Giant Radio Ear: Remembering Voyager 2’s Encounter With Neptune, 30 Years On (Part 1)

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.

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.

The second part of this four-part article will appear tomorrow.

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