In January 1986, NASA's Voyager 2 became the first, and so far the only, spacecraft to explore Uranus, the second to last stop on its journey through the outer solar system. NASA’s Jet Propulsion Laboratory in Pasadena, California, manages Voyagers 1 and 2, twin spacecraft launched in 1977 to explore the outer planets.
Initially planned to fly by Jupiter and Saturn only, Voyager 2 took advantage of a rare planetary alignment that occurs once every 175 years to complete two additional encounters in the outer solar system. Following Voyager 1’s successful exploration of Saturn and Titan in November 1980, on Jan. 8, 1981, NASA approved Voyager 2 to maintain a trajectory that, following its encounter with Saturn in August 1981, would then take it past Uranus in 1986, and if the spacecraft was still functioning, fly by Neptune in 1989.
Each Voyager spacecraft carried a suite of 11 instruments, including:
- an imaging science system consisting of narrow-angle and wide-angle cameras to photograph the planet and its satellites.
- a radio science system to determine the planet’s physical properties.
- an infrared interferometer spectrometer to investigate local and global energy balance and atmospheric composition.
- an ultraviolet spectrometer to measure atmospheric properties.
- a magnetometer to analyze the planet’s magnetic field and interaction with the solar wind.
- a plasma spectrometer to investigate microscopic properties of plasma ions.
- a low energy charged particle device to measure fluxes and distributions of ions.
- a cosmic ray detection system to determine the origin and behavior of cosmic radiation.
- a planetary radio astronomy investigation to study radio emissions from Jupiter.
- a photopolarimeter to measure the planet’s surface composition.
- a plasma wave system to study the planet’s magnetosphere.
At the time of the Voyager encounter, Uranus had five known moons and a set of dark rings first observed the year the spacecraft left Earth. Astronomers had named the moons, in order of distance from the planet, Miranda, Ariel, Umbriel, Titania, and Oberon, after characters in works by William Shakespeare and Alexander Pope. Because of Uranus’ great distance from the Sun, engineers made changes to Voyager’s imaging techniques to accommodate light levels only 25% of what they were during the Saturn encounter. Engineers programmed image motion compensation techniques into Voyager’s computer to maintain clear photographs at the required 15-second exposure times coupled with the spacecraft’s velocity. NASA also upgraded the ground-based tracking antennas of the Deep Space Network to increase their sensitivity to receive Voyager’s signals from Uranus’ distance.
Voyager 2 began to observe Uranus on November 4, 1985, by creating a series of time-lapse videos of the planet and its surroundings. Due to Uranus’ axial tilt of nearly 98 degrees to its orbital plane (the planet is basically lying on its side), Voyager 2’s encounter resembled aiming at a bull’s eye, with the planet at the center and its moons and rings orbiting around it. Due to this unusual orientation, the Sun illuminated only the southern hemispheres of the planet and its moons. It also required Voyager 2 to complete its close encounter observations in just a few hours, compared with several days for the Jupiter and Saturn flybys. On Dec. 30, Voyager 2 discovered its first new moon, eventually named Puck, orbiting closer to Uranus than Miranda. Engineers at JPL resolved an issue with the spacecraft’s imaging system that resulted in highly streaked photographs just three days before the closest encounter.
- John Uri
NASA Johnson Space Center