Discovered in 1781 by German-born British astronomer, William Herschel, Uranus was the first planet in our Solar System to be discovered since ancient times. In the present day, astronomers continue to hunt for new planets but this time orbiting distant stars. Among the thousands of extrasolar planets found to date are an ever increasing number of worlds the size of Uranus and its near-twin, Neptune, categorized today as “ice giants”. The study of the ice giants in our Solar System has been given a high priority by the planetary science community so that we can better understand this important class of planet and their role in the formation of planetary systems including our own Solar System. Despite the interest in Uranus, only a single mission has visited this planet – NASA’s Voyager 2 which flew by Uranus on January 24, 1986 long before it was appreciated how important its study would become.

Origin of the Uranus Mission

During the early years of the Space Age, as the United States and the Soviet Union were launching their first probes to the Moon and planets, Uranus was not being seriously considered as a near-term target for exploration. Because of its remoteness, the trip time from the Earth to Uranus by conventional chemical propulsion would be over a decade and require enormous rockets to loft tiny payloads. Realistically, it was felt that Uranus could only be reached directly from Earth by means of nuclear-based or other advanced propulsion systems that might not be available for a decade or more.

But in 1965, Gary Flandro, then a NASA summer intern, discovered that as a result of a rare alignment of the outer planets that occurs only once every 176 years, it would be possible to send a probe to Jupiter in the late 1970s and then have it flyby successively more distant planets for a fast reconnaissance of all the outer planets. While the largest rockets available were just capable of sending a respectable payload directly to Jupiter with a travel time of 18 months or so, this giant planet’s gravity would provide the extra boost needed to reach all of the planets beyond with flight times of just years instead of decades.

In 1969 NASA formally began design studies of what they called the Grand Tour mission. They envisioned building a set of highly advanced, nuclear-powered probes based on a study model called TOPS (Thermoelectric Outer Planet Spacecraft) capable of enduring a decade-long mission—many times longer than had been demonstrated with any earlier spacecraft. In one of the many options considered, a pair of Grand Tour spacecraft would be launched in August 1977. They would reach Jupiter around January 1979, Saturn about August 1980, and finally distant Pluto (still considered a planet then) in December 1985. A second pair of probes would depart the Earth in November 1979 and arrive at Jupiter around March 1981. Jupiter would then slingshot the spacecraft to Uranus in February 1985 and Neptune in February 1988 to complete the Grand Tour.

In the end, NASA’s plans for the Grand Tour proved to be too ambitious and far too expensive with an estimated price tag of $900 million or over $5 billion in today’s money. In 1972 NASA scaled back the mission to have a single pair of Mariner-class spacecraft take advantage of the 1977 launch window to Jupiter and Saturn. Originally designated Mariner Jupiter-Saturn 1977 (MJS 77), this more modest mission was managed by the Jet Propulsion Laboratory (JPL) and could be completed with a flight time of four years at an estimated cost of $250 million. By the time of their launch in the summer of 1977, the MJS 77 spacecraft were renamed “Voyager”.

In order to preserve as much of the original Grand Tour as possible, NASA also began studies for a Mariner Jupiter-Uranus 1979 (MJU 79) mission. MJU 79 would use modified Voyager spacecraft equipped with upgraded systems and instruments optimized for the longer trip to Uranus. Later, more ambitious versions of this proposal even included an entry probe which had been studied earlier as part of the proposed Pioneer Saturn/Uranus missions (see “The Future That Never Came: The Pioneer Saturn/Uranus Probe”). In the end, the MJU 79 proposal was not approved in part because of NASA’s ever-tightening budgets in the post-Apollo era.

Even after the cancellation MJU 79, NASA still had a chance of getting a mission to Uranus with a much less expensive option. In 1974, it was discovered that in addition to the Jupiter-Saturn launch window in the late summer of 1977, there also existed an overlapping launch window that allowed a spacecraft to follow a slightly slower trajectory that could continue on to Uranus and Neptune after passing Saturn. It was possible to have one of the already-approved MJS 77 spacecraft continue on to Uranus after its encounter with Saturn, assuming that it had enough propellant left and its vital systems survived that long.

