The Next Encounter
NASA’s Voyager 2 spacecraft flew closely past distant Uranus, the seventh planet from the Sun, in January 1986.
At its closest, the spacecraft came within 81,500 kilometers (50,600 miles) of Uranus’s cloud tops on Jan. 24, 1986.
Voyager 2 radioed thousands of images and voluminous amounts of other scientific data on the planet, its moons, rings, atmosphere, interior and the magnetic environment surrounding
Since launch on Aug. 20, 1977, Voyager 2’s itinerary has taken the spacecraft to Jupiter in July 1979, Saturn in August 1981, and then Uranus. Voyager 2’s next encounter is with Neptune in August 1989. Both Voyager 2 and its twin, Voyager 1, will eventually leave our solar system and enter interstellar space.
Voyager 2’s images of the five largest moons around Uranus revealed complex surfaces indicative of varying geologic pasts. The cameras also detected 10 previously unseen moons. Several instruments studied the ring system, uncovering the fine detail of the previously known rings and two newly detected rings. Voyager data showed that the planet’s rate of rotation is 17 hours, 14 minutes. The spacecraft also found a Uranian magnetic field that is both large and unusual. In addition, the temperature of the equatorial region, which receives less sunlight over a Uranian year, is nevertheless about the same as that at the poles.
Nearly 3 billion kilometers (1.8 billion miles) from Earth, Uranus is the most distant object yet visited by a spacecraft. Uranus is so far away that scientists knew comparatively little about it before Voyager 2 undertook its historic first-ever encounter with the planet.
Indeed, since its discovery by William Herschel in 1781, Uranus had remained largely a mystery throughout the ensuing two centuries. Five moons – the first discovered in 1787, the last in 1948 – were visible only as tiny points of light. A system of nine narrow rings went undetected until 1977. The planet’s rate of rotation could be estimated only roughly and was believed to be anywhere from 16 to 24 hours. Before Voyager, there were indirect indications of a magnetic field at Uranus, although the evidence was not conclusive.
Scientists were not sure what to expect from Uranus’s strange orientation. The planet is tipped on its side, with its orbiting moons and rings forming a giant celestial bull’s-eye. As a result, the northern and southern polar regions are alternatively exposed to sunlight or to the dark of space during the planet’s 84-year orbit around the Sun.
Voyager 2’s encounter of Uranus began Nov. 4, 1985 with an observatory phase. Activity built to a peak in late January 1986, with most of the critical observations occurring in a six- hour period in and around the time of closest approach. The spacecraft made its closest approach to Uranus at 9:59 a.m. PST on Jan. 24.
To prepare for the flyby of this unusual planetary system, engineers extensively reprogrammed Voyager 2’s onboard computers via radio control from the ground. They endowed the spacecraft with new capabilities that would enable it to return clear, close-up pictures despite the dim light and high velocity at which Voyager would be passing its targets. (Uranus receives about 1/400th of the sunlight that falls on Earth.)
In addition, giant antenna receiving stations on Earth were linked electronically in order to capture and enhance Voyager’s faint radio signal.
Voyager 2 obtained clear, high-resolution images of each of the five large moons of Uranus known before the encounter: Miranda, Ariel, Umbriel, Titania and Oberon. The two largest, Titania and Oberon, are about 1,600 kilometers (1,000 miles) in diameter, roughly half the size of Earth’s Moon. The smallest, Miranda, is only 500 kilometers (300 miles) across, or just one- seventh the lunar size.
The 10 new moons discovered by Voyager bring the total number of known Uranian satellites to 15. The largest of the newly detected moons, named Puck, is about 150 kilometers (about 90 miles) in diameter, or larger than most asteroids.
Preliminary analysis shows that the five large moons are ice-rock conglomerates like the satellites of Saturn. The large Uranian moons appear, in fact, to be about 50 percent water ice, 20 percent carbon- and nitrogen- based materials, and 30 percent rock. Their surfaces, almost uniformly dark gray in color, display varying degrees of geologic history. Very ancient, heavily cratered surfaces are apparent on some of the moons, while others show strong evidence of internal geologic activity.
