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Giant Radio Ear: Remembering Voyager 2's Encounter With Neptune, 30 Years On (Part 1)
By Ben Evans, on August 17th, 2019

Neptune and its large moon, Triton, as seen by Voyager 2 in August 1989, three days after closest approach. Photo Credit: NASA

Thirty years ago, this month, all eyes were on the outermost reaches of the Solar System, as humanity braced itself for its last, first-time, close-up glimpse of a new planet in the 20th century. NASA’s Voyager 2 spacecraft, launched in August 1977, had already completed a breathtaking exploration of Jupiter and Saturn—together with twin, Voyager 1—and had pushed the boundaries of knowledge further with a whistlestop tour of distant Uranus. Both Uranus and Neptune were poorly understood and in early 1984 scientists gathered in Pasadena, Calif., to develop a comprehensive set of observations for Voyager 2. And for a period of several weeks in the summer of 1989, the small amount of data about Neptune was multiplied many times over as this unknown world suddenly became known.

NASA’s identical Voyager 1 and 2 spacecraft, launched more than four decades ago, provided a comprehensive glimpse of the four outer planets in the Solar System. Image Credit: NASA

Discovered by the German astronomer Johann Galle of the Berlin Observatory in September 1846, following calculations independently made by the French mathematician Urbain Leverrier and the English mathematician John Couch Adams, Neptune lay some 2.8 billion miles (4.5 billion km) from the Sun and a full billion miles (1.6 billion km) more distant from the Sun than Uranus. Two moons were detected—Triton, found only 17 days after Neptune itself, and Nereid, discovered a century later in 1949—but Neptune’s apparent size in Earth’s skies was so small that even stellar occultations could only discern usable data a handful of times each year.

As Voyager 2 headed beyond Uranus into the darker depths of the Solar System, it became far harder to communicate with the tiny spacecraft and acutely necessary to enhance the capabilities of ground stations to handle dramatic reductions in lighting levels. By August 1989, Neptune was the Solar System’s outermost planet—a placeholder it would retain until 1999, when diminutive Pluto, then still classed as a “planet” in its own right—moved outward in its highly eccentric orbit to reclaim this title.

NASA’s Deep Space Network (DSN) facility in Canberra. Neptune was almost directly above this station during the Voyager 2 encounter in August 1989. Photo Credit: NASA

In readiness for the Neptune flypast, Voyager’s software was reprogrammed to take exposures 96 seconds in length and, to avoid jolting the spacecraft and potentially ruining the images, each attitude-control thruster firing was shortened to under four milliseconds. Experience from visiting gloomy Uranus in January 1986 had taught the Voyager Imaging Team that every movement by the spacecraft (even tape recorder vibrations) could nudge the camera off-target and blur close-range imagery.

The result was Nodding Image Motion Compensation (NIMC), which would restrict Voyager 2’s movements to the bare minimum during Neptune observations. When the camera shutter was open, the entire spacecraft would be rotated extremely slowly by short thruster bursts to track the motion of specific targets. It would then close the shutter and turn its high-gain antenna towards Earth, enabling images to be transmitted to NASA’s Deep Space Network (DSN) tracking stations. Voyager 2 would then “nod” back to its target, re-open its shutter and prepare to take its next image.

Voyager 2 begins its journey of exploration, with a rousing liftoff atop a Titan IIIE-Centaur booster from Launch Complex (LC)-41 at Cape Canaveral on 20 August 1977. Photo Credit: NASA

For all the benefits afforded by this methodology, all would be fruitless without improvements to the DSN itself, whose three main Voyager tracking stations were sited in Goldstone, Calif., Madrid in Spain and just outside Canberra in Australia. Their antennas were expanded to 210 feet (64 meters) before the Uranus encounter and were increased yet further to 230 feet (70 meters) for Neptune. Canberra was of particular importance, for Voyager 2 would execute its closest approach to Neptune almost directly above Australia. As such, NASA also retained the services of the 210-foot (64-meter) Parkes Radio Telescope in New South Wales, which was electronically linked to Canberra via a 250-mile-long (400 km) microwave communications infrastructure.

Still, Voyager 2’s weak signal was estimated to be less than a billionth of a billionth of a single watt and more receiving power was still needed. Two other tracking stations—the 210-foot (64-meter) Usuda complex on Japan’s Honshu Island and the 27 dishes of the Very Large Array (VLA), near Socorro, N.M.—were electronically linked to Goldstone to maximise receiving capacity. The result was that Earth would be listening for Voyager 2’s signal with a colossal radio “ear” covering the entire Pacific Ocean.

