When Voyager 2[i] made its 1989 fly-by of Neptune[ii], our knowledge of this distant planet was increased many-fold, and spectacular images and reams of data were captured.  Prior to the fly-by, little information was able to be gleaned, a direct result of Neptune’s enormous distance (4.5 billion kms) from the Sun (and hence Earth).  Its axial tilt and orbital period were known, as well as the existence of two satellites (one particularly large), and a small ring system (with possibly incomplete rings).  Preliminary spectroscopic analysis allowed a rough breakdown of its atmospheric constituents, but even details as basic as its diameter and polar flattening (and hence its density and likely overall composition) could not be accurately determined from Earth-based telescopes.

Figure 1 - Voyager 2 spacecraft[iii] Voyager (see Figure 1) radically changed the depth of our understanding of Neptune.  With its instruments allowing image capture in the visible light, Infra-Red and Ultraviolet spectrums, as well as measuring thermal radiation, energetic particles, magnetic fields and radio emissions[iv], an unprecedented level of analysis was possible.  Some of the key discoveries about Neptune by Voyager include:

• Neptune has an active and dynamic atmosphere.  Whilst not as tumultuous as Jupiter or Saturn, the bland atmosphere of Uranus that greeted Voyager in 1986 led mission scientists to expect a similar visage on Neptune[v].  Instead, a series of weather systems, including a large storm pattern similar to the “Great Red Spot” on Jupiter (and unimaginatively name the “Great Dark Spot”) were found (see Figure 2).  The Great Dark Spot was roughly the size of Earth, and was proportionally of similar relative size and location (22° South) to the Great Red Spot.  Other features included a high-level[vi] methane cirrus cloud formation, a smaller dark spot (“D2” - 55° South), and a bright feature called “The Scooter” (42° South).

Figure 2 - Atmosphere of Neptune[vii]

• Its atmosphere is composed of 85% hydrogen, 13% helium, 1-2% methane and small traces of acetylene[viii].  As with Uranus, the absorption of red-spectrum light by methane gives the planet its distinctive blue coloring, its slightly higher methane level making it blue rather than green.
• Neptune radiates 2.6 times as much thermal energy as it receives from the Sun, indicating an internal heat source.  Although the heat source is unknown, it is believed that this is the primary driver of weather systems on Neptune.
• Neptune consists of several temperature zones, with the poles and the equator being warmer than the mid-latitudinal regions, another factor likely to influence weather patterns.  Neptune has the fastest known winds in the solar system, reaching speed of 2000km/hr[ix].
• Key properties of Neptune were accurately determined[x]:

• The average density figure suggests a rocky or icy core, (about the size of Earth[xi]) surrounded by a liquid ice mantle, and a higher proportion of heavy elements than Jupiter or Saturn.  Given the greater abundance of hydrogen and helium compared to heavier elements as the distance from the sun increases, this is unexpected, and possibly indicates that Neptune (and Uranus) were formed later than the other two planets, when solar winds had reduced hydrogen and helium in this region[xii].
• Neptune has a magnetic field, although it is quite small when compared other gas giants and Earth[xiii].  The field is tilted by 47° to the rotational axis, and offset from the planetary center by .55 Neptunian radii[xiv].  These results were similar to those of Uranus, but unlike any other planet (see Figure 3).  The rotation of the magnetic field was used to determine the planet’s rotation period.

Figure 3 - Magnetic axis tilt and location[xv]

• An addition to the two known satellites – the very large, retrograde-orbiting Triton, and the small Nereid, six additional small satellites[xvi] were discovered by Voyager[xvii].  Titan was the last solar body encountered by Voyager, and was smaller than originally predicted (2705km in diameter).  Its high density (twice that of water) indicates a two-thirds rock, one-third ice composition[xviii].  Unlike most other satellites, there are very few crater marks, indicating a young surface.  Its retrograde orbit strongly suggests that it was captured by Neptune, and the young surface may have been relayered by the forces of this event.  Triton has a very tenuous nitrogen atmosphere[xix], and is the coldest body in the solar system (-235° C) [xx].  Voyager was able to closely examine Triton’s surface, which showed unusual landscape patterns, and the existence of icy geysers.

