M. Colleen Gino

A quarter of a century ago a pair of spacecraft embarked on a journey to study the outer gas giant planets of the Solar System. The Voyager Mission returned an unprecedented amount of scientific data, data which frequently raised as many questions as it answered. Such was the case with the information gained from Voyager 2’s encounter with Neptune, the smallest and most distant of the gas giants. More than a decade has passed since the Neptune flyby, yet despite the plethora of discoveries which yielded enough material to rewrite the book on planetary science, many questions remain unanswered. These questions can only be answered by a return mission to Neptune.

The Voyager Mission consisted of twin spacecraft, Voyager 1 and Voyager 2 ⁽¹⁾. Although the mission scientists believed that a tour of all four gas giants was possible, budgetary constraints resulted in a limited mission involving only the first two planets, Jupiter and Saturn. With this end in mind, the craft were designed with a five year life expectancy. The spacecraft were launched a few weeks apart in 1977 on slightly different flight paths, with Voyager 1 on a quicker and more direct route. Optimistically anticipating the extension of the mission, however, the mission scientists chose a trajectory for Voyager 2 that would allow the craft to not only visit the first two gas giants, but continue on to Uranus and Neptune should the spacecraft remain operational and funding become available.

Voyager Spacecraft
http://voyager.jpl.nasa.gov/

That optimism paid off. After completing its mission at Jupiter and Saturn in 1979 to 1980, Voyager 1 left the plane of the Solar System and headed out into space. The craft is still communicating with Earth today. Four months later, Voyager 2 followed behind, visiting Jupiter and Saturn and returning more stunning images. But instead of leaving the plane of the Solar System as Voyager 1 had done, Voyager 2 continued on to Uranus and Neptune, the outermost gas giant planets.

A dozen years after setting sail on its history-making journey, Voyager 2 reached Neptune’s neighborhood. Passing a mere 5000 kilometers above the north pole of the planet at its closest approach, Voyager 2 was able to image Neptune with amazing detail and clarity. This was our first good look at the planet. Due to Neptune’s relatively moderate size (about one-third the diameter of Jupiter, the largest of the gas giants) and its vast distance from the Earth (nearly 4.5 billion kilometers), resolving features on the disk of the planet had been a difficult task even for the largest Earth-based telescopes. Also, the images Voyager had taken of Uranus, a planet close in size and composition to Neptune, showed no evidence of features in the upper atmosphere until the images were highly processed ⁽²⁾. Since Neptune was much further from the Sun and therefore even less likely to have an energy source to power atmospheric activity, the scientists were not prepared for what they discovered.

Breathtakingly detailed images revealed an extremely active upper atmosphere, with several dark spots of varying sizes revolving around the planet at different velocities. The largest of these dark formations bore a striking resemblance to the Great Red Spot on Jupiter, a huge cyclone-like storm three times the size of Earth. Dubbed the Great Dark Spot, the storm was located at the same southern latitude and was comparable in size in relationship to Neptune as the Great Red Spot. Fast-moving winds near the swirling storm were found to blow westward, opposite the rotational direction of the planet, with speeds up to 2000 kilometers per hour. Moreover, the size and shape of the Great Dark Spot changed significantly during Voyager’s brief visit ⁽³⁾.

Unlike the Great Red Spot on Jupiter, which we know from direct observations to have existed for at least 300 years and probably much longer, the Great Dark Spot may have been a relatively short-lived phenomenon. When the Hubble Space Telescope imaged Neptune in 1994, only 5 years after Voyager’s visit, no trace of the dark spot was seen ⁽⁴⁾. One year later a new dark spot had appeared in the northern hemisphere of the planet. It appears as though the huge storms on Neptune are transitory in nature.

A number of other cloud formations were observed on Neptune in addition to the dark storm systems. A triangular-shaped white cloud formation was discovered moving around the planet every 16 hours in the southern hemisphere, below the Great Dark Spot. This cloud was dubbed a "scooter", in honor of its speedy motion around the planet. Wispy white cloud streaks reminiscent of cirrus clouds on Earth were observed around the edges of the Great Dark Spot. These clouds changed in appearance in as little as four hours ⁽⁵⁾.