Voyager 2 encountered Neptune 30 years ago this month. Video Credit: CBS News/YouTube

And that ear needed to be at its sharpest on the night of 24 August 1989, when Voyager 2 performed not only its most distant planetary rendezvous, but also its closest approach to any celestial body since it left Earth 12 years earlier. The campaign, to be outlined in tomorrow’s AmericaSpace history feature, would involve significant daring which balanced the potential for scientific riches with a very real possibility of losing Voyager 2 itself.


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Remembering Voyager 2’s Visit With Neptune, 30 Years On (Part 2)
By Ben Evans, on August 18th, 2019

Artist’s concept of Triton and its thin atmosphere, with Neptune and the distant Sun in the background. Image Credit: European Southern Observatory (ESO)

Thirty years ago, this month, humans and technology steeled themselves for the last, first-time, close-up glimpse of a new planet in the 20th century. NASA’s Voyager 2 spacecraft, launched in August 1977, had already conducted a breathtaking exploration of the giant gaseous worlds Jupiter, Saturn and Uranus, but as it headed deeper into the Solar System, bound for Neptune, the potential for failure multiplied. As outlined in yesterday’s AmericaSpace history feature, various techniques were implemented to keep the spacecraft steady whilst taking photographs in the low-light conditions at Neptune and new technologies allowed the worldwide Deep Space Network (DSN) to listen for Voyager 2’s weak signal with greater acuteness than ever before.

Early plans for a flypast of Neptune envisaged sending the spacecraft directly over the giant planet’s north pole, in order to establish the proper conditions for a close encounter with its large moon, Triton. The trajectory called for Voyager 2 to sweep a mere 6,200 miles (10,000 km) above Triton’s surface. However, in the early 1980s, ground-based observations revealed ring material around Neptune—at first suspected to be incomplete ring “arcs”—and the possibility of the incurring damage to the spacecraft forced a rethink. Instead, the polar range was increased to pass some 3,000 miles (4,900 km) over Neptune’s royal-blue cloud-tops and the Triton flypast distance was expanded to 25,000 miles (40,000 km).

Ironically, this fourth and final planetary rendezvous actually promised to be the least risky of Voyager 2’s career, since it had no more visits ahead of it. “It gave us the freedom to choose a flyby geometry that was best for the studies of Neptune and Triton,” said Dr. Ellis Miner of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., “without having to worry about where the spacecraft would be going thereafter.” An extended mission to tiny Pluto had been ruled out, because it no longer lay on either Voyager’s flight path and, in any case, too much additional propellant would be required to reach it.

Neptune and its large moon, Triton. Image Credit: NASA

Top priority at Neptune was getting as close as possible to the planet and flying sufficiently close to Triton, in order to acquire map-quality imagery of its frozen surface. Yet the question in the early 1980s remained: What was the nature of the planet’s rings. Were they complete rings or merely incomplete arcs, shepherded around Neptune by the gravitational influence of tiny, as-yet-unseen moons? The situation was further compounded by a desire to gather occultation data for both Neptune and Triton, by passing Voyager 2’s radio signal through their respective atmospheres as they passed in front of Earth and the Sun on two occasions apiece. Each occultation would yield temperature and pressure measurements, together with the ultraviolet signatures of gases emitted into space. The result would be an improved level of understanding of their internal chemical dynamics.

But to complete four occultations, and get close enough to both Neptune and Triton for detailed imagery, trajectory planners were obliged to adopt a highly risky maneuver. After hurtling over the giant planet’s north pole, Voyager 2 would plunge southwards, behind Neptune, pass some 25,000 miles (40,000 km) from Triton about five hours later, then depart the Solar System at southern mid-latitudes. It was an audacious plan which promised, on the one hand, a substantial reward in the event of success, balanced against the risk of totally losing Voyager 2 in the circumstance of failure.

The paths of Voyager 1 and Voyager 2 through the outer Solar System. Image Credit: NASA

Even after adopting this trajectory, there remained serious concerns that an even closer flyby of Triton was needed to acquire high-resolution images. Such a move would also improve the chances of success for the Earth occultations, which could be timed to occur directly over the Deep Space Network (DSN) ground station at Canberra in Australia. NASA accordingly postponed Voyager 2’s arrival at Neptune until early on 25 August 1989. On St. Valentine’s Day in 1986, three weeks after leaving Uranus, the first of six trajectory correction maneuvers were executed to chart a course for Neptune. Only four of these were ultimately required, thanks to the better-than-expected accuracy of the earlier ones, together with more precise observations from Earth, which gave trajectory planners a clearer idea of where the suspected ring material lay.

