WHILE celebrations recall the arrival of men on the moon, one of the most remarkable unmanned missions - the Voyager ``Grand Tour'' of the outer planets - is closing in on its final target, the giant blue planet Neptune. Images sent back by the Voyager 2 spacecraft already show planetary details that are invisible from Earth. They reveal changing cloud patterns that suggest an active atmosphere whose weather may be driven by an internal heat source. Methane in the atmosphere absorbs red light, making Neptune appear blue. The images have also picked up a previously unknown small moon.
On July 20, Voyager 2 was 51,510,000 kilometers (32,007,000 miles) from Neptune. It also was 4,325,234,000 km or 2,687,575,000 miles from the sun. It is rushing toward the blue planet at 67,680 km (42,054 miles) an hour.
The National Aeronautics and Space Administration (NASA) points out that these ``gee whiz'' numbers imply two important facts. Voyager 2 is on course, as planned, to fly close by Neptune at 9 p.m. Pacific Daylight Time Aug. 24 (midnight EDT). That's very nearly 12 years after the spacecraft left Earth on Aug. 20, 1977. Also, at such a great distance, it takes around four hours (4 hours and 6 minutes at Neptune) for radio signals to pass between spacecraft and Earth. Voyager 2 will be on autopilot when it makes its last planetary inspection.
The Neptune encounter began officially on June 5 and will last through Oct. 2. This is the period when Voyager's instruments are dedicated to Neptune studies. Mary Beth Murrill, information officer for the NASA/Jet Propulsion Laboratory that manages Voyager, says the spacecraft is working well.
The high point so far has been discovery of the new moon. Neptune has two major moons - Triton and Nereid. Saturn and Uranus, however, have more moons than Earth-base telescopes detect.
Confirmation of the existence of a third moon - temporarily called 1989 N1 - on July 5 shows that Neptune has such moons, too.
Voyager imaging-team member Stephen P. Synnott says that early estimates of the new moon's diameter range between 200 and 644 km (125 to 400 miles).
N1 moves in a nearly circular equatorial orbit 117,000 km (73,000 miles) from the planet's center.
Dr. Synnott's team is on the lookout for other moons. One of the key questions concerning Neptune is whether it has rings - or partial rings - as do the other giant planets. Telescopic observations suggest such rings exist. If that is so, planetary scientists expect that small moons could be the source of the ring material. Such moons could also act as ``shepherds'' in organizing the material into rings, as happens with Saturn.
Voyager 2 and its twin, Voyager 1, have both visited Jupiter and Saturn, as did two Pioneer probes. But Voyager 2 will be the only spacecraft to make the ``Grand Tour'' of all four giant planets for an indefinitely long time. It visited Jupiter on July 9, 1979; Saturn on Aug. 25, 1981; and Uranus on Jan. 24, 1986. NASA notes that the entire Voyager 1 and 2 mission cost only $556 million.
This tour was made possible by a rare alignment of planets that enabled the gravitational field of each giant planet, in turn, to deflect Voyager 2 on to the next planet. This alignment occurs about once in 175 years.
NASA has boosted the sensitivity of its tracking system 58 percent to make the most of this unique planetary encounter. This will enable Voyager 2 to transmit data from Neptune as fast as it did from Uranus, even though Neptune is much farther from Earth.