Voyager Photographs Show Third Moon Orbiting the Planet Neptune

A third moon of Neptune, about one-fifth the size of Earth’s moon, has been discovered in photographs taken by the Voyager 2 space probe, JPL researchers said Friday. Only two moons, Nereid and Triton, were previously known.

The newly found moon, temporarily designated 1989 N1, is an estimated 125 to 400 miles in diameter and orbits Neptune’s Equator at an altitude of about 57,000 miles. It was discovered by JPL astronomer Stephen P. Synnott as a “small, bright smudge” on photographs of the giant blue-green planet taken from 45 million miles away as the probe raced toward its Aug. 24 rendezvous with Neptune and Triton.

Researchers expect to discover as many as 10 moons at Neptune. “Most of the outer planets have a lot” of moons, said Voyager project manager Norm Haynes of JPL, “and it would be unusual if Neptune had only two or three.”

Synnott first saw the moon in mid-June in television images transmitted to Earth by Voyager 2. Four or five images obtained over the next few weeks allowed Synnott and his colleagues to calculate an orbit for the object.

Another image obtained Wednesday showed the object to be precisely where they had predicted, confirming that it was a moon. “It just came in right dead smack on the button,” Synnott said.

“I never really jumped and screamed,” Synnott added, “but I clenched my fist, nodded my head to myself, and said, ‘Yup, that one’s real!’ Ninety-nine percent of the time, this discovery process is frustration, so it is finally nice to dig a nugget out of all of the mud.”

Actually, Synnott and his JPL colleagues have already dug out several nuggets. Voyager 2 and its twin, Voyager 1, which were launched in 1977, have discovered at least 18 moons so far: three at Jupiter, at least four and possibly as many as seven at Saturn, 10 at Uranus and now one at Neptune.

Dark Bands Seen

Earlier photographs from Voyager had also revealed the existence of a 6,200-mile-wide “dark spot” on Neptune similar to the giant red spot on Jupiter and believed to be caused by a giant storm. The photographs have also revealed 2,700-mile-wide dark bands, thought to be wind-blown clouds, near the poles similar to those seen on other gaseous planets.

The discovery of 1989 N1--which will be officially named by the International Astronomic Union, probably sometime within the next year--raises some new questions about Neptune.

Triton, which was discovered in 1846, is about the size of the Earth’s moon, but it has what is known as a retrograde orbit, traveling around Neptune in the opposite direction from most other moons in the solar system. For that reason, astronomers have speculated that it was a wandering asteroid that came too close to Neptune and was captured.

Gravitational Attraction

Had that occurred, however, the asteroid would have passed through the low orbit of any pre-existing moon such as 1989 N1, most likely colliding with it or sweeping it up by gravitational attraction. But the fact that 1989 N1 is in such an orbit suggests that Triton is probably not an asteroid and was most likely formed when Neptune was formed.

“The difficulty we now have is that we’ve found a moon (1989 N1) in a place we didn’t think one should have existed,” JPL’s Ellis Miner said. “That’s forcing us to revise our thinking on how Triton came into its present orbit.”

The newly found moon is too faint to appear in printed photographs, but will become more visible as Voyager 2 nears the planet. It cannot be seen from Earth because it is obscured by the glare from Neptune itself.

Voyager 1’s trajectory carried it out of the plane of the solar system after it encountered Saturn in 1981. But the Voyager team at JPL was able to use Saturn’s gravity to whip Voyager 2 into a course that carried it past Uranus in January, 1986, and on to Neptune. It will fly by the planet at a distance of about 3,000 miles about 9 a.m. on Aug. 24. A gravitational boost from Neptune will change its trajectory so that it will also make a close approach to Triton.