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Four times every 700 years, the planets align in a particular way. No, we are not suddenly headed for astronomy. This rare planetary configuration, which occurs once every 175 years, enables scientists to make the best use of gravity assists.
Voyager 2, which was actually the first of the two Voyager spacecraft to be launched, used exactly this. By using one planet’s gravity to propel itself to the next, Voyager 2 not only visited its initial targets of Jupiter and Saturn, but also made its way to Uranus and Neptune.
This August 1977 photo shows engineers working on the Voyager 2 spacecraft a few weeks before its launch on August 20. | Photo Credit: AP Photo/NASA
Launched on August 20, 1977, Voyager 2 passed through the asteroid belt between December 1977 and October 1978. During this period, its primary radio receiver failed in April 1978, and since then Voyager 2 has been operating using its backup receiver.
Encounter with Jupiter
Months after Voyager 1’s closest approach to Jupiter, Voyager 2 made its closest approach to the planet on July 9, 1979, as part of a four-month encounter with the solar system’s largest planet. It then set off on a 29-month journey toward Saturn, using Jupiter’s gravity to assist it.
Since Voyager 1 had successfully encountered Saturn, Voyager 2 was tasked with studying some of Saturn’s moons, which its twin had been unable to do, before speeding off toward Uranus and then Neptune. Long-range observations of Saturn began on June 5, 1981. As it got closer, the quality of the images sent back improved, including pictures of Saturn’s moons Prometheus, Pandora, Iapetus, Hyperion, Titan, Enceladus, Janus and Tethys.
View of Saturn and its ring system taken by Voyager 2 on August 11, 1981, when the spacecraft was 8.6 million miles from the planet. The shadow of the ring system is clearly visible in the equatorial region. | Photo credit: AP/The Hindu Archives
Looks at Saturn
Three days after its closest approach on Aug. 25, 1981, Voyager 2 turned its cameras on Saturn and returned spectacular images of the partially backlit planet. By Sept. 28, when Voyager 2’s observations of the Saturn system were complete, the spacecraft had returned 16,000 images, including those of the planet, its rings and moons.
The next day, Voyager 2 fired its thrusters, enabling it to make a course correction to send it toward Uranus. More than four years later, in January 1986, Voyager 2 became the first and so far only spacecraft to visit Uranus and its system, coming within 81,500 km of the planet’s cloud tops during closest approach. Before taking a gravity assist from Uranus and heading toward its final planetary encounter, Voyager 2 discovered 10 new moons for Uranus, as well as two new rings and a magnetic field more powerful than Saturn’s.
The low light challenge
About 30 times farther from the sun than Earth, Neptune receives about one-thousandth as much sunlight as Earth. This was a challenge for Voyager 2, as its camera would need long exposures to take good pictures in such low light. The spacecraft’s speed meant that such a long exposure would result in a blurred image. To deal with this problem, the team programmed Voyager 2’s thrusters so that they would fire slowly during the close approach, as well as rotate the spacecraft so that the camera stayed focused on its target, without changing the spacecraft’s speed and direction.
The “Great Dark Spot,” a bright, pale blue patch of clouds that accompanies a storm and hurricane in the atmosphere, can be seen on the planet Neptune in this image taken by Voyager 2 on August 25, 1989, less than five days before its closest approach to the planet. | Photo Credit: NASA/JPL-Caltech/Handout via REUTERS
The Deep Space Network that was used to communicate with Voyager extended its dish, allowing engineers not only to hear Voyager 2 clearly despite the great distance, but also to receive more data from the spacecraft during the Neptune flyby. Despite coming of age before the Internet, the team involved made a point of giving frequent updates to the public in the week before the historic August 1989 flyby. In addition, a program called “Voyager All Night” broadcast regular updates on the day it made the closest approach.
The First Neptune
On August 25, 1989 – just eight years after a similar flyby of Saturn – Voyager 2 made its closest flyby of Neptune, giving us humans our first glimpse of the solar system’s eighth and farthest planet. Voyager 2 showed that Neptune is very similar to Jupiter and Saturn, except it is blue in color due to the presence of methane. Six new moons and four rings were discovered, as well as a giant storm called the “Great Dark Spot,” similar to Jupiter’s Great Red Spot. Voyager 2 also flew close to Triton, Neptune’s largest moon.
Two 10-minute exposures of Neptune’s rings clearly show the two main rings as well as the inner faint ring and a faint band that extends toward the planet about halfway between the two bright rings. These images were taken 1 hour and 27 minutes apart using a clear filter on Voyager 2’s wide-angle camera. | Photo credit: Voyager 2 for NASA
Voyager 2’s success story extends far beyond August 25 and its flyby of Saturn and Neptune. The only spacecraft to visit each of the gas giants, Voyager 2 has now entered interstellar space to meet its twin in an unknown location. Both probes continue to report back to Earth, helping humanity learn about a region where no other spacecraft has flown.
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