NASA’s Pluto-bound New Horizons spacecraft has traversed the orbit of Neptune. This is its last major crossing en route to becoming the first probe to make a close encounter with distant Pluto on July 14, 2015.
The sophisticated piano-sized spacecraft, which launched in January 2006, reached Neptune’s orbit — nearly 2.75 billion miles (4.4. billion kilometers) from Earth — in a record eight years and eight months. New Horizons’ milestone matches precisely the 25th anniversary of the historic encounter of NASA’s Voyager 2 spacecraft with Neptune on Aug. 25, 1989.
“It’s a cosmic coincidence that connects one of NASA’s iconic past outer solar system explorers, with our next outer solar system explorer,” said Jim Green, director of NASA’s Planetary Science Division, NASA Headquarters in Washington. “Exactly 25 years ago at Neptune, Voyager 2 delivered our ‘first’ look at an unexplored planet. Now it will be New Horizons’ turn to reveal the unexplored Pluto and its moons in stunning detail next summer on its way into the vast outer reaches of the solar system.
Neptune’s tiny, innermost moon, Naiad, has now been seen for the first time since it was discovered by Voyager’s cameras in 1989. Dr. Mark Showalter, a senior research scientist at the SETI Institute in Mountain View, California, announced the result today in Denver, Colorado, at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society. He and collaborators Dr. Jack Lissauer of the NASA Ames Research Center, Dr. Imke de Pater of UC Berkeley, and Robert French of the SETI Institute, also released a dramatic new image of Neptune’s puzzling rings and ring-arcs, which were first imaged by Voyager.
“Naiad has been an elusive target ever since Voyager left the Neptune system,” said Dr. Showalter. From Earth, Neptune is 2 million times brighter than Naiad, and the two are separated by only one arcsecond. “This is equivalent to the width of a human hair from 50 feet away,” noted collaborator Lissauer. The team of astronomers needed to develop new techniques to suppress Neptune’s glare. Naiad was finally revealed, moving across a sequence of eight images taken during December 2004.
Strangely, Naiad appears to have veered significantly off course. The astronomers are puzzled by the fact that Naiad is now far ahead of its predicted orbital position. They wonder whether gravitational interactions with one of Neptune’s other moons may have caused it to speed up, although the details remain mysterious. Further observations will be needed in order to understand Naiad’s motion.
NASA’s Hubble Space Telescope has discovered a new moon orbiting the distant blue-green planet Neptune, the 14th known to be circling the giant planet.
The moon, designated S/2004 N 1, is estimated to be no more than 12 miles across, making it the smallest known moon in the Neptunian system. It is so small and dim that it is roughly 100 million times fainter than the faintest star that can be seen with the naked eye. It even escaped detection by NASA’s Voyager 2 spacecraft, which flew past Neptune in 1989 and surveyed the planet’s system of moons and rings.
Mark Showalter of the SETI Institute in Mountain View, Calif., found the moon July 1, while studying the faint arcs, or segments of rings, around Neptune. “The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system,” he said. “It’s the same reason a sports photographer tracks a running athlete — the athlete stays in focus, but the background blurs.”
Similar to the giant gas planets Jupiter and Saturn, their smaller cousins, Uranus and Neptune, have long been known to harbor swirling clouds and violent winds churning up their atmospheres. Massive bands of jet streams encircling the entire planet have been observed in both cases. But given that Uranus’ atmosphere is believed to be thick enough to swallow the entire Earth, it was not known just how far the weather perturbations reach into the planet’s interior.
Now a team of planetary scientists with the University of Arizona’s Lunar and Planetary Laboratory, including William Hubbard and Adam Showman, has published the results of new analyses that put an upper limit to the weather zone on Uranus and Neptune. According to their data, reported in the journal Nature, the atmosphere on both planets goes from screaming winds of infernal violence to dead-quiet at a much shallower depth than previously thought.
“Our analyses show that the dynamics are confined to a thin weather layer no more than about 680 miles deep,” said Hubbard. “This number is an upper limit, so in reality, it is possible that the atmosphere quiets down even shallower than that.”