At 12:59 p.m. EST (5:59 p.m. GMT) today, exactly three decades passed since NASA’s Voyager 2 spacecraft became the first machine fashioned by human hands to visit Uranus, the seventh planet in line from the Sun. Frequently berated as the “butt” of much off-color humor, knowledge of Uranus was a virtual blank before it was explored at close quarters on 24 January 1986, just days before the loss of Space Shuttle Challenger changed the face of space exploration forever. Hurtling silently just 50,640 miles (81,500 km) over the aquamarine-hued Uranian cloud-tops, Voyager 2’s accurate delivery was equivalent to sinking a golf-putt from a distance of 2,250 miles (3,630 km). Less than three hours later, scientists and trajectory planners shrieked with delight as images and data—having traveled across 1.8 billion miles (2.9 billion km) to reach the Deep Space Network (DSN) antenna near Canberra in Australia—were translated into data and images at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. A new era of planetary exploration had begun.
That said, the images from Voyager 2 belied a misleadingly quiet world, seemingly void of any major activity in its deep atmosphere. Unlike the spacecraft’s two previous planetary destinations, Jupiter and Saturn, there was no evidence of multi-colored clouds, bands, and eddies, no vast rotating spots, and Uranus’ rings were thin and indistinct. Several members of the Voyager imaging team wryly dubbed themselves “The Imagining Team,” but in false-color the picture was quite different and revealed cloud patterns beneath the upper-level haze. Uranus was composed principally of hydrogen and helium—about 83 percent and 15 percent, respectively—together with a proportionately greater abundance of methane than Jupiter or Saturn. Temperatures were fairly uniform, at about -216 degrees Celsius (-356 degrees Fahrenheit), although the equator proved somewhat warmer than the south pole, which was in direct sunlight at the time of Voyager 2’s encounter. This raised the first suspicions that an as-yet-undetermined internal heat-transfer mechanism exists within the atmosphere.
As outlined in yesterday’s AmericaSpace history article, Uranus has a rotational tilt of 98 degrees, effectively orbiting the Sun on its “side,” like a celestial beer barrel, and it has been speculated that this strange orientation came about following a collision during its youth. The planet’s aquamarine hue is caused by the presence of methane, which absorbs red light from the weak incident sunlight and reflects back colors at the blue-green end of electromagnetic spectrum. It has been suggested in recent years that the high internal temperatures and pressures within both Uranus and its near-twin, Neptune, could produce interesting atmospheric effects, such as diamond “rain,” falling like stones through honey.
Current thinking is that these “ice giants” formed more slowly, perhaps due to the primordial solar nebula being less dense at its outer edges and requiring longer periods of time to draw together planetary objects. Findings from Voyager 2, together with theoretical predictions and subsequent data from the Hubble Space Telescope (HST), suggest that Uranus’ atmosphere extends to a depth of at least 1,800 miles (3,000 km), to a slushy “ocean” of ammonia, methane, water, and other volatiles, below which could exist a rocky core, about the size of Earth.
At the time of the Voyager 2 encounter, five moons were known to orbit Uranus, all about its equatorial plane and all discovered through ground-based telescopic observation before the dawn of the Space Age. In fact, Uranus’ discoverer, the astronomer William Herschel, found Titania and Oberon in 1787, just six years after the planet, and two others—Ariel and Umbriel—were identified by William Lassell in 1851, before Miranda was first spotted by Gerard Kuiper in 1948. All of the moons are named in honor of characters from the works of William Shakespeare and Alexander Pope, a convention which owes its genesis to Lassell. However, the quintet could be seen as little more than fuzzy dots until Voyager 2 entered its two-month “Observatory” phase in November 1985 and literally swamped astronomers with new data … and new moons. By the end of January 1986, ten more natural satellites had been added to Uranus’ tally, but their intrinsic darkness and tiny size made it increasingly likely that many more existed.
Utilizing its ultraviolet spectrometer, Voyager 2 examined the signatures of gases escaping from the moons’ surfaces and revealed an approximate chemical composition of around 50 percent water-ice, 30 percent rock, and 20 percent “other elements,” including carbon and nitrogen. It was suggested that the magnetic field of Uranus—which remained undetected by the spacecraft’s instruments until a few days before Closest Approach—could be responsible for the profound darkening of their surfaces. Discovery of the effects of an intrinsic magnetic field allowed accurate determinations of the length of Uranus’ day at about 17 hours and 14 minutes, which in turn enabled measurements of the speed of internal winds. “You have to measure how fast the ‘solid’ planet is moving to compare against,” explained Michael Kaiser of NASA’s Goddard Space Flight Center in Greenbelt, Md. “Also, the people who measure energetic particles and magnetic fields need to know how fast the planet is rotating to properly analyze their data [and] for spacecraft operations, things are generally scheduled around the planet’s length of day.”
