Remembering Voyager 2’s Visit With Neptune, 30 Years On (Part 2)

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.

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

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.

  • The third part of this four-part article will appear next weekend.

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