Neptune reveals secrets

Press, 30 August 1989, Page 15

Neptune reveals secrets

Voyager 2, 12 years after launch, made the closest approach to any major planet by a spacecraft as it flew by Neptune in August. It will return about 8000 photos of this distant body and provide our first real knowledge of the conditions on Neptune. The wonderful nature of this feat can be gauged by remembering that the return of this data to Earth requires more than four hours to transmit. It is equivalent to observing a 20 watt light at a distance of 4.3 billion kilometres! Neptune was discovered in 1846, although it had been seen on several previous occasions but mistaken for a star. It is the most distant known planet of the solar system and will remain so until the year 2007, because of the eccentricity of Pluto’s orbit which brings it closer than Neptune at present. Neptune has a diameter of 48,600 km making it slightly smaller than Uranus and vastly inferior to the two largest planets, Jupiter and Saturn. It lies at a mean distance of 4497 million kilometres from the Sun and makes one orbit every 165 years. It rotates on its axis in about 19 hours.

William Lassell, an English amateur astronomer, discovered in 1846 a moon, Triton, orbiting Neptune in a backwards direction. It is presumed that as planets and their satellites condensed out of the solar nebula they would rotate in the same direction as that of the original nebula. Triton does not and it is suggested that it was originally a comet, or other small body, that was captured by Neptune and thrown into an orbit around the planet. Its diameter is 3500 km making it almost the same size as our Moon. A second satellite was discovered in 1949 by the American astronomer, Gerard Kuiper. This is a very small body about 300 km in diameter. Voyager 2 has already added a further three satellites to the Neptune system. These range in diameter from 96km to 300 km and orbit the planet in circular paths around the planet’s equator in the direction of the rotation of Neptune. They lie at distances of 27,300 km and 48,000 from the planet. The closest whizzes around the planet once every eight hours and 10 minutes, while the outermost requires 13hr 30min to complete one orbit. The discovery of these three moons was not unexpected because it was believed that a collection of small moons was likely once it had been found that the other major planets all had a retinue of satellites. These new discoveries have as yet not been named and are simply referred to as N2, N 3 and N 4. It also came as no suprise that Voyager 2 had detected rings around Neptune in the form of

broken arcs rather than complete rings. These had been seen from Earth during times when a star was occulted by Neptune. These observations showed that the star, as Neptune approached it, was dimmed but suffered no loss of light after the occultation. On other occasions the reverse happened. This suggested that such rings as might exist were incomplete, forming thin, broken arcs.

There is a long history of supposed sightings of rings around Neptune dating back to 1846. Lassell and other astronomers considered that they could see rings round the planet and actually produced drawings of their positions. However, these were eventually regarded as defects in the telescopes of that era- and discarded as non-existent. Tidal forces close to a planet have the effect of breaking up larger bodies into smaller ones. This increases the chances of collisions between the particles that remain. The same forces prevent these tiny bodies from forming into larger ones. It can be suggested that Lassell and others, who were all excellent and careful observers, did actually see rings around Neptune as the result of a recently captured comet breaking up into small pieces under the influence of tidal forces.

Theories predict that rings could be formed into arcs rather than complete rings by the influence of satellites close to the planet. These would tend to sweep the tiny particles into regions at certain distances ahead of and behind the satellites. One such theory predicts that Voyager 2 will discover at least six satellites which are necessary to produce this configuration.

It was Voyager 2’s discovery of rings around Uranus which sparked interest in observing stellar occultations by Neptune in the hopes of detecting rings around that planet. On April 7, 1968, Ed. Guinan was observing such an occultation from the Mount John University Observatory. He was interested in the planet’s atmosphere and paid little regard to a decrease in the brightness of the star just before the actual occultation. It was only a decade later after the dis-

covery of the arcs around Neptune. Since then several observations have been made of such arcs so there was no surprise when Voyager 2 detected these arcs.

Nothing was really known about the conditions in the cloud belts that surround Neptune. Voyager 2 has reported a 10,000 km dark spot which might be the centre of a hurricane-like storm. This might be like the Great Red Spot on Jupiter, although this will not be certain until the photos have been studied in greater detail. Such a storm centre would imply that there must be some internal heat source. It has been suggested that at a depth of several thousand kilometres there is a super-hot mixture of rocks and water heated from the core.

Neptune, because of its vast distance from the Sun, receives little sun-> light. The temperature at the top of its mainly hydrogen and helium atmosphere is about -212 deg. C. There is also a certain amount of methane, which, because it absorbs red light, gives the planet a pale blue-greenish appearance.

The passage of Voyager 2 marks the end of the first space age during which men have landed on the Moon and all the planets have been visited by space probes. The 1990 s will' bring the new generation of spacecraft designed for a variety of special missions. These will not only be launched by N.A.S.A. but also by the European Space Agency, the Soviets and Japan. Some are already in orbit.

Voyager 2, like Voyager 1 and Pioneer 10 and 11, will now travel to the far confines of the solar system. It is hoped that they will continue to transmit data for another decade. And that they will cross the shock wave where the solar wind meets the stellar wind from inter-stellar space. They have already completely revised our knowledge of other planets and it is on that information that the new generation of spacecraft will build.

Evening skies in September will be dominated by the brilliant Venus. It will set around 9.10 p.m. on September 1 and at 10.15 p.m. on September

30. Mercury will also be visible in the western evening sky during the first 10 days of September, setting close to 8 p.m. Then it will dip down steeply towards the Sun and be lost in evening twilight. It will reach inferior conjunction on September 25 and pass into the morning sky. Mars is now lost in the evening twilight and will reach conjunction with the Sun on September 30. Jupiter, in Gemini, will rise at 4 a.m. on September 1 and at 2 a.m. on September 30. Saturn, in Sagittarius, will remain in an excellent evening viewing position throughout the month, setting well after midnight.