As identified by Voyager 2, Uranus’ magnetic field extended a mere 370,000 miles (600,000 km) sunward, but wound about 6.2 million miles (10 million km), in an extraordinary corkscrew-like “magnetotail,” beyond the giant planet. Ultraviolet observations of polar aurorae revealed the field’s magnetic axis to be skewed at a 59-degree angle to Uranus’ rotational axis, a peculiarity that—on Earth—would be akin to our world’s north magnetic pole residing in Florida. Together with the planet’s 98-degree rotational tilt, this could provide an explanation for the nature and dynamics of the magnetotail. The field carried a powerful sting, in the form of intense, trapped radiation, which could have quickly (within 100,000 years or so) broken down and darkened any methane in the surfaces of the moons, leaving a thick carbon “dusting.” Specifically, Umbriel’s extreme darkness could be an indicator either that it originally harbored a substantial methane content or that it received the worst of the Uranian radiation.
However, the approach profile of Voyager 2 to Uranus meant that only the southern pole was in direct sunlight, and in some cases the majority of the moons’ surfaces were in darkness, with only 35 percent visible in one instance. One moon which did fall under intensive scrutiny was Miranda, the smallest of the five, discovered less than four decades earlier by Kuiper. “Uranus’ gravity increased the velocity of Voyager 2 by about 1.24 miles/sec (2 km/sec),” explained Ellis Miner of JPL. “The flyby distance was determined by the need to go on to Neptune and was close to the orbital distance of Miranda. It was for that reason that Voyager 2 was able to obtain high-resolution images of Miranda.”
Measuring about 310 miles (500 km) in diameter, the tiny moon would not be a disappointment. In fact, its surface was a veritable jigsaw of activity, which may have seen it endure tumultuous geological upheaval over the millennia. Traces of internal melting and the occasional “upwelling” of internal icy material were identified by Voyager 2, with vast, fault-like canyons, up to 12 miles (20 km) deep, as well as oval, racetrack-like features, running like systems of scratches across the surface. These were juxtaposed against strange, “terraced” regions, with old and young, bright and dark, heavily and lightly cratered terrain types. Inverness Corona, in particular, exhibited a pale, chevron-like feature, betwixt darker layers, suggestive of a re-aggregation of fragments of Miranda’s original surface having been pulled apart and forcibly hammered back together.
Obviously, further clues will have to await a future mission to the Uranian system, although Voyager 2’s data led to speculation that the coronae developed either through tectonic activity or icy volcanism, following an initial spate of heavy micrometeoroidal bombardment. The other moons surrendered their own secrets grudgingly. Umbriel, whose name, propitiously, means “dark angel,” was found to reflect only a few percent of the sunlight which struck its surface, but exhibited a bright ring—nicknamed “the Cheerio”—which may represent an icy crater-floor. “We suppose that it was the result of some sort of cryovolcanism which coated the floor of a relatively fresh crater, except for the central peak area, with relatively fresh, reflective water-ice,” explained Ellis Miner. “There are probably other places where such things have occurred, but they are either buried by subsequent deposits of meteoritic material, have evaporated away or were not large enough to show up in the images.” Of the other moons, Titania was the largest, at almost 1,000 miles (1,600 km) in diameter, characterized by huge faults and winding canyons, some of them extending 1.24-3 miles (2-5 km) deep and around 930 miles (1,500 km) in length, whilst heavily-cratered and ridged Ariel and dark Oberon both showed the signs of meteoriodal bombardment.
Ten new moons were found by Voyager 2, the largest of which was named “Puck,” and it could be supposed that the next discoveries would be made either by a subsequent spacecraft visitor or by advanced optics from Earth. Indeed, in the fall of 1997 the Hale telescope on Mount Palomar in California was employed to identify two new moons, but Voyager 2 data was still being analyzed … and it turned up another moon from the original images. In May 1999, Erich Karkoschka of the University of Arizona in Tucson found the tiny object, just 25 miles (40 km) in diameter, which had been photographed by Voyager on 23 January 1986, just 19 hours before Closest Approach. Its small size and intrinsic darkness meant that it was not seen again for 13 years, when Voyager imagery was compared with Hubble Space Telescope imagery and it was named “Perdita.” As well as being a character from Shakespeare, and therefore in-family with Uranian satellite-naming conventions, Perdita actually has an apt Latin meaning: Lost.
Nine years before Voyager 2 reached Uranus, the planet’s thin system of five rings were discovered by telescopic observations fromn Earth. The arrival of the spacecraft allowed for the identification of six more rings, many of which are no more than 7.4 miles (12 km) across, and even the outermost extends no wider than 62 miles (100 km), suggesting their relative youth when compared to those of Saturn.
Despite efforts to implement a Uranus Orbiter and Atmospheric Probe concept as part of a future Planetary Science Decadal Survey, it remains to be seen when the next mission to this strange aquamarine world and its network of moons—which stood at five on the eve of Voyager 2’s arrival, 15 in the immediate aftermath of the spacecraft’s departure, and is currently 27-strong—will get underway. It remains to be seen when the misleadingly quiescent facade of Uranus’ bland atmosphere will be seen for what it really is: an active world of high winds and dynamic cloud features. It remains to be seen when the mysteries of what caused Miranda to suffer such formative upheaval will be understood. It remains to be seen when charcoal-gray Umbriel and its snow-white Cheerio will again be seen.
It remains to be seen.
This is part of a series of history articles, which will appear each weekend, barring any major news stories. Next week’s article will focus on the 30th anniversary of the loss of Space Shuttle Challenger, which changed the face of human spaceflight forever.