Titania, for example, is marked by huge fault systems and canyons that indicate some degree of geologic activity in its history. These features may be the result of tectonic movement in its crust. Ariel has the brightest and possibly the geologically youngest surface in the Uranian moon system. It is largely devoid of craters greater than about 50 kilometers (30 miles) in diameter. This indicates that low-velocity material within the Uranian system itself peppered the surface, helping to obliterate larger, older craters. Ariel also appears to have undergone a period of even more intense activity leading to many fault valleys and what appear to be extensive flows of icy material. Where many of the larger valleys intersect, their surfaces are smooth; this could indicate that the valley floors have been covered with younger icy flows.
Umbriel is ancient and dark, apparently having undergone little geologic activity. Large craters pockmark its surface. The darkness of Umbriel’s surface may be due to a coating of dust and small debris somehow created near and confined to the vicinity of that moon’s orbit.
The outermost of the pre-Voyager moons, Oberon, also has an old, heavily cratered surface with little evidence of internal activity other than some unknown dark material apparently covering the floors of many craters.
Miranda, innermost of the five large moons, is one of the strangest bodies yet observed in the solar system. Voyager images, which showed some areas of the moon at resolutions of a kilometer or less, consists of huge fault canyons as deep as 20 kilometers (12 miles), terraced layers and a mixture of old and young surfaces. The younger regions may have been produced by incomplete differentiation of the moon, a process in which upwelling of lighter material surfaced in limited areas. Alternatively, Miranda may be a reaggregation of material from an earlier time when the moon was fractured into pieces by a violent impact.
Given Miranda’s small size and low temperature (-335 degrees Fahrenheit or -187 Celsius), the degree and diversity of the tectonic activity on this moon has surprised scientists. It is believed that an additional heat source such as tidal heating caused by the gravitational tug of Uranus must have been involved. In addition, some means must have mobilized the flow of icy material at low temperatures.
All nine previously known rings of Uranus were photographed and measured, as were other new rings and ringlets in the Uranian system. These observations showed that Uranus’s rings are distinctly different from those at Jupiter and Saturn.
Radio measurements showed the outermost ring, the epsilon, to be composed mostly of ice boulders several feet across. However, a very tenuous distribution of fine dust also seems to be spread throughout the ring system.
Incomplete rings and the varying opacity in several of the main rings leads scientists to believe that the ring system may be relatively young and did not form at the same time as Uranus. The particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects.
To date, two new rings have been positively identified. The first, 1986 U1R, was detected between the outermost of the previously known rings – epsilon and delta – at a distance of 50,000 kilometers (31,000 miles) from Uranus’s center. It is a narrow ring like the others. The second, designated 1986 U2R, is a broad region of material perhaps 3,000 kilometers (1,900 miles) across and just 39,000 kilometers (24,000 miles) from the planet’s center.
The number of known rings may eventually grow as a result of observations by the Voyager 2 photopolarimeter instrument. The sensor revealed what may be a large number of narrow rings – or possibly incomplete rings or ring arcs – as small as 50 meters (160 feet) in width.
The individual ring particles were found to be of low reflectivity. At least one ring, the epsilon, was found to be gray in color. Important clues to Uranus’s ring structure may come from the discovery that two small moons –Cordelia and Ophelia – straddle the epsilon ring. This finding lends credence to theories that small moonlets may be responsible for confining or deflecting material into rings and keeping it from escaping into space. Eighteen such satellites were expected to have been found, but only two were photographed.
The sharp edge of the epsilon ring indicates that the ring is less than 150 meters (500 feet) thick and that particles near the outer edge are less than 30 meters (100 feet) in diameter.
The epsilon ring is surprisingly deficient in particles smaller than about the size of a beachball. This may be due to atmospheric drag from the planet’s extended hydrogen atmosphere, which probably siphons smaller particles and dust from the ring.