Navigating a path to Neptune was far more complex than for Jupiter, Saturn and Uranus. For each journey, the Voyagers utilized optical navigation techniques to reach their targets, acquiring long-exposure images of each planet’s moons, against a known “star field”, which provided a determination of the exact position of each object. However, whereas Uranus was known to possess at least five moons before 1986, and Jupiter and Saturn both had in excess of a dozen apiece, Neptune—at the time—was thought to have only two. The smaller of these, Nereid, discovered by astronomer Gerard Kuiper in 1949, is a maverick in a highly eccentric orbit and takes 360 days to orbit its host. Triton, on the other hand, occupies a more stable, relatively circular, though retrograde, orbit, circling Neptune every six days, which made it more useful as a navigational tool

Neptune’s tenuous rings, here seen from Voyager 2, were not discovered until shortly before the encounter. Photo Credit: NASA

Having just one reliable point of celestial reference to navigate a journey of a billion miles (1.6 billion km) beyond Uranus was risky at best. However, mission planners were keenly aware that more moons would likely be found as Voyager 2 drew closer. As a result, optical navigation details were programmed into the spacecraft’s computer, but omitted to specify their targets, which would be updated at short notice when new moons were discovered and their parameters accurately plotted.

Fortunately, the discovery of six new moons came thick and fast in the summer of 1989, leading to a trajectory correction firing on 2 August to precisely direct Voyager 2 to Neptune. Ironically, one of these new moons—later named “Larissa”—had actually been seen from Earth during an occultation in 1981, but could not be confirmed in more than one sighting. All six new moons were irregular, like battered potatoes, dark as soot, and were confined to the region around Neptune’s equator.

In addition to observing the moons, Voyager 2 had begun observing the planet itself for some considerable time. In May 1988, from a distance of 425 million miles (684 million km), the spacecraft’s images were already of better quality than anything acquired from Earth, resolving Triton for the first time as a pale reddish smudge. However, the 12 weeks from June to September 1989 would produce the most spectacular observations of a planetary system like no other: a beautiful, sky-blue world, four times the size of Earth, with a misleadingly calm appearance, beyond whose roiling clouds lurked ferocious storms, wild weather and some of the fastest winds ever seen in the Solar System.


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Wild World: Remembering Voyager 2's Encounter With Neptune, 30 Years On (Part 3)
By Ben Evans, on August 24th, 2019 [AS]

The Great Dark Spot and Bright Companion, together with the chevron-like “Scooter” are visible in this Voyager 2 image of Neptune. Photo Credit: NASA

Thirty years ago this weekend, NASA’s Voyager 2 spacecraft swept silently above Neptune’s royal-blue cloud-tops and revealed a world on the ragged edge of the Solar System which yielded more questions than answers and more mysteries than solutions. Coming 42 months after it passed Neptune’s near-twin, Uranus, it was expected that the two giant planets—so akin to one another in size and chemical composition—would share many similarities. Yet whilst Uranus proved to be a quiet world, with a misleadingly calm atmosphere, it became abundantly clear in the months prior to Voyager 2’s visit that Neptune was quite the opposite. Its dynamic atmosphere sprang into the worldwide headlines in early 1989, when NASA revealed the discovery of large cloud structures. These far-off observations corroborated similar Earth-based studies, but more was to come as the tiny spacecraft drew closer.

Uranus and Neptune: the Solar System’s unique ice giant planets, of which we only got brief glimpses during the Voyager 2 flybys in the 1980’s, beckon for further, more detailed exploration. Image Credit: NASA

By March 1989, a large oval feature was detected in Neptune’s southern hemisphere, which came to be labeled the “Great Dark Spot”, by analogy with the centuries-old storm on Jupiter. The spot paralleled its Jovian counterpart in several regards: its diameter relative to the size of its host planet was broadly the same, it too rotated counter-clockwise—implying that it too was a high-pressure, fast-rotating atmospheric vortex—and it occupied the same latitudinal co-ordinates (22 degrees South) as the Great Red Spot.