One of the major drivers behind the decision whether or not to exercise this option was Saturn’s largest moon, Titan (see “Voyager 1: The First Close Encounter with Titan”). Making close-up observations of Titan with its dense, hazy atmosphere was a primary objective of the Voyager mission. Unfortunately, there were no trajectory options at this time that allowed a close pass by Titan while preserving an option to reach Uranus. Mission planners decided that if the first Voyager spacecraft met its objectives at Titan and if the second spacecraft was still in good health with sufficient consumables remaining, it would exercise the option to send the second Voyager to Uranus and possibly even Neptune. But given that the encounters with Uranus and Neptune would not occur until more than 8 and 12 years after launch, respectively, the chances of success were deemed to be very low. The option for Voyager to continue on to Uranus was publicly downplayed by JPL in the months leading up to launch, while the Neptune option was barely even mentioned.

The Voyagers

Even though they were less sophisticated than the originally proposed TOPS spacecraft, the Voyagers were still the most advanced interplanetary spacecraft of their day. The pair of identical probes had a mass of 825 kilograms each at launch. Their appearance was dominated by a large white dish antenna with a diameter of 3.7 meters that was used to communicate with the Earth. This was mounted on top of the ten-sided main body of Voyager, which was derived from earlier Mariner spacecraft that had explored the inner planets of the Solar System.

All of the spacecraft’s electronic equipment was housed in the various compartments of the main body, including three pairs of redundant reprogrammable computers that controlled all the functions of the spacecraft and its instruments. The most important computer system was the Command Control Subsystem (CCS), which was the heart of Voyager’s control system. Next was the Attitude and Articulation Control Subsystem (AACS), which controlled propulsion, attitude control, and instrument pointing. Finally there was the Flight Data System (FDS), which controlled the instruments, stored data, and prepared all the science and engineering data for transmission back to Earth. While these computers are primitive by today’s standards, they gave the Voyagers much more flexibility than any earlier planetary spacecraft.

Voyager sported a number of appendages. One was a boom carrying a set of three radioisotope thermoelectric generators (RTGs) that supplied Voyager with up to 390 watts of electricity. Opposite the RTGs was another boom with the pointable Science Scan Platform mounted on the end. Various boresighted sensors for infrared radiation, polarimetry, ultraviolet spectroscopy, as well as wide- and narrow-angle cameras were fitted to this platform. Instruments for measuring the electromagnetic fields, particles, and the plasma environment were fixed to the body of the spacecraft. The other appendages on Voyager included a slender 13-meter long boom carrying sensitive magnetometers and a pair of 10-meter whip antennas shared by the plasma wave and planetary radio astronomy instruments.

The Voyagers were launched using the Titan IIIE – Centaur D-1T, which was the most powerful American launch vehicle available after the retirement of the Saturn rockets at the end of the Apollo program. To give Voyager the extra kick needed to get to Jupiter, a Thiokol TE 364-4 solid rocket motor, similar to the type used in the third stage of the Delta as well as other launch vehicles of the day, topped off the launch vehicle.

The Mission Starts

After dealing with a host of last minute issues, Voyager 2 was successfully launched on August 20, 1977 following the slower trajectory designated “JSX” (where there were the “X=T” option to fly close to Titan and “X=U” to continue on to Uranus instead). While the launch was nearly perfect, minor problems with the spacecraft would continue to crop up after lift off. About an hour after launch, the boom that carried the Science Scan Platform was suppose to extend but the signal was never received that it had fully deployed. After several attempts to lock the boom in place apparently failed, engineers at JPL suspected the problem was not with the boom but the extension sensor. Test images acquired by the cameras of the spacecraft itself confirmed that the science boom had fully extended to within a fraction of a degree.