The discovery of the active atmosphere posed a new set of questions for the scientists. Here on Earth, energy from the Sun in the form of light is primarily responsible for fueling atmospheric activity. The heating of the surface during the day followed by the cooling off at night, along with the planet’s rotation, all contribute to create a flowing dynamic pattern, better known as weather. But Neptune is 30 times farther from the Sun than the Earth is, and receives 900 times less solar energy. The planet radiates three times more energy than it receives from the Sun, indicating that the atmospheric activity is fueled by an internal source of energy. One theory suggests that the energy comes form heat generated by the continuing gravitational collapse of the planet ⁽⁶⁾.

It was known from Earth-based observations that Uranus has an unusual tilt to its axis of rotation – it is close to its orbital plane. Basically, the planet appears to be tipped over on its side. Scientists believe this to be the result of a collision with a large body soon after the planet’s formation ⁽⁷⁾. But when Voyager 2 flew near Uranus in 1986, three years before the Neptune encounter, they found something that they did not expect: Uranus has a magnetic field, and an atypical one at that. The field is not aligned closely with the planet’s axis of rotation, as is the case with Earth’s magnetic field, but instead is angled 60° away from it. Also unlike Earth’s, the planet’s magnetic is offset from the center of the planet. At the time, scientists believed this to be related in some way to the fact that its rotational axis is so unusual, or thought perhaps that the magnetic field was going through a reversal.

Comparison of Earth, Uranus and Neptune magnetic field lines.
University of Oregon, Online Physics Course, http://zebu.uoregon.edu/uop.html

However, Voyager found that Neptune has a similarly misaligned magnetic field, one that is offset from the center of the planet by at least half the radius of the planet and tilted 46° from Neptune’s axis of rotation. This finding suggested that the orientation of the magnetic field on Uranus is not related to its sideways axis of rotation, but more likely is a characteristic of the interior flows of both Uranus and Neptune ⁽⁸⁾.

Two Neptunian satellites had been observed from Earth, but Voyager found six more. The most striking discovery involved one of the previously known satellites, Triton. The images showed evidence of current geologic activity, with a multitude of geyser-like eruptions peppering the southern hemisphere. These geyser were observed to spew a mixture of nitrogen gas and dust particles several kilometers into Triton’s thin atmosphere. Such icy volcanism had not been observed on any other body in the Solar System.

Unlike Neptune’s other satellites, Triton is in a highly tilted, circular orbit opposite to the direction of the planet’s rotation. This retrograde orbit combined with the extreme density and frigid surface temperature of the satellite (38 Kelvin) is evidence that Triton did not form near Neptune, but is more likely a captured object from the outer reaches of the Solar System ⁽⁹⁾.

In a matter of days, Voyager 2 supplied us with more information about Neptune and its environs than had been gathered in the previous 150 years since the planet’s discovery.

In a recent publication on their mission strategy for future Solar System exploration, NASA rated a mission to Neptune as a ‘top priority’ for the 2008-2013 term ⁽¹⁰⁾. Falling under NASA’s Roadmap Missions theme, in its current incarnation the Neptune Orbiter is a small, lightweight spacecraft utilizing innovative power and communication systems. Advancements in solar array technology make possible the use of a solar-electric propulsion system (SEP). The large arrays necessary for collecting solar energy will be inflatable, maintaining the lightweight standard of the craft. Solar concentrators will be used to focus the sunlight on the collecting arrays, increasing their efficiency. An optical communication system that utilizes lasers of the appropriate wavelength rather than radio waves will result in higher bandwidth communications. Such a system is smaller, weighs less and consumes far less power. As with the solar arrays, the large diameter optical communications antenna will be inflatable ⁽¹¹⁾.