Figure 4 - Voyager image of Triton[xxi]

• A feint ring system had been speculated prior to Voyager[xxii], but was thought to be possibly incomplete (arcs).  Observations by Voyager revealed 4 rings[xxiii] (see Figure 5), primarily made of dust and quite insignificant in comparison to Uranus[xxiv].  The outermost ring contains three bright arcs, where a greater density of dust exits.  Shepparding satellites were not detected, but the gravitational influence of some orbiting bodies is believed to support the ring system’s stability.[xxv]

Figure 5 - Rings of Neptune[xxvi] Despite the wealth of information gathered, Neptune was always a bonus in the Voyager program, with Voyager 2’s primary purpose to act as a back-up for its twin Voyager 1[xxvii].  Had Voyager 1 not made a successful rendezvous with Saturn’s large moon Titan, Voyager 2 would have been redirected towards this satellite, and entered a trajectory that would have made a Neptune (and Uranus) fly-by impossible[xxviii].  For this reason, our one and only encounter with Neptune was serendipitous, and utilized equipment that was designed with a focus almost exclusively for use in conditions around Jupiter and Saturn.  Since that time, dedicated orbital probes have launched to revisit Jupiter (Galileo) and Saturn (Cassini[xxix]).

Why go back?

Orbital probes, because they position themselves in an orbit around the target planet, are able to gather much more data, exploring aspects of that planet that may not be in correct position at the time or trajectory of a fly-by.  They also have a much longer time period to carry out a larger number of experiments, and have the capacity to perform far more advanced experiments[xxx].  A single planet orbital mission can carry equipment and experiments that are uniquely developed to study that planet (using information gained from the Voyager fly-by), rather than more generalized versions.  Technical advancements since the Voyager probes were constructed (in the mid-70’s) would also boost the scientific value of the mission. The orbital positions of the four gas giants in 1977 allowed a trajectory to be entered that, with the use of planetary gravitational assist, allowed all four of these planets to be visited within the limitations of the Voyager’s on-board fuel supplies.  Such a mission would be impossible today because of the altered planetary positions (see Figure 6 and Figure 7). Figure 6 - Planetary orbits at Voyager launch Figure 7 - Planetary orbits at present time[xxxi] With Jupiter and Saturn being the subject of current or recent study, a further outer planet mission should focus on either Uranus or Neptune, which as stated, are now (and will remain for several hundred years) in orbital positions that would make a dual-planet visit impossible.  With an unlimited budget, individual missions could be undertaken to each planet, but were a mission endorsed, it is likely that only one of these planets would be selected.  We must therefore ask ourselves which of these two planets would offer the greater returns in our knowledge of the solar system for the money invested in the mission? In addition to the Voyager fly-by, our knowledge of Uranus and Neptune have been augmented by visual data gathered by the Hubble Space Telescope (HTS) and other orbital observation platforms operating in other regions of the electro-magnetic spectrum.  These have expanded our knowledge of these two planets, but have also raised many questions that can only be answered by an orbital mission.

The case for Uranus

From a visual perspective, Uranus is one of the dullest objects in the Solar System, being an almost uniform yellow-green color.  It displays none of the weather patterns of the other three has giants(although Voyager images showed some very minor cloud differentiation[xxxii]).  Uranus has a number of small satellites, but no large ones.  The two most unique properties of Uranus are its highly tilted rotational angle to the ecliptic (98°), giving it extreme seasons that are driven by its orbital period (the poles of Uranus spend 60 years in darkness).  This tilt is unique to this planet[xxxiii].  Uranus also appears to have no significant internal heat source, or alternately, this heat source is prevented from contributing to the thermal output of the planet.  The other three gas giants all radiate out more energy than they receive, and have a higher surface temperature than if they relied on the Sun’s radiant input alone. The high rotational inclination is unlikely to be answered by observation, with leading theories suggesting a very large impact (with an object greater in size than earth), of which there is unlikely to be any remaining evidence.  Such a collision may also account for the lack of internal heating (the heat was radiated into space after the collision), but determining why something is not happening is likely to be harder than observing something that is happening (Neptune’s internal heating). In their 1999 paper “Exploration of the Solar System: Science and Mission Strategy”, NASA identified a Neptune orbiter as the top priority in the 2008-2013 time frame.  Although no solid proposals in this area have been made since the release of that document, we must concur that this is the better choice for a mission.