On 5 June, the multi-month campaign to study Neptune began in earnest, with an initial “Observatory” phase. At this point, other features on the planet had become apparent, including a dark longitudinal “band”, spanning much of the southern hemisphere; similar banding would be detected later in the 1990s by the Hubble Space Telescope (HST) and the ground-based Keck-II instruments atop Mauna Kea in Hawaii. These features are thought to represent active regions of the atmosphere, rather than singular, short-lived storms. During the Observatory period, Voyager 2 acquired five narrow-angle images during each rotation of Neptune. These were then combined to produce time-lapse movies of the atmosphere in mesmerizing action, allowing planetary meteorologists to trace the formation and dissipation of cloud structures and calculate wind speeds.

On terrestrial planets like our Earth, such imaging aids the determination of rotation rates, but Neptune’s high winds—which vary considerably at different latitudes—tend to hide its bulk motion. Moreover, different regions of its atmosphere rotated at different speeds. As a consequence, Voyager 2 “observed” Neptune’s core rotate by measuring interactions between charged particles trapped in its magnetic field and solar wind particles streaming from the Sun. Early estimates for the length of a Neptunian “day” ranged from eight to 24 hours, but had been refined to about 17 hours through ground-based observations. Voyager 2 pegged the bulk rotation rate, with an accuracy of better than a minute, at 16 hours and 6.7 minutes. Such a fast rate is typical of the giant gaseous planets and causes Neptune’s poles to appear flattened (a phenomenon known as “oblateness”), which in some areas makes its polar radii up to 250 miles (400 km) less than its equatorial radius.

Neptune’s tenuous rings, as seen by Voyager 2 from a distance of 175,000 miles (280,000 km). Photo Credit: NASA

By mid-August 1989, the Observatory phase shifted into the Far Encounter phase. As Neptune’s bulk began to more than fill the spacecraft’s narrow-angle lens, Voyager 2 began to resolve greater detail through wide-angle imagery. They revealed that the planet possessed not one dark spot, but two, as well as enigmatic clouds which raced through its atmosphere. The most noteworthy of these was the “Scooter”, a chevron-shaped, westward-moving structure, named in honor of its immense velocity. It took less than 16 hours to circle the planet. Even today, its nature remains imperfectly understood, although it appears to represent some kind of plume emanating from a deeper cloud deck.

Although the source of the Scooter likely originated below Neptune’s main layer of methane-rich cloud, the Great Dark Spot seemed to occupy a far loftier position. From close range, it appeared to be at least 10 percent darker than its sky-blue environs, implying that it sat as high as 30 miles (50 km) above the visible cloud deck. The spot had been too small to be seen with ground-based instruments, but a bright cirrus-like cloud—probably of frozen methane—had been seen from Earth as early as January 1989. It “hovered” along the edge of the Great Dark Spot and it was nicknamed the “Bright Companion”. Voyager 2 also revealed that it changed shape during each rotation period of the giant planet.

Strangely, though, it always remained inextricably tied to the spot, usually lingering close to its southern rim. Yet the Great Dark Spot itself lay some 60 miles (100 km) “below” the Bright Companion, which prompted several atmospheric specialists to liken it to lenticular cloud forms, found in mountainous regions on Earth. Clouds of this type form as a result of rapid cooling, when winds are pushed to higher altitudes by the presence of mountains, but at Neptune—a world devoid of any solid surface—the sole available “mountain” seemed to be the rising column of the Great Dark Spot itself.

High-altitude clouds of Neptune, as seen by Voyager 2. Photo Credit: NASA

Other, less pronounced cirrus streaks of methane ice also lay high in the atmosphere, forming and dissipating every few hours. At low northern latitudes, around 27 degrees North, Voyager 2 watched them casting long shadows on the main cloud, 30-60 miles (50-100 km) below. This was the first occasion the cloud “shadows” had ever been photographed on one of the giant gaseous planets. The streaks appeared to be 30-120 miles (50-200 km) across, casting shadows between 20-30 miles (30-50 km) wide. However, they did not form a continuous opaque layer and this gave Voyager 2 an unhindered view as far down as the main cloud deck.

The Great Dark Spot and its Bright Companion stretched and contorted restlessly as they traveled across Neptune’s disk. The storm was clearly nowhere near as stable as Jupiter’s Great Red Spot—which had endured largely unchanged for more than 300 years—and was described by one astronomer as “kind of floppy”. The material within the spot, including its bright central core, spun counter-clockwise every 18 hours and the entire storm, clipping along at 680 mph (1,100 km/h), took eight Earth-days to travel around the atmosphere.

