Twenty-five years ago, this week, 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 conducted a breathtaking exploration of Jupiter and Saturn—together with its twin, Voyager 1—and had undertaken our species’ first visit to Uranus. In readiness for the Uranus and Neptune rendezvous, a conference in Pasadena, Calif., in February 1984, allowed scientists to look at what was known about the two mysterious planets and identify a comprehensive series of observations about them. Although a reasonable amount of valuable data existed about Neptune, the enormous quantity of new information which would flow back to Earth in August 1989 enriched our understanding of this strange, distant world.
Discovered by astronomer Johann Galle of the Berlin Observatory in September 1846, following calculations independently made by the French mathematician Urbain Le Verrier and the English mathematician John Couch Adams, Neptune lay at a mean distance of 2.8 billion miles (4.5 billion km) from the Sun and a billion miles (1.6 billion km) farther from Earth than Uranus. This reduced its apparent size in our skies and, as a consequence, it was only possible to use it for “stellar occultations” a handful of times each year. Still, in the 143 years betwist its discovery and its first visit by an emissary of humanity, a certain amount of useful information was gleaned about Neptune, including two moons: Triton (discovered by William Lassell in September 1846, just 17 days after the planet itself, and Nereid (detected more than a century later, by Gerard Kuiper, in 1949).
As Voyager 2 headed beyond Uranus, it became far harder to communicate with the spacecraft and increasingly necessary to enhance its capabilities to handle dramatic reductions in lighting levels. By August 1989, Neptune had for a decade been considered the Solar System’s outermost planet, a title it would retain for another decade, until 1999, when the eccentric orbit of tiny Pluto—at the time still formally classified as a “planet” itself—moved out to reclaim this status. In anticipation of the Neptune rendezvous, Voyager 2’s software was extensively reprogrammed to take exposures up to 96 seconds in length, and, in order to avoid jolting the spacecraft and ruining these priceless images, each attitude-control thruster firing was shortened to less than four milliseconds. Experience from visiting gloomy Uranus had already taught the Voyager Imaging Team that, in such a dark environment, every movement by the spacecraft, including tape recorder vibrations, could nudge the camera off-target and impair close-range imagery.
A new technique was developed, known as “Nodding Image Motion Compensation” (NIMC), in support of Neptune operations. This was intended to hold Voyager 2 as steady as possible and restrict its motions to the bare minimum when photographing the planet. During each exposure, whilst the camera shutter was open, the entire spacecraft would be “turned” 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 toward Earth, allowing the image to be transmitted back to NASA’s Deep Space Network (DSN) tracking stations, with the need for the tape recorder. Voyager 2 would then “nod” back to its target, open its shutter once again, and prepare for its next image.
For all the benefits provided by these methods, it was recognized that they would be of limited use without significant enhancements to the worldwide DSN itself, whose three main Voyager tracking complexes were based in Goldstone, Calif., Madrid in Spain, and just outside Canberra in Australia. Their receiving antennas had been increased in diameter to 210 feet (64 meters) in readiness for Uranus and were expanded to 230 feet (70 meters) prior to the Neptune encounter. The Canberra station was particularly vital, for Voyager 2 would make its closest approach almost directly above its position. For this reason, NASA also opted to retain the services of the 210-foot (64-meter) Parkes Radio Telescope in New South Wales, which had been electronically linked to Canberra via a 250-mile-long (400-km) microwave communications system.
Although these enhancements were expected to improve the chances of satisfactorily picking up the spacecraft’s weak radio signal—estimated to be less than a billionth of a billionth of a single watt—it was clear that more receiving power was necessary. Two other tracking networks promptly offered their support: The 210-foot (64-meter) Usudu station on Japan’s Honshu Island, run by the Institute of Space and Astronautical Science (ISAS), was tasked with gathering radio science occultation data, whilst the 27 dishes of the Very Large Array (VLA), near Socorro, N.M., were electronically linked to Goldstone on a daily basis to maximise receiving capability. The result was that the Voyager 2 would be listening to their spacecraft with the equivalent of a gigantic radio “ear” which covered the entire Pacific Ocean basin.
That ear needed to be at its sharpest on the night of 24 August 1989, when Voyager 2 was set to perform its closest approach to Neptune and, indeed, its closest approach to any celestial body since it left Earth, more than 12 years earlier.
The second part of this article will appear tomorrow.