Uncertain Journey: 30 Years Since Voyager 2’s Encounter With Uranus (Part 1)

Visible only as a crescent, Uranus recedes from Voyager 2 in the days after Closest Approach. Photo Credit: NASA
Visible only as a crescent, Uranus recedes from Voyager 2 in the days after Closest Approach. Photo Credit: NASA

Thirty years ago, today and tomorrow, NASA’s Voyager 2 spacecraft gave humanity its first close-up glimpse of Uranus. Known for more than two centuries, and frequently the “butt” of many off-color jokes, this giant gaseous world had been discovered by the astronomer William Herschel, but until Voyager 2’s encounter its physical characteristics and its five known moons—Ariel, Umbriel, Titania, Oberon, and Miranda—were a virtual blank. In just a few days, the tiny spacecraft revealed far more about the seventh planet in line from the Sun, and the first to be discovered in modern times, than had been achievable through telescopic observations from Earth. And yet Voyager 2’s mission to Uranus might not have happened at all. That it did was the sum of the efforts of scientists, engineers, and trajectory specialists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., dating back to the 1970s.

A “Grand Tour” of the outer planets—Jupiter, Saturn, Uranus, and Neptune—had been proposed in the mid-1960s by JPL aeronautics postgraduate Gary Flandro, utilizing earlier calculations by mathematician Michael Minovich. This tour proposed that a spacecraft launched in 1976-1978 could employ the Jovian gravitational field to reach all four gaseous planets, with Jupiter encountered in 1979, Saturn in 1980, Uranus in 1984, and finally Neptune in 1986, with a relatively small demand for propellant expenditure. It was, quite literally, a chance of three lifetimes; after the closure of the 1976-1978 window, a similar “spreading-out” of the giant planets along the ecliptic plane, allowing a spacecraft to skip from one to the next, would not occur for 176 years.

Early proposals for Grand Tour missions were sketched-out, and may have borne fruit through the Thermoelectric Outer Planet Spacecraft (TOPS), but ultimately fell foul to budget cuts, and when Voyager 1 and Voyager 2 were launched in the summer of 1977, their original mandate was that both would rendezvous with Jupiter and Saturn. Voyager 1 would then depart the Solar System, with the option left open for its twin to complete the Grand Tour by encountering Uranus in January 1986 and Neptune in August 1989. “Those closest to the project knew from the start that the possibility of Uranus and Neptune flybys existed and expected NASA to approve that extension, assuming the spacecraft were still fully functional,” Ellis Miner of JPL explained in 2002. “The specifics were not laid out until after the Voyager 1 Saturn flyby, but each of the instrument providers did their best to prepare their instruments to be able to observe these more distant planets, although they had specific instructions not to add hardware costs purely for the sake of Uranus and Neptune.”

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. Photo Credit: NASA
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. Photo Credit: NASA

By the end of August 1981, as Voyager 2 left Saturn behind, and all systems continued to perform well, NASA formally approved a Uranus “leg” of the mission. The Voyager Uranus Interstellar Mission (VUIM) officially began on 1 October of that year, as the hardy spacecraft was readied for an 870-million-mile (1.4 billion km) trek out to the distant reaches of the Sun’s realm. According to Miner, however, funding for this planetary encounter was considerably less than for Jupiter or Saturn, with projected staff levels reduced to about 50 percent, “ostensibly with the idea that we could take twice as long to prepare for Uranus.” In early 1984, a hundred planetary scientists met at JPL to discuss the current state of knowledge about Uranus and plan a structured program of observations.

In the meantime, Voyager 2 chugged along without difficulty, although its journey was not entirely smooth-sailing. Since it would be imaging a world in a far darker region of the Solar System than it had been designed—with one team member likening it to taking photographs of charcoal briquettes at the foot of a Christmas tree, dimly lit by a single-watt bulb—several enhancements had to be made to the spacecraft. In the almost friction-free environment of space, its every motion, including turning its tape recorder on and off, induced “nodding” motions which tended to blur imagery. However, by allowing Voyager 2 to “settle” for longer after maneuvers, scan platform movements, or tape recorder activities, as well as halving the duration of its attitude-control burns, these nodding motions were subtly corrected.