Voyager 1, following the faster “JST” (for Jupiter-Saturn-Titan) trajectory, was finally launched on September 5 after a series delays to deal with its own last minute problems. Like its sister, Voyager 1 experienced a number of growing pains after leaving the Earth. Ultimately the problems were resolved and the probes’ computers were reprogrammed as needed to correct these issues and optimize spacecraft performance. In particular, changes to AACS programs and the probes’ cruise attitude led to a significant reduction in hydrazine consumption for attitude control. Combined with near optimum launch dates, excellent launch vehicle performance and accurate navigation, Voyager 2 would now have more than sufficient propellant reserves to reach Uranus.

By far the most serious crises experienced by Voyager 2 started on April 5, 1978, when engineers discovered that the spacecraft’s primary receiver had failed. As programmed, the CCS had switched to the backup receiver when the problem was detected, but the backup was also found to be faulty. It was discovered that the backup receiver’s tracking loop capacitor was malfunctioning and the receiver was unable to lock onto the Doppler-shifted transmissions from controllers on Earth. When the engineers switched back to the primary receiver, it experienced a power surge within 30 minutes and blew a fuse, permanently disabling the receiver.

Over the course of the next week, engineers worked feverishly to address the problem. On April 13 Voyager 2 was finally contacted by ground controllers and slowly regained regular contact with the receding probe. But with only one balky receiver and no backup, controllers were at risk of losing contact with Voyager 2. As insurance, on June 23, 1978 Voyager 2 was programmed for a bare bones backup mission to automatically flyby Saturn. By October 12 similar instructions were uploaded for a minimum encounter at Jupiter as a backup. The prospects for a mission to Uranus dimmed significantly.

But as the months and years unfolded, the partially deaf backup receiver continued to function and engineers slowly learned how to predict what frequency to transmit from Earth so that the faulty receiver could accept commands. In the mean time, Voyager 1 made its closest pass by Jupiter on March 5, 1979, with Voyager 2 following on July 9. Both spacecraft met their objectives and returned a treasure trove of images and other scientific data about Jupiter and its system of moons.

After leaving the Jovian system, both Voyagers pushed on to Saturn, with Voyager 1 reaching its distant target on November 13, 1980. After Voyager 1 successfully achieved all its objectives during its close encounter with Titan and Saturn, Voyager 2 was now free to exercise the “X=U” option of its “JSX” trajectory to continue on to Uranus. And with the continued good performance of Voyager 2 despite the loss of its primary receiver, Voyager program scientists and engineers even began to believe they could exercise the option of having Voyager 2 flyby Neptune in August 1989 after its Uranus encounter. If Voyager 2 could survive the dozen years needed to reach this remote world, four out of the five original Grand Tour targets would have been reached by the less sophisticated Voyager.

On August 25, 1981, Voyager 2 hit its target 101,000 kilometers from Saturn to slingshot on to Uranus. But 100 minutes after closest approach, the Science Scan Platform jammed. Within a couple days, engineers at JPL were able to free the platform but much of the post-flyby data was already lost. In time the problem was traced to excessive use of high slew rates in azimuth that forced lubrication from the actuator’s gears. It was felt that restricting the number of slews and the slew rate of the platform would prevent a recurrence of the problem. If the azimuth actuator seized again, a technique to free it using thermal cycling was developed. But just in case the platform permanently froze in azimuth, the entire spacecraft could be rolled to point the instruments while still using the platform’s elevation actuator. It was estimated that Voyager 2 had enough propellant for 150 such roll maneuvers at Uranus but the Neptune option would be in danger.

On to Uranus

During Voyager’s quiet four and a half year cruise from Saturn to Uranus, program scientists and engineers were busy back on Earth preparing for the challenging encounter with Uranus. Because of its greater distance, the solar illumination at Uranus is only one quarter of that at Saturn. The greater distance also meant that Voyager’s transmissions to Earth would also be that much weaker. As a result, much less data could be transmitted from Uranus than either Jupiter or Saturn.