In fact, it was in the vicinity of the spot and its companion that the fastest-known winds ever recorded in the Solar System were measured: close to 1,240 mph (2,000 km/h), surpassing even blustery Saturn, the previous record-holder. As it moved its way through the atmosphere, the Great Dark Spot was also slowly migrating northward, heading for the equator, at a rate of about 15 degrees of latitude per year. According to predictions made at the time of Voyager 2’s encounter, it should have reached the equator, and its powerful, westward-blowing jet stream, by 1991-1992. However, HST imagery in 1994 revealed that both the spot and the Bright Companion had vanished completely.

Moreover, the smaller spot (dubbed “D-2” by the Voyager team) was also gone. Although it was tiny in comparison to the Great Dark Spot, it was still the size of our Moon and shared several similarities with its big brother: a bright, rapidly-moving core and attendant clouds of methane ice, carried along by 400 mph (640 km/h) winds. It was assumed that the spots and their entourage did not survive passage through Neptune’s equatorial region. However, in November 1994, another storm was spotted through HST images in the northern hemisphere. It was virtually a mirror image of the Great Dark Spot and led to astonishment at the atmospheric dynamism of such a cold world, so far from the Sun.

In fact, many of Neptune’s atmospheric phenomena endured for several years. It is unknown how “old” the first Great Dark Spot was when photographed in 1989—the Bright Companion was first spotted in January—but its northern successor lasted for more than two years. HST imagery from mid-1996, together with infrared imaging from ground-based observatories, revealed the second spot to be alive and well, as was Neptune’s powerful equatorial jet stream. These findings clearly demonstrate a highly active planet.

Neptune emits 2.6 times as much heat as it receives from absorbed sunlight. This “energy balance” is more than twice that of its near-sister, Uranus, and when set in proportion to Neptune’s size and mass, is larger than that of any other planet in the Solar System. The nature of whatever internal heat source is responsible for driving this active weather remains a mystery, in spite of insights yielded by Voyager 2. Even today, with advanced “adaptive optics”, most scientific thinking remains on a theoretical level and must await a future mission to the planet to be uncovered in more detail.

A series of images of Neptune, which illustrate the dynamic weather features that dominate the planet. Image Credit: Lawrence A. Sromovsky/University of Wisconsin-Madison/NASA

It is assumed that Neptune possesses a rocky core, although this is far from certain. If such a core exists, it is likely two or three times the size of Earth and constitutes only a few percent of the giant planet’s mass. Opinion as to its temperature remains divided, with estimates ranging from 6,000-10,000 Kelvin. It might then be surrounded by a thick, slushy “ocean” of icy and rock, comprising more than 90 percent of the mass, and finally overlaid by a gaseous region, 1,800-3,600 miles (3,000-6,000 km) deep, of hydrogen and helium, together with smaller traces of water, ammonia, and methane. Whether there is a sharp or gradual transition from the deeper, hotter layers into this outer gaseous envelope is unknown. Others, however, argue that planets with extremely dense, rocky cores should have much shorter rotation rates than the 16 hours clocked by Voyager 2’s instruments for Neptune. This has led to increased speculation that the planet’s icy and rocky elements might actually be mixed quite uniformly in its interior. In fact, the only “ice” in large enough quantities to be directly seen by Voyager 2’s radio science occultations was methane, although, like Uranus, water and ammonia ice were predicted at greater depths.

Further parallels with Uranus came with the detection of hydrogen, helium, and methane in Neptune’s atmosphere, together with acetylene, hints of ethane and possibly haze-forming polyacetylenes. However, several fundamental differences between Uranus and Neptune were identified by Voyager 2, the most obvious of which are their respective colors: aquamarine, versus sky-blue. As on Uranus, the absorption of sunlight by atmospheric methane is primarily responsible for Neptune’s overwhelmingly blue hue, but its deeper coloration implies a slightly different agent. The nature of this coloring agent is unknown, although it has been suggested that a blue-tinted cloud deck in the troposphere, perhaps composed of hydrogen sulphide, might be a key driver.

Neptune also differs from Uranus in that its axial inclination is fairly normal, inclined at 29.6 degrees to the plane of its orbit, which causes it to receive considerably more solar heating at its equator than at its poles. It was initially suspected that this would produce a noticeable temperature differential between the near-twins. However, the actual difference is lower than predicted, implying that Neptune possesses some form of internal mechanism which transfers heat from its equator to its polar regions. It would seem that its convective interior is somehow connected to the parts of its atmosphere where sunlight is absorbed. This mechanism then blocks excess internal heat from escaping at the equator, pushing it instead to higher latitudes, where it helps to warm the poles.