New command routines were implemented to make better use of improved image motion compensation techniques, enabling the spacecraft to “pan” its cameras under gyroscopic control as they tracked their targets. This served to minimize the blurring which would be induced by Voyager 2’s blistering 39,000 mph (64,000 km/h) passage through the Uranian system. Conversely, this improvement to the imaging capability also meant that as well as picking up minute features on the planet and its moons, it also threatened to reveal inherent damage to the cameras’ optics, including irritating blobs caused by dust on the lenses. Worse, in mid-January 1986, only days before Closest Approach, a computer error caused light and dark streaks to crop up on intermittent images. Although later corrected, the problem highlighted the number of unknowns awaiting resolution.

Voyager 2’s data would be received by the Deep Space Network (DSN) station near Canberra in Australia, due to the extreme southerly latitude of Uranus in Earth’s skies during the winter of 1985-1986. Beefed-up with a 250-mile-long (400 km) microwave link to the Parkes radio telescope in New South Wales, the Canberra site already benefited from its main 210-foot-diameter (64-meter) antenna and subsidiary (34-meter) dishes and would be capable of tracking Voyager 2 for 12 hours per day, as opposed to around eight in the case of the other DSN stations near Madrid, Spain, and at Goldstone, Calif.

The two Voyagers both executed flybys of Jupiter and Saturn, with Voyager 2 pressing onward to Uranus in January 1986 and Neptune in August 1989. Image Credit: NASA
The two Voyagers both executed flybys of Jupiter and Saturn, with Voyager 2 pressing onward to Uranus in January 1986 and Neptune in August 1989. Image Credit: NASA

The planet upon which so much attention would be lavished in January 1986 was known to have a rotational tilt of 98 degrees, with speculation that its unusual orientation may have been the result of a collision during Uranus’ accretion period. Its five large moons orbited their host within the equatorial plane, suggestive of their formation much later, and as Voyager 2 arrived—approaching the sunlit southern hemisphere of the planet—it would have barely six hours to conduct the majority of its observations. From November 1985 until early January 1986, the spacecraft conducted an “Observatory” phase, examining Uranus from afar, with specific focus upon characterizing its elusive magnetic field. A Far and Near Encounter phase from 10-26 January would have all instruments running continuously, followed by the Post Encounter phase (likened by Miner to “the cleaning crew that does its work the morning after an all-night party”) until 25 February.

Even at the start of the Observatory phase, Voyager 2’s photographs exceeded those taken from Earth; in fact, as soon as July 1985, from a distance of 153 million miles (247 million km), resolved atmospheric features as small as 2,900 miles (4,600 km) across. That said, the process of planning the imaging campaign at Uranus was intricate and difficult. “For all the planets, we had a prediction of where the spacecraft would be at each point in time,” said Andrew Ingersoll of California Institute of Technology. “We told the engineers what latitude and longitude we wanted to look at, and they told the camera to take a picture. These commands had to worked out and radioed to the spacecraft weeks in advance. We had to predict the positions of interesting weather features weeks in advance.”

Moreover, Uranus’ south pole would be permanently facing Voyager 2 throughout the encounter, changing very little as the planet rotated, which led to plans to run a series of 38-hour “movies” to track atmospheric cloud motions and accurately determine wind velocities. At 5:59 p.m GMT on 24 January 1986—30 years ago, tomorrow—the spacecraft swept silently just 50,640 miles (81,500 km) over the Uranian cloud-tops. The impressive accuracy of the trajectory profile was just 62 miles (100 km) off-target, which equated to sinking a golf-putt from a distance of 2,250 miles (3,630 km).

A little less than three hours later, engineers, scientists, and trajectory planners shrieked with delight as images and data, transmitted across 1.8 billion miles (2.9 billion km) of space to Canberra and from thence to JPL in Pasadena, reached them in the form of the first data and images on their monitors. Just four days before the loss of Challenger was to change the face of human space exploration forever, Voyager 2 had scored a tremendous victory for robotic space exploration and secured its niche in the annals of space science.


The second part of this article will appear tomorrow.




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