Voyager 2 approaches Neptune

Press, 4 April 1989, Page 42

Voyager 2 approaches Neptune

Twelve years after its launching on August 20, 1977, Voyager 2 will make its closest approach to the distant planet, Neptune. The spacecraft has observed the planet for the last year, but detailed observations will begin in June and continue for the next four months.

Neptune, the fourthlargest planet, has a diameter of 48,600 km and orbits the sun once every 165 years at a distance of 4497 million kilometres.

Viewed from Earth, Neptune reveals a small, bluish-green disk, devoid of any features. It is not visible to the naked eye and attracts little attention even with large telescopes. It was discovered in 1846 by J. G. Galle at the Berlin Observatory, although it had been seen on several previous occasions but always mistaken for a star.

Its discovery was a triumph for mathematical calculations. In 1845, J. C. Adams, in England, and Urbain Leverrief, in France, independently

predicted the existence of Neptune from their investigations into the discrepancies of the orbit of Uranus. They correctly predicted the position in which Neptune could be found.

Very little is known about Neptune at present. It is presumed that it has a magnetic field and it appears to have an atmosphere, which is topped by methane clouds over ice crystals. The planet itself is thought to be composed mainly of hydrogen, helium and methane.

It is probable that Neptune generates some heat, because its temperature is about the same as that of Uranus, despite the fact that the intensity "of sunlight is 2.5 times less than at Uranus.

Neptune has two moons, Triton, with a diameter of 3500 km, and Nereid, a mere speck estimated to be about 300 km. Triton is unique in moving in a retrograde orbit, meaning that it travels in the oppo-

site direction to the planet’s rotation. This motion means that this moon is steadily and very slowly falling into Neptune. Ultimately, in many millions of years, it will be so close to the planet that it will be torn apart by tidal forces. Its surface is probably covered with methane ice and possibly shallow lakes of liquid nitrogen.

The discovery that Uranus was encircled by a system of rings led astronomers to think that possibly Neptune also had such a system. The only method of solving this riddle was to carefully observe occultations of stars by the planet. Rings would reveal their presence by successive fadings and brightenings of the star before it finally disappeared behind the planet.

Several such events have been observed and appear to show that Neptune does not have complew rings of particles around it. but a few in-

complete rings of horse-shoe-shape. That means that only arcs of these have been seen as they appear to exist on one side of the planet only. The individual particles in these arcs range in size from dust to pebbles. Voyager 2 has already returned spectacular views of other planets and has had Neptune under observation for a year. In June, it will be close enough to start intensive observations which will continue for four months.

The closest approach will occur on August 25, when the spacecraft will pass over the north pole of the planet at a distance of 4400 km, the nearest it will have come to any planet in its 12-year journey. Five hours later, the spacecraft will survey Triton from a distance of about 40,000 km. The path of the spacecraft is designed to avoid any damage to it from encounters with the ring particles, or radiation from Neptune’s magnetic

field. The distance of the arcs of rings from Neptune is uncertain, so there is some danger that the spacecraft could be hit by ring particles. This will be overcome by adjusting the flight path as Voyager gets nearer to the planet. Voyager’s radio instruments will probe the atmosphere of Neptune, while other instruments will investigate the magnetic field and charged particles. Conditions on Triton will also be investigated. The resultant data will give us our first real knowledge of this planet and will also, no doubt, pose more questions than it answers.

The mere feat of having Voyager still operating is tremendous. It has travelled a vast distance and in the process has survived many hazards and is still working perfectly. The fact that this aging spacecraft will still be able to return high resolution photographs of Neptune and Triton is a tribute to its designers.

Long exposures will be necessary to obtain these images since the light available at this vast distance is very low.

The tracking of the spacecraft and the receipt of its data will be solved by using radio telescopes from several countries in an array of instruments. The on-board transmitter has a power of 20 watts, so that its signals are faint and get fainter as its distance increases.

Australia will again be one of the main receiving stations, where the Deep Space Station will work in an array with the Parkes Radio Telescope, and also with Usuda Observatory in Japan. Other arrays will be in California, New Mexico and Spain.

After the encounter with Neptune, Voyager 2 will continue to investigate the heliosphere, that area of space influenced by the Sun and then pass into interstellar space. It is expected to continue to flection well into the next century.