The dark-textured surface of Neptune’s moon Proteus, as seen by Voyager 2. Photo Credit: NASA

The planet’s long-awaited rings also turned out to be quite different from those of Uranus and, indeed, also those of Jupiter and Saturn. On 11 August 1989, two weeks before closest approach, Voyager 2’s cameras spotted a pair of ring “arcs”—6,000 miles (10,000 km) and 30,000 miles (50,000 km) long—extending partway around Neptune. In July 1989, NASA had announced the detection of three new moons (later named Galatea, Larissa and Despina), which seemed to interact gravitationally with the ring arcs. These moons ranged from 90-125 miles (150-200 km) in diameter, remarkably close to the predicted sizes of the kind of moons which had be theorized to “shepherd” such ring-arc material.

As August wore on, it became clear that there were not two arcs, but four, and that they did, in fact, run fully around the planet. However, they were uneven in composition and somewhat “clumpy” in places. The outer ring, named in honor of the English mathematician John Couch Adams—who had been one of the first to independently predict Neptune’s existence in the 1840s—appeared to be accompanied by three to five clumps of material, each measuring up to 30 miles (50 km) wide. In subsequent years, it was argued that debris from destroyed moons might be responsible for this uneven spread of ring material. This theory was reinforced by a pair of tiny moons, Thalassa and Naiad, which both measure about 25 miles (40 km) in diameter and may be strong contenders to be themselves torn apart in the future to form part of the rings.

Neptune’s rings are intrinsically dark; so dark, in fact, that protracted, 10-minute-long exposures were demanded of Voyager 2 in order to resolve them. The spacecraft’s best images came in the small hours of 25 August, when it dipped “behind” the planet and acquired a high-resolution profile of their radial extent with its radio science instrumentation. The rings turned out to be very narrow, with the core of the Adams ring measuring only about 10.5 miles (17 km), which implies that they are also quite young.

Early on 25 August 1989, a quarter-century ago, Voyager 2 passed silently over Neptune’s north pole, then turned south, and, an hour later, crossed the ring plane at 16,100 mph (26,000 km/h). Its plasma wave instrument recorded about 300 “hits” per second of ring particles during the quarter-hour on either side of the crossing. Yet its whistle-stop tour of Neptune was not over. Hurtling southward, a few hours later, it encountered the large moon, Triton, which—as tomorrow’s AmericaSpace history feature will show—would turn out to be one of the most enigmatic objects in the entire Solar System.


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Shrinking Triton: Remembering Voyager 2's Encounter With Neptune, 30 Years On (Part 4)
By Ben Evans, on August 25th, 2019 [AS]

Neptune, as seen by Voyager 2 in 1989. No other missions have yet returned to this enigmatic world. Photo Credit: NASA/JPL

Thirty years ago today, NASA’s Voyager 2 spacecraft swept silently over the royal-blue clouds of Neptune at the ragged edge of our Solar System and revealed a world unlike any other: a world of wild weather, a reservoir of energy and a place quite unlike its near-twin, Uranus. As outlined in yesterday’s AmericaSpace history feature, the tiny spacecraft’s observations of Neptune turned up many more questions than answers; as would its close passage by one of the Solar System’s strangest moons, Triton.

After passing over Neptune’s north pole, Voyager 2 headed southwards, hurtling through the giant planet’s ring plane towards Triton, which had grown steadily bigger over the preceding 15 months. By 25 August 1989, the moon covered more than half of the spacecraft’s narrow-angle lens. But in spite of its size, it soon became apparent that Triton was far smaller than anticipated: only 1,680 miles (2,700 km) in diameter, some three-quarters as large as our Moon, yet still one of the Solar System’s biggest natural satellites. Pre-Voyager mass estimates—based upon the amount of sunlight Triton reflected—had yielded erroneous results. “Triton has been shrinking as we approached,” joked Brad Smith, leader of the Voyager 2 imaging team, “until we feared that by the time we arrived, it might be gone!”

