The Voyager 2 spacecraft has discovered three more moons orbiting Neptune although the small craft is still 21 million miles and three weeks away from its close encounter with the distant planet, the Jet Propulsion Laboratory announced Thursday.
That brings to six the total number of moons known to be circling Neptune, including one discovered by Voyager last month. All of the newly discovered moons are small and in orbits close to the giant, gaseous planet.
Voyager is due to pass within about 3,000 miles of Neptune's cloud tops Aug. 24, the culmination of a grand tour of the four outer planets.
The 12-year-old spacecraft is 2.7 billion miles from Earth, closing in on Neptune at a speed of 17 miles per second.
The discovery of the moons was not unexpected because Neptune is believed to have partial rings, called "ring arcs," in the region where the satellites were found. Scientists believe the gravitational fields of small moons probably cause dust and gas to form in long streamers, creating rings--or in this case partial rings--around the planet.
Neptune has only two large moons, Triton and Nereid. Triton is particularly interesting to scientists because it orbits in the opposite direction of all other large satellites in the solar system.
Voyager could provide valuable data on the exact relationship between the satellites and the ring arcs, according to Peter M. Goldreich, a Caltech astrophysicist and a leading theorist on Neptune. Unlike Saturn, Neptune does not have complete rings, current evidence indicates.
Scientists at the Jet Propulsion Laboratory in Pasadena said the newly discovered moons showed up on images transmitted back to Earth by Voyager late last week, but the announcement was held up until the finding could be confirmed. After tracking the moons for five days and determining that they were following predicted orbits, the laboratory made the announcement.
The moons range from about 60 to 125 miles in diameter, making them much smaller than Triton, Neptune's largest satellite, which is about the same size as Earth's moon. The four moons found by JPL have been temporarily named in order of their discovery--1989 N1 (discovered by Voyager last month), 1989 N2, 1989 N3 and 1989 N4.