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Neptune - Before Voyager 2
By Greg Bryant
 Published in the June 1989 issue of Universe

Voyager 2's final planetary encounter, within this Solar System anyway, will be with Neptune on the 25th August this year.  This space probe is a true wonder of technology.  Launched on August 20, 1977, Voyager 2 visited Jupiter on July 9, 1979, Saturn on August 25, 1981, and Uranus on January 24, 1986.  We all know the highlights of these previous encounters, but don't think that the Neptune encounter will be an anticlimax.  As the probe can't be gravitationally assisted to an encounter with Pluto (Voyager 2 would have to pass well within the atmosphere of Neptune to have a remote chance of achieving this goal), mission planners have been able to point Voyager 2 in a direction which is independent of future encounters.

However, the proposed trajectories have had to be critically examined in light of a number of factors.  Voyager 2 is an explorer, and Neptune is new territory.  As way back as 1980, the trajectory experts were calculating possible flight trajectories.  Although Neptune's ring system had not been discovered at this time, it was decided that Voyager 2 should cross Neptune's equatorial plane at least 1.3 Neptune radii from the planet, thus leading to a Triton flyby distance of 44,000 km.  In September of 1985, pressure from a number of scientists pushed the Triton flyby distance to 10,000 km, and hence the Voyager 2 space probe would pass a mere 3,400 km from the atmosphere.

Later on that year, however, came a number of discoveries that affected the Neptune plans in the worst possible way.  New models indicated that Neptune's mass was 1.5% smaller than previously estimated, the radius was 1,000 km larger, the axial inclination was 4 degrees larger than previously thought, and an improved ephemeris of Triton placed it 8,000 km from its expected position.  Based on this data, in order for the Triton flyby distance to be maintained at 10,000 km, the new trajectory was scheduled to pass inside the outermost arc region before passing only 1,250 km above the atmosphere!  Naturally, the rings and atmosphere posed severe problems to Voyager's continued functioning.

New calculations, based on the failure of the International Ultraviolet Explorer to detect Neptune in ultraviolet radiation, assumed that Neptune's atmosphere would only have a temperature of 500K.  Besides the problems of atmospheric drag, the possibility of high-voltage arcing was considered.  As such, it was decided that Voyager 2 could not be allowed closer than 27,300 km from Neptune's centre, only 2,550 km above the one microbar atmospheric level.

Neptune's rings also present a major problem.  From ground-based observations of occultations, it has been found that Neptune is orbited by at least 3 rings.  Their distances from Neptune range from 41,000 km to 67,000 km.  Impact with even a very small particle would cause serious damage.  Although the probability of impact can't be ruled out, it has been decided for the Voyager 2 space probe to pass outside the outermost ring arc, thus giving it a closest approach to Neptune of 28,950 km, and a Triton flyby of 38,400 km.  After a number of other factors were considered, such as the position of Voyager 2 for the radio occultation of the Sun and Earth by Neptune, the final decision is : Voyager 2 will fly through Neptune's equatorial plane at a distance of 71,000 km, skim above the north pole at 5,000 km from the atmosphere, dive 48 degrees below the ecliptic plane, and encounter Triton 5 hours later at 40,000 km.

What do we know about the Neptune system at present?  It is interesting to take a brief look at its history.  Neptune was the first planet to be discovered by mathematical calculations.  Based on observed planetary orbital perturbations, John Adams predicted Neptune's position in 1845, but his findings did not receive much encouragement.  Le Verrier made the same calculations.  His results received a better response, and Neptune was discovered in 1846.  Both Adams and Le Verrier are credited with the discovery.

Neptune's orbital distance from the Sun ranges from 4.456 to 4.537 billion kilometres.  (In this article, a billion is defined as a thousand million.  It has been argued in a recent article that we should adopt the British definition rather than the American definition.  However, this is Australia - it has been decided that we should adopt the American definition, and a billion = a thousand million is gaining wider recognition amongst the scientific community globally.)  At this distance, it takes Neptune 164.8 years to orbit the Sun.  Neptune's equatorial diameter is 49,500 km, and it has a rotation period of 15.8 hours.