Aims of a Neptune orbiter

The key questions surrounding Neptune relate to observing and understanding its weather patterns.  It is interesting to note that when the HST took photographs of Neptune in 1995, the storm had disappeared, (in stark contrast to the Great Red Spot, which has been visible since 1664[xxxiv]), and a new storm of similar proportions has appeared in the northern hemisphere[xxxv].  Attempting to construct a detailed model of the dynamics of Neptune’s atmosphere would be a key goal.  This would include determining the gradient and composition of the atmosphere’s vertical cross section, and mapping the movement, structure and temperatures of the upper atmosphere, across UV- IR spectrums[xxxvi].  And given the low of solar thermal input and modest heat output, why are these winds the fastest in the solar system? Other questions include:

• What drives the internal processes that cause Neptune to give off more heat than it absorbs[xxxvii]?
• As well as methane, what chromophore gives it its distinctive bluish tinge[xxxviii] ?
• What processes give the rings their twisted structure and different brightness levels? What drives their ongoing stability?
• What causes the asymmetric magnetic field generated by Neptune?

Triton

Possibly the most compelling reason for selection of Neptune over Uranus is its large satellite Triton.  Because of its retrograde orbit, Triton was most likely captured, and may therefore be a large representative of the Kuiper belt objects (of which Pluto is also believed to be a member).  In depth examination of this body (and how it differs from other satellites, such as the four large Jovian moons) can provide vital insights into the formation of the solar system.  It can also possibly offer a number of insights into the Pluto-Charon[xxxix] system without requiring a direct mission (which would have far less scientific return than a Neptune orbiter). Other areas of interest include it seasons (combined with Tritons  retrograde orbit, Neptune has a 30 inclination to its orbital plane, giving Triton a Uranus like seasonal dynamic), the  apparent youth of its surface, the composition of its atmosphere, and the dynamics of its icy volcanoes.  Triton’s gravitational mass could also be used by the orbiter to achieve altered orbital trajectories, without large fuel expenditures[xl]. Although we have focused on Neptune, it should be noted that Uranus and Neptune share a number of similarities, and naturally fall into their sub-class within the gas giants group (they are sometimes referred to as the Ice Giants, with Jupiter and Saturn the true Gas Giants).  Thus a mission to Neptune would potentially offer answers to questions that concern Uranus, such as the off center and a strong deviance from the axis of rotation magnetic poles and long corkscrew magnetic fields, the high methane and lower abundances of helium and hydrogen, the lack of distinct core layers and the lack of pronounced polar hazes (all features of Jupiter and Saturn). A practical Neptune orbiter still has one major difficulty to overcome.  Unlike a fly-by, an orbiter has to enter a stable planetary orbit.  This involves achieving a significant deceleration[xli].  Utilizing new propulsion technologies (e.g. ion drives) and planetary gravity-assists means higher velocity, and shorter travel times, but also a need for greater deceleration.  A fuel based deceleration mechanism would not be feasible, leaving aero braking (using Neptune’s atmosphere to achieve the required deceleration) the only viable option.  This will increase the weight of the orbiter (which will need a very substantial heat shield), as well as the risk of the mission (you only get one shot, and the forces places upon the craft are much greater).  Providing these technical hurdles can be addressed, a window for Jupiter gravity-assist in about 2019.  Given the vast amount of information, related not only to the planet, but also our understanding of the formation of the solar system, such efforts are to be encouraged.

Bibliography

Hunt, G. & Moore, P., “Atlas of Neptune”, Cambridge University Press, 1994 Hey, N., “Solar System”, Weidenfeld & Nicholson, 2002 Beatty, J., Peterson, C., Chaikin, A., “The New Solar System”, Cambridge University Press, 1999 Kaufmann, W., Freedman, R., “Universe”, W.H. Freeman and Co, 1999 Hammel, H., Oirci, C., Rages, K., “The Case for a Neptune Orbiter/Multi-Probe Mission”, 2001

[i] Herein referred to as just Voyager.  Voyager 1 will be specifically named as such when referred to.