The incorrect estimations originated in the early 1980s, when ground-based infrared observations revealed the spectroscopic signature of nitrogen gas and ice on Triton. This prompted some investigators to wonder if the moon’s diameter and mass were smaller than presumed. Triton was discovered by astronomer William Lassell in October 1846, only a few weeks after Neptune itself, but remained nameless until the early 20th century. One of its peculiarities, noted Lassell, was its highly inclined, retrograde orbit, which caused it to circle Neptune “backwards”—or, more properly, in a clockwise direction as viewed from the north—and this led to several suggestions about its possible origin.

It has long been theorized that Pluto, which seems to be compositionally similar to Triton, is an ancient moon of Neptune which escaped its gravitational clutches. When astronomers began to ponder the retrograde orbit, they speculated that some ancient event had sent Pluto careening away into space and disturbed the path of Triton around Neptune. However, since the discovery of the “Kuiper Belt”—a broad swarm of debris beyond Neptune, which might harbor chunks of primordial material from the Solar System’s formation—it is generally thought that both Pluto and Triton may originate from this region. It is particularly noteworthy that the six “new” moons found by Voyager 2 occupy “well-behaved” equatorial orbits and unless they formed significantly later than the hypothetical ancient event their paths would be expected to have been similarly disturbed. This makes a Kuiper Belt origin for Triton more likely.

In fact, the sizes of the 14 moons found to date—Triton, Nereid, six others detected by Voyager 2 and six more identified earlier this century by ground-based astronomers and by the Hubble Space Telescope (HST)—offer persuasive evidence that some catastrophic sequence of events may have occurred in Neptune’s distant past. Unlike the other three gas giants, the planet does not have a system of “mid-sized” moons, with diameters ranging from roughly 300-1,000 miles (500-1,600 km). Triton, which consumes about 99.5 percent of all the mass in orbit around Neptune, is by far the largest and falls “above” the upper limit for this breed of moons, whilst Nereid and the others fall short of its lower qualifying size. Some astronomers have suggested that the planet did once possess a family of mid-sized moons, but that the arrival of Triton either scattered them, destroyed them or ejected them from the Neptunian system altogether. In support of this possibility, Triton lies about 15 Neptune radii, or 230,000 miles (370,000 km), from the planet’s center, which happens to be squarely in the middle of the domain occupied by the mid-sized moons of Jupiter, Saturn and Uranus. If such an event did happen, the debris from the mid-sized moons may have coalesced into the “second generation” seen today.

If Triton was a late arrival to the Neptunian system, the origins of Nereid remain more of a mystery. Some researchers have argued that if it was not thrown into its eccentric orbit by the arrival of Triton, then it is probably a captured asteroid, even though its path is not retrograde. Voyager 2 saw Nereid is a distant blob—the spacecraft passed no closer than 2.9 million miles (4.7 million km) from the moon—and was unable to resolve any detail. Indeed, after observing it intently for 12 full days, it was not possible to even determine Nereid’s rotation rate, so indistinct was its surface. Certainly, the moon’s shape and highly eccentric orbit is consistent with a captured body. Nereid orbits Neptune as close as 807,780 miles (1.3 million km) and as far as 5.9 million miles (9.6 million km). Voyager 2 was unable to pinpoint its exact shape, but it is not spherical, which again hints at an asteroidal origin. However, recent spectroscopic observations revealed the presence of dirty water-ice on its surface, suggestive of it being an “original”, pre-Triton moon of Neptune.

For Voyager 2, these mysteries were eclipsed by its flyby of Triton, which came a few hours after closest approach to the planet itself. Image motion compensation techniques, trialed at Uranus, came into their own and held the spacecraft sufficiently steady to acquire some of the most astonishing, map-quality photographs of the entire mission. Moving at 39,600 mph (63,730 km/h), Voyager 2 passed within 24,730 miles (39,800 km) of Triton’s center, revealing a world of profound complexity and a spectacular conclusion to a 12-year, four-planet “Grand Tour”.

Two minutes after closest approach, the spacecraft’s particles and ultraviolet sensors observed Triton pass in front of the star Gomeisa, in the constellation of Canis Minor, and later our own Sun, which allowed for details of the moon’s composition to become apparent. At 37 Kelvin, it has the coldest known surface in the Solar System, although temperatures climbed slightly higher to 93 Kelvin at the top of its 500-mile-deep (800-km) atmosphere. This incredibly thin mixture can only really be termed an “atmosphere” in the loosest sense of the world. Its low pressure is just one-70,000th of the surface pressure on Earth and is very nearly vacuum and only barely capable of sustaining thin nitrogen ice clouds and haze. These clouds “hang” eight miles (13 km) above Triton’s surface, whilst the haze may constitute a sort of photochemical “smog”.

