Pioneer terrestrial mission closes on Neptune

Press, 11 July 1989, Page 21

Pioneer terrestrial mission closes on Neptune

By

STEVEN SMITH

VOYAGER 2 is about to make its historic rendezvous with the planet Neptune, at this time the most distant planet from the sun. The American spacecraft, about the size of a Volkswagen, last flew by a planet in January, 1986, when it was the first manmade object to visit Uranus in the outer solar system. It will make its closest approach to Neptune, seen as only a fuzzy glow through the best Earth telescopes, on August 25, at 4 p.m. New Zealand Standard Time. The craft will come within 4000 kilometres of Neptune’s northern polar cloud tops at 28 kilometres per second. During the last year, Voyager 2 has been intermittently taking its first pictures of the planet and its largest moon, Triton, beaming them back to Earth. The spacecraft started a four-month continuous observation of the planet on June 5. This will continue during the close encounter and end on October 2. Voyager 2 and its sister craft, Voyager 1, were launched in 1977 by NASA to fly by giant outer planets Jupiter and Saturn. Both craft sent back thousands of stunning images of the planets and their moon systems, never seen before in such detail. After its encounter with Saturn, Voyager 1 continued into interplanetary space. But Voyager 2, because of its trajectory past Saturn and an increased budget allocation, continued on toward Uranus. The craft has had its fair share of technical difficulties. During the initial stages of the journey to Jupiter, one of the twin receivers on board Voyager 2 died, leaving only one functioning for the entire mission. After the Saturn encounter, scientists at NASA’s monitoring centre, the Jet Propulsion Laboratory, found that Voyager’s camera wasn’t pointing towards the planet but out into the blackness of space. It was discovered that the gear mechanism controlling the camera’s drive had been stripped, probably due to the wrong grade of oil lubricant being used. Scientists had to devise a technique to correct this problem — it meant moving the

whole spacecraft to compensate for its motion during each photograph. During the Uranus encounter, this technique worked perfectly, producing clear, sharp images of the planet and its newly-discov-ered rings, plus its host of enigmatic moons. The same technique will also be used at Neptune. Main risks

to the spacecraft at Neptune, however, will be damage from encountering unknown rings, radiation and atmospheric drag on the structure during its close approach. The craft’s longevity is testimony to the skill of scientists and programmers at JPL and the intelligence of the onboard computer.

j As it zooms near Neptune, Voyager 2 will take hundreds of photographs and measure the planet’s magnetic field and its environment. It will also use radio waves to probe deep into the atmosphere to determine its structure and composition. Neptune, the eighth planet, orbits at a distance of 4500 million kilometres from the sun,

30 times greater than the solar distance to the Earth. It takes nearly 165 years to complete one orbit about the sun. At this distance, the intensity of sunlight is about 1000 times less than on Earth, and pictures obtained will take 4 hours and 6 minutes to travel back to Earth. Neptune’s greenish atmosphere is probably similar to that of Uranus, a chilly mixture of hydrogen,' helium and methane racing about the planet at hundreds of kilometres an hour. Neptune’s diameter of 49,400 km is nearly four times the Earth’s. Neptune’s existence was determined independently in 1845. by John Adams, then a recent Cambridge graduate student, and in 1846 by a Frenchman, Charles Leverrier. It was deduced mathematically by its effect on the orbit of its neighbouring planet, Uranus. The position of Uranus had been observed to deviate from its orbital path due to the presence of a large, undiscovered planet outside its orbit Neptune was subsequently identified in 1846 by a German amateur astronomer. Although Neptune is normally the second most distant planet from the sun, the orbit of Pluto is very elongated and at present takes it well within Neptune’s. So for the moment at least, Neptune is the most distant planet from the sun and marks the edge of the known solar system. Observations from Earth indicate that Neptune may be encircled by three sets of broken rings or ring-arcs, each IQ to 20km wide and orbiting 52,000 km above the planet’s chilly (—220 deg.C) green cloud tops. Voyager 2 will photograph this long-conjectured ring system: . i ■ As well as the planet itself, the 800 kg spacecraft will have an excellent opportunity to view Neptune’s two known moons, Triton and Nereid. Little is known about these'two cold and lonely

bodies. Triton is the larger of the two and similar in size to our moon, although slightly larger, with a diameter of about 2000 km. Earth-bound studies indicate that Triton has an atmosphere. Huge lakes of liquid nitrogen may cover its surface. Five hours after its closest approach to Neptune, Voyager 2 will sweep past Triton at a distance of 40,000 km, enabling details as small as a kilometre across to be seen on its cold, dark surface. Voyager will also probe Triton’s atmosphere, using radio waves to determine its structure. Triton is the solar system’s only large satellite that orbits its parent body in the opposite direction to the planet’s rotation. This will ultimately lead to its demise because it will fall into Neptune in some 10,000 million years. Nereid is another mystery altogether. Astronomers are unsure about what type of object it really is. Its reddish colour is unusual for a natural satellite and may be due to a layer of sulphur coating its surface. Added to this is the fact that astronomers on Earth have seen Nereid’s brightness fluctuate by a factor of four times over about eight hours as it orbits Neptune. It is much smaller than Triton with a diameter of about 300 km and is probably shaped like a potato. Voyager will close to within 4.5 million km of Nereid during the fly-by. ■s A search will also be made for new rings and moons orbiting the planet.