In order to improve the data transmission rate, NASA upgraded its Deep Space Network (DSN) stations. The most aggressive improvements were made to the DSN station near Canberra, Australia, in part because it would have the best view of Uranus from its vantage in the southern hemisphere. The 64-meter antenna usually used for communications with Voyager was electronically linked with two 34-meter antennas at the complex as well as the Parkes 64-meter radio astronomy antenna located 320 kilometers away. The 64-meter antennas at the DSN sites in Goldstone, California, and near Madrid, Spain, were arrayed with a 34-meter antenna at each site to improve reception from Uranus. Voyager 2 would now be capable of transmitting data at a rate as high as 21.6 kilobits per second from Uranus, or just half the rate used at Saturn.

Because of the flexibility afforded by Voyager’s computers, engineers at JPL were able to upgrade some of the spacecraft’s capabilities remotely. For example, the unused backup FDS on Voyager 2 was reprogrammed to compress raw imaging data. Instead of transmitting the full 8-bit brightness value for each image pixel, only the full brightness value of the first pixel in each row was transmitted, with just the brightness differences saved for the rest of the pixels in the row. This compression technique resulted in an average of only 3 bits per pixel. To protect the compressed data from bit errors during transmission, a Reed-Solomon encoder that was carried on board was used on the compressed images as well as the other science data. Previously images were unencoded and non-imaging science data were passed through one of a redundant pair of Golay encoders also carried by the Voyagers. These steps required significantly more work back on Earth to decode the data and reconstruct the images but they allowed up to 200 images to be transmitted each day.

Because of the dim illumination and dark targets expected at Uranus, the exposure times needed to acquire usable images were much longer than at Saturn. The AACS was reprogrammed to steady Voyager more effectively, resulting in a drift rate of only half an arc minute per minute, or less than a quarter of that experienced at Saturn. While a steady platform was needed for observations of distant targets, images of closer targets would smear during long exposures. The AACS was programmed with a new motion compensation technique that accurately slewed the spacecraft during observations at rates as high as two degrees per minute to compensate for the relative movement between Voyager 2 and a close target.

The Uranus Encounter

During its encounter, Voyager 2 viewed Uranus nearly pole on with its rings (which were discovered only months before Voyager’s launch in 1977) and the orbits of its five moons then known looking like a cosmic bulls eye. As a result, only the southern half of Uranus and its moons would be illuminated by the Sun. Unlike the situation at Jupiter and Saturn where the close encounters with the planet and various moons took place over the course of a day or more, the close encounters at Uranus would take place in rapid succession over a just a few hours.

In order to deflect its trajectory towards Neptune, Voyager was directed towards a specific aim point 81,600 kilometers above the cloud tops of Uranus. The timing of the close encounter events was chosen so that they could be observed from the DSN station in Australia. As luck would have it, this allowed Voyager to pass only 29,000 kilometers from the smallest and innermost Uranian moon then known, Miranda. This was the closest Voyager 2 passed by any planet or moon up to this point in its mission.

In case the faulty backup receiver failed before the encounter, Voyager 2 was programmed to automatically execute a barebones backup mission that promised to return at least some data from Uranus. Other backup encounter sequences were also prepared for transmission to Voyager 2 in case of a failure in one of the computers or if the scan platform seized again, as well as a host of other contingencies.

On November 4, 1985, Voyager 2 started its observatory sequence where it created a series of time lapse movies of Uranus and its surroundings. In December Voyager spotted its first new moon—eventually called Puck—inside the orbit of Miranda. On January 10, 1986, Voyager 2 started its faster-paced far encounter sequence. As Voyager moved closer, it became apparent that Uranus had a blander appearance than was expected. Only subtle variations in color and extremely faint cloud features were detectable even after extensive image processing. It was later found that high altitude haze layers were obscuring the banded deck of clouds present and so readily visible on Jupiter and Saturn. On January 18 Voyager 2 started returning images that were heavily streaked. The problem was quickly traced to a bad element in the FDS used for image compression. New instructions were uploaded to avoid the faulty element and normal images were received again starting January 21—just three days before closest approach.