The main source of Triton’s atmospheric gases seems to be the slow evaporation of nitrogen, methane, carbon monoxide, carbon dioxide and water, which exist as ices on its frozen surface. Of these, nitrogen is the most abundant, constituting 99 percent of the atmosphere. Winds, presumably caused by the movement of these gases, are thought to carry fine dust particles and deposit them on the surface up to 30 miles (50 km) away. Ground-based observations from 1981 also suggested that atmospheric ice or frost periodically showers Triton’s surface with frozen nitrogen “snow.” Such large quantities of surface ice cause the moon to reflect 85-95 percent of the sunlight which strikes it; by comparison, our Moon reflects, on average, a mere 11 percent. This variation in Triton’s reflectivity is caused by its highly inclined orbit around Neptune, which causes its seasons to change quite considerably in their severity.

Voyager 2’s highest resolution images covered only a third of Triton’s surface and revealed a curious, ubiquitous, greenish terrain, which was nicknamed “cantaloupe”, due to its textural similarity to the ridged skin of a cantaloupe melon. Visually, this took the appearance of a series of roughly circular, interlocking depressions, known as “cavi”, each about 15-18 miles (25-30 km) across. The cantaloupe terrain is criss-crossed by long, interconnected ridges, which are thought to have been caused by one or more epochs of melting and near-complete resurfacing of Triton. Speculations continues to abound that the heat source for this melting comes most likely from the tremendous heat associated with the moon’s capture by Neptune and the subsequent circularization of its orbit. It is quite possible that this melting might have kept Triton’s surface in a liquid state for up to a billion years after its capture and perhaps created the right conditions for the development of tiny, microbial life.

Neptune’s largest moon Triton has unusual “cantaloupe” terrain and geysers of nitrogen. Photo Credit: NASA/JPL-Caltech

The immense forces associated with Triton’s capture by Neptune are also thought to be responsible for substantial crustal fractures identified by Voyager 2. These forces caused localized melting of the moon’s surface, which left large, smooth regions or filled basins with lava-like flows, probably of water ice. At a much later date, “outgassing” in the form of volcanic vents or other subsurface processes led to the gradual formation of its thin atmosphere. Some scientists believe that Triton had a thick atmosphere during its formative period, but that eventually temperatures close to its surface began to cool and that this led to the development of surface ice to reflect away the already-weak sunlight and triggering a significant temperature decline. More surface ice formed, overlying large sections of the cantaloupe terrain, and temperatures to such a point that elements of the tenuous atmosphere froze and formed extensive polar caps.

Voyager 2 revealed the southern polar cap to be strange, pinkish color, possibly due to the evaporation of methane and nitrogen ice. Beyond the edge of the cap was a bluish crustal area, probably composed of water ice, which was pockmarked with interlocking cellular features, similar to melted chain mail. Crossing this wasteland were vast canyons and long, straight ridges with central furrows, perhaps representative of subsurface material having been pushed up through crustal fractures. Most of the features seen by Voyager 2 are believed to be solid water ice, which is frozen so hard that it has the consistency of rock at these frigid temperatures. The largest of its visible craters, a monster named “Mazomba”, measures 16.8 miles (27 km) in diameter, although Triton has relatively few craters, indicating the relative youth of its present surface.

It has been argued that liquid might still be emerging, cryovolcanically, from the depths of Triton, freezing onto the surface and obliterating all but the very youngest craters. Volcanism had previously been seen only on Earth and on Jupiter’s moon, Io, but in August 1989 icy plumes of geysers and their after-effects were clearly detected on Triton. Voyager 2’s imagery revealed large quantities of dark material and a “powder” of rock particles, transported up to 30 miles (50 km) across the surface by the prevailing winds. The spacecraft’s photographs showed dark smudges, caused by this material, which ran across the southern polar ice cap. Detailed analysis of two plumes, nicknamed “Hili” and “Mahilani”, revealed that they changed over a 90-minute period and that winds close by blew at more than 30 mph (50 km/h).

Following its visit to Triton, Voyager 2’s exploration of the Neptunian system came to an end. The spacecraft continued to make observations until early October 1989, after which the giant planet’s gravity “bent” its trajectory inexorably southward and a new mission. For the remainder of its life, Voyager 2 would embark on an exploration to depart the Solar System forever and identify the very edge of the Sun’s realm. Thirty years later, that mission continues to inspire and astound us.


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