Ground-Orbiter Optical Link at Table Mountain Facility.
NASA Roadmap Missions: Neptune Orbiter
http://sse.jpl.nasa.gov/site/tech/c/information_tech.html

The Neptune Orbiter has a number of science objectives. The dynamics, structure and composition of the atmosphere remains poorly understood. What is the energy source responsible for powering the high speed winds and variable storm systems? What is the composition of the atmosphere at different altitudes and in the different light and dark features? What accounts for the relatively high percentage of methane and lack of hydrogen and helium? What happened to the Great Dark Spot? These are but a handful of the unresolved issues regarding the atmosphere ⁽¹²⁾.

The curiously misaligned magnetic field poses another set of questions. Why is the field oriented at such a large angle to the planet’s axis of rotation? Likewise, why isn’t the field centered on the rotation axis? What kind of internal processes could generate such an off-kilter magnetic field?

Triton harbors yet another set of mysteries to be solved. What is the composition of the satellite? Was Triton formed near Neptune, or is it a captured object from the outer reaches of the Solar System? What causes the geologic activity, and has the distribution of the ice geysers changed dramatically since the Voyager flyby?

To help answer these questions, the Neptune Orbiter will use a battery of advanced instrumentation. A multispectral imaging system will allow images from a wide range of wavelengths, from the ultraviolet (UV), through the visible spectrum to the infrared (IR), to be produced. Mapping the movement, structure and temperatures of the upper atmosphere of Neptune will thus be possible. Similar multispectral imaging and thermal mapping of Triton will reveal any changes in its surface features as well. The orbiter will be equipped with a magnetometer, an instrument to study the puzzling magnetic polar alignment and magnetic field. In addition, the craft will be capable of conducting radio telemetry experiments to probe the atmosphere, and use radiometry to measure the intensity of electromagnetic radiation from the UV to the IR ⁽¹³⁾.

While a mission to Uranus is most certainly on someone’s wish list, NASA has no current plans for a return mission in the mid-term (2008 – 2013). Neptune seems to hold greater allure, offering a more interesting scientific return. The disparity between the amount of energy Neptune receives from the Sun and the energy it radiates is a puzzle that begs to be solved. And the probability that Triton is a captured object promises new insights into the formation and evolution of the Solar System. There has been talk of piggybacking a visit to Uranus and Neptune onto a Pluto exploration mission ⁽¹⁴⁾. But considering that the fate of a Pluto mission itself is currently uncertain due to budget cutbacks, the likelihood of adding Uranus and Neptune to the mission is not high.

Between 1979 and 1989 the Voyager spacecraft visited four planets and the dozens of satellites associated with them. The knowledge we gained from the twin Voyager spacecraft revolutionized our understanding of planetary science. A generation of scientists and engineers involved in the planning and execution of the Voyager Mission and the subsequent reduction of the returned data, built their careers on the Voyager Mission. Now, the next generation of scientists are waiting in the wings for their turn to study Neptune more closely, and have their opportunity to add to the ever increasing body of knowledge about the history and formation of our Solar System.

References:

(2) NASA Planetary Photojournal Catalog Page: Uranus
http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA01360

(3, 8, 9) NASA Facts: Voyager Mission to the Outer Planets
www.jpl.nasa.gov/news/fact_sheets/voyager.pdf

(4) APOD: 2001 August 21, Dark Spots on Neptune
http://antwrp.gsfc.nasa.gov/apod/ap010821.html

(5) NASA Welcome to the Planets
http://pds.jpl.nasa.gov/planets/welcome.htm

(6) Kaufmann, W.J. and Freedman, R. A., 1999, Universe, W.H. Freeman and Company

(7) NASA Solar System Exploration: Uranus
http://sse.jpl.nasa.gov/features/planets/uranus/uranus.html

(10) NASA (1999) Exploration of the Solar System: Science and Mission Strategy

(12) Hammel, H.B. et al, 2001, The Case For A Neptune Orbiter/Multi-Probe Mission, Innovative Approaches to Outer Planetary Exploration 2001-2020
www.lpi.usra.edu/meetings/outerplanets2001/pdf/4095.pdf

(14) Moomaw, Bruce, 2001, The Perils of Pauline, Space Daily
http://www.spacedaily.com/news/outerplanets-01d.html