As expected, the dominant constituents of the atmosphere are hydrogen and helium. But the amount of helium – about 15 percent – was considerably less than the 40 percent that had been suggested by some Earth-based studies. Methane, acetylene and other hydrocarbons exist in much smaller quantities. Methane in the upper atmosphere absorbs red light, giving Uranus its blue- green color.
Voyager images showed that the atmosphere is arranged into clouds running at constant latitudes, similar to the orientation to the more vivid latitudinal bands seen on Jupiter and Saturn. Winds at mid-latitudes on Uranus blow in the same direction as the planet rotates, just as on Earth, Jupiter and Saturn. These winds blow at velocities of 40 to 160 meters per second (90 to 360 miles per hour); on Earth, jet streams in the atmosphere blow at about 50 meters per second (110 mph). Radio science experiments found winds of about 100 meters per second blowing in the opposite direction at the equator.
A high layer of haze – photochemical smog – was detected around the sunlit pole.
The sunlit hemisphere also was found to radiate large amounts of ultraviolet light, a phenomenon that Voyager scientists have dubbed “dayglow.”
The average temperature on Uranus is about 60 Kelvin (- 350 degrees Fahrenheit). The minimum near the tropopause is 52 K (-366 F) at the 0.1-bar pressure level. (The tropopause is the boundary between the stratosphere and the troposphere, the lowest level of atmosphere, comparable to the region on Earth where life abounds. One bar is the average pressure at sea level on Earth.)
Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature below the tropopause. Voyager instruments did detect a somewhat colder band between 15 and 40 degrees latitude, where temperatures are about 2 to 3 K lower. The temperatures rise with increasing altitude, reaching 150 K (-190 F) in the rarified upper atmosphere. Below this level, temperatures increase steadily to thousands of degrees in the interior.
Radio emissions detected several days before closest approach provided the first conclusive indication that Uranus actually possesses an magnetosphere.
Not only does a Uranian magnetic field exist; it is intense and skewed with its axis tilted at a 60-degree angle to rotational axis. At Earth, by comparison, the two axes are offset by about 12 degrees.
The intensity of the magnetic field at Uranus’s surface is roughly comparable to that of Earth’s, though it varies much more from point to point because of its large offset from the center of Uranus. The magnetic field source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent. The magnetic fields of Earth and other planets are believed to arise from electrical currents produced in their molten cores.
As at Mercury, Earth, Jupiter and Saturn, there is a magnetic tail extending millions of miles behind Uranus. Voyager measured the magnetotail to at least 10 million kilometers (6.2 million miles) behind the planet. The extreme tilt of the magnetic axis, combined with the tilt of the rotational axis, causes the field lines in this cylindrical magnetotail to be wound into a corkscrew shape.
Voyager 2 found radiation belts at Uranus of an intensity similar to those at Saturn, although they differ in composition. The radiation belts at Uranus appear to be dominated by hydrogen ions, without any evidence of heavier ions (charged atoms) that might have been sputtered from the surfaces of the moons. Uranus’s radiation belts are so intense that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles.
Voyager detected radio emissions from Uranus that, along with imaging data, helped narrow the planet’s rate of rotation to about 17 hours, 14 minutes.
The Uranus encounter officially came to an end on Feb. 25, 1986. Eleven days earlier, project engineers took a major step toward the encounter at Neptune by commanding Voyager 2 to fire its thrusters for a course-correction maneuver lasting more than 2-1/2 hours.
Voyager 2 will fly about 1,300 kilometers (800 miles) over the north pole of Neptune at 9 p.m. PDT on Aug. 24, 1989. Five hours later, Voyager 2 will encounter Neptune’s moon, Triton -- the spacecraft’s final destination before heading toward the boundary of our solar system.
The Voyager project manager is Norman R. Haynes of JPL, and George P. Textor, also of JPL, is the deputy project manager. Dr. Edward C. Stone of the California Institute of Technology is the project scientist. Dr. Ellis D. Miner of JPL is the deputy project scientist. JPL manages the Voyager Project for NASA’s Office of Space Science and Applications.