute them; detection of methane cloud layer in the atmosphere of Uranus; and the nature of Uranus's coal-black rings.

The experiment has also provided data on the densities of moons at planets the Voyagers have visited, as well as the planets' gravity fields. In concert with the Voyagers' infrared interferometer spectrometer and radiometer (IRIS) instrument, the radio science experiment additionally has determined the ratio of hydrogen to helium, the two overwhelmingly dominant elements in the atmospheres of each of the planets.

As Voyager 2 has headed into the outer solar system, the strength of its radio signal at Earth has become fainter. During the Neptune flyby, the radio's signal strength at Earth will be 1/36th of what it was when the Voyagers flew by Jupiter.

The collaboration with Japan's ISAS will help offset the diminished signal, Tyler explained.

During Voyager 2's closest approach to Neptune, radio science data will be recorded at the NASA/JPL Deep Space Network Station in Canberra, Australia; at the Parkes Radio Observatory in Australia; and at the Usuda center in Japan.

Each receiver is linked to an atomic clock which gives an exceedingly precise time-stamp to the received radio signal. This allows the recorded signals from all the stations to be combined later so that they mesh extremely closely, with the peaks and troughs of the radio waves matching virtually exactly.

The combined signal is then analyzed to yield the experiment's science data. Factors such as the chemistry and temperature of planet's atmosphere will have minute, but measurable, effects on the signal's frequency and strength.

Tyler noted that his team studies frequency changes that amount to only one one-hundredth of cycle per second (1/100th Hz) in signal from Voyager 2 being transmitted at frequency of billions of cycles per second. That is similar, he said, to measuring change in position of 1 millimeter (1/25th inch) from an observing position 5 billion kilometers (3 billion miles) away.

"The best ear of any trained musician can detect change of partial musical step, which is some number of cycles per second," Tyler added. "Needless to say, the changes we study in Voyager's radio signal are much more subtle than that."

Opened in October 1984, the Usuda Deep Space Center is set in the mountains of Japan's Nagano Prefecture at 1,456 meters (4,777 feet) above sea level, some 100 kilometers (about 60 miles) northwest of Tokyo.

In addition to performing tracking, telemetry and commanding for Japan's solar system missions, such as the Suisei and Sakigake spacecraft which flew by Comet Halley in 1986, the Usuda center supports experiments involving sounding rockets and balloons.

The Deep Space Network, which includes complexes in the California desert and Spain in addition to the Australian site, is managed by the Jet Propulsion Laboratory for NASA's Office of Space Operations.

The Parkes Radio Observatory in Australia is operated by the Commonwealth Scientific and Industrial Research Organization.

JPL manages the Voyager Project for NASA's Office of Space Science and Applications.