At a glance, Neptune and Uranus are twin planets.  Although Neptune is slightly smaller, it is more massive.  Despite its greater distance, Neptune has the same effective temperature as Uranus, 57 K.  Neptune's interior is thought to consist of a rocky core of metals and silicates; an icy mantle of methane, water and ammonia; and a hydrogen/helium "crust".  Why Neptune emits excessive heat and Uranus doesn't is one of the many mysteries that will hopefully be solved by Voyager.

Neptune's satellite system consists of two members at present.  The outermost satellite is Nereid, discovered in 1949.  It orbits Neptune at a distance of 5.56 million km, and has an orbital period of just under a year.  Some estimates have placed Nereid's diameter at about 500 km.  However, very little is known about the moon, and Voyager 2 will pass Nereid at an expected distance of 4.7 million km.  Indeed, its orbit is not known accurately.  As such, a 1 arc second error in the location of Nereid translates to a positional error of 20,000 km, and a pointing error for Voyager 2's cameras of 0.5 degrees.  In June of 1987, a number of observations were made to help pinpoint Nereid's orbit.  Four accurate positions were obtained, which differed from those predicted by the Jet Propulsion Laboratory by 3 arc seconds in right ascension, and 0.2 arc seconds in declination.  During these observations, it was noticed that Nereid had a very reddish colour, and that its magnitude varied by 1.5 magnitudes during the observing run.  These brightness changes could be explained by the rotation of either albedo differences on Nereid's surface, or by a nonspherical shape.  Whatever the case, Voyager 2 will hopefully enlighten us.

Neptune's big attraction, however, is the moon Triton, first seen a few weeks after the discovery of Neptune.  Orbiting at a distance of 353,000 km from Neptune, very little is known about Triton.  We do know that, on the orbital side, that Triton revolves in a retrograde (backward) direction around Neptune, and that its orbital inclination to Neptune's equatorial plane is 21 degrees.  Given its orbital and axial inclination, Triton is presented with a changing cycle of mild and very extreme season.  The most extreme seasons occur at intervals of about 700 years, and the next "major" summer is in 2007.

Equatorial diameter estimates range from 2,200 km to 5,000 km, depending on whether Triton has bright frost deposits (2,200 km) or dark hydrocarbon deposits (5,000 km).  Of some certainty, however, is the presence of methane in some state on Triton.  Some doubt does exist about the actual physical state of the methane.  Frozen methane on the surface could explain the spectral features, as could an atmosphere of methane gas.  However, as methane ice is quite volatile, methane ice on the surface means that there must be methane gas in the atmosphere.  Methane ice is colourless, and yet Triton is definitely orange.  As such, it may be that there are complex organic compounds on Triton's surface, like the organic compounds that colour Titan's atmosphere.

At present, it is also thought that nitrogen may exist.  Triton is cold enough that nitrogen may exist as a solid.  Once again, it is quite volatile, so it would seem that nitrogen would be present in its atmosphere.  There are only two other solar system bodies that have nitrogen atmospheres : Earth and Titan.  Some estimates have placed Triton's atmospheric pressure at 10% of Earth.  This would make it thinner than Titan's (1.5 times Earth's!) but thicker than Mars'.  It is also speculated that Triton has seas of liquid nitrogen!  Triton rotates once every 5.877 days, so each side of the moon receives nearly 3 days of constant sunlight followed by nearly 3 days of constant darkness.  The resulting temperature fluctuations could alternatively melt and freeze some of Triton's nitrogen.

Is it any wonder that Voyager 2's final port of call will be a mission of discovery, to complete an incredible planetary tour?  Later on this year, and into the new year, I will give a complete roundup of the Voyager missions, giving particular emphasis to Neptune.  Voyager should be able to resolve cloud features that have been observed in infrared observations from Earth, and accurately determine the position of Neptune's ring system.  Triton's results are anyone's guess.  To close this preview, can we imagine that after closest approach with Triton, Voyager will be able to look back and see sunlight glinting off nitrogen lakes and illuminating haze layers in Triton's atmosphere.