Around 10.6 hours before closest approach, Voyager 2 entered Uranus’ magnetosphere, which was first detected remotely via its radio emissions just five days earlier. These same radio emissions also allowed scientists to pin down the rotational period of the interior of Uranus for the first time – 17.9 hours or 2.4 hours longer than originally estimated by tracking faint cloud features from Earth. The difference was found to be due to wind speeds as great as 100 meters per second. The magnetic field of Uranus was also found to correspond to a dipole tilted 59° to the planet’s axis of rotation and offset from the center of mass by a third of a planetary radius. This arrangement was unique among the planets explored to date (although five years later, Voyager 2 found a similarly odd magnetic field associated with Uranus’ near-twin, Neptune).

As Voyager 2 neared Uranus, it continued observations not only of the planet but its five larger moons which, contrary to all expectations, displayed a surprising range of geologic activity. About an hour before closest approach, Voyager 2 made its closets approach to Miranda. The motion compensation technique worked perfectly and the images returned revealed a rich variety of features across the face of the tiny moon. In the end, a total of ten additional moons inside the orbit of Miranda were spotted during approach.

On January 24, 1986, at 17:58:24 UT, Voyager 2 made its closest approach to Uranus and its trajectory was deflected towards Neptune as intended. As Voyager 2 passed behind Uranus, its radio transmissions were able to probe the structure of the planet’s atmosphere. Observations of the Uranus ring system under strongly backlit lighting conditions (which highlights smaller ring particles) revealed new details and a pair of previously unknown rings. Voyager continued to make observations for the next month as Uranus receded. In the end, almost 7,000 images and a mass of other scientific data were returned about Uranus, its moons and its rings that are still being studied.

To this day Voyager 2 is the only spacecraft to have visited Uranus. While this planet has been deemed a high priority target for further study by the planetary science community, there has been no support in recent years for a number of proposed follow on missions including a Uranus orbiter. Mars and Jupiter have been considered higher priority targets and received the bulk of funding instead. Fortunately, in 2015 NASA began feasibility studies for orbiter missions to Uranus and Neptune. But with it now too late to take advantage of the low-energy Jupiter gravity assist (JGA) windows to Uranus in the 2019 to 2022 time frame and with the next favorable JGA windows not expected for another decade and a half, mission planners in the US and Europe are now considering non-JGA options for launch in the late 2020s to 2030s to increase flexibility. It could be the 2040s before Uranus is visited once again – over half a century after the first Uranus encounter by Voyager 2.

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Related Video

This is a JPL-produced documentary from 1990 on the Voyager mission to the outer planets.

Related Reading

“Voyager 1: The First Close Encounter with Titan”, Drew Ex Machina, November 12, 2015 [Post]

“The Future That Never Came: The Pioneer Saturn/Uranus Probes”, Drew Ex Machina, August 26, 2015 [Post]

“The Grand Tour Finale: Neptune”, The Space Review, Article #2586, August 25, 2014 [Article]

Bibliography

E.C. Kohlhase and P. A. Penzo, “Voyager Mission Description”, Space Science Reviews, Vol. 21, pp. 77-101, 1977

Ellis D. Miner, “Voyager 2 and Uranus”, Sky & Telescope, Vol. 70, No. 5, pp 420–423, November 1985

David Morrison and Jane Samz, Voyages to Jupiter (SP-439), NASA, 1980

David Morrison, Voyages to Saturn (SP-451), NASA, 1982

Bruce Murray, Journey in Space: The First Thirty Years of Space Exploration, W.W. Norton & Co., 1989

Bruce A. Smith, “NASA Reconfigures Voyager 2, Ground Stations for Uranus Flyby”, Aviation Week & Space Technology, Vol. 122, No. 20, pp 65–68, May 20, 1985

E.C. Stone and E.D. Miner, “The Voyager 2 Encounter with the Uranian System”, Science, Vol. 233, No. 4759, pp 39–43, July 4, 1986

Voyager to Jupiter and Saturn (SP-420), NASA, 1977

“Voyager 1986 Press Kit”, Release No. 85-176, NASA, January 1986

“Voyager 2 Managers Resolve Imagery Problem Before Uranus Encounter”, Aviation Week & Space Technology, Vol. 124, No. 4, pp 24-25, January 27, 1986