Thirty-five years ago today, NASA launched Voyager 2, a 1,600 pound space probe, from Cape Canaveral, Florida. Despite its numerical designation, Voyager 2 was the first of the Voyager probes to be launched. The Voyager twins' mission is to explore our solar system's outer planets and study interstellar space beyond.
The idea for the Voyager probes dates back to the late 1960's. Aerospace engineer Gary Flandro of the Jet Propulsion Laboratory conceived a mission (called the Planetary Grand Tour) requiring four probes that would be launched in the mid- to late 1970's. That time frame would take advantage of an alignment of Jupiter, Saturn, Uranus, Neptune, and Pluto, an event that would not happen again for 175 years. Two of the probes would fly by Jupiter, Saturn, and Pluto. The other two would pass by Jupiter, Uranus, and Neptune.
After the last Apollo mission in 1972, NASA faced budget cuts that spelled doom for many planned programs, including the Planetary Grand Tour. But money remained for two probes, originally meant to be a continuation of the Mariner Project, which explored the inner solar system. After the probes' design was finalized, it was decided they needed their own name because they were a generation ahead of the Mariner probes. Thus, the Voyagers were born.
The Voyager spacecraft, as they came to be called instead of probes, were not the first craft sent from earth to the outer reaches of our solar neighborhood. That honor goes to the Pioneer 10 and 11 missions, which passed by Jupiter and Saturn. Pioneer 11 arrived at Saturn almost a year before Voyager 1 and was used to test the larger spacecraft's route. While the two Pioneers gained valuable data, the Voyager spacecraft carried a wider array of scientific instrumentation and would pass by every planet in the outer solar system with the exception of Pluto, which was still considered a full-fledged planet at that time. The world had never seen Jupiter, Saturn, Uranus, and Neptune like they would see them in the coming years.
In December, 1977, Voyager 1 passed Voyager 2, so we will discuss Voyager 1's journey first. After exiting the asteroid belt in September, 1978, Voyager 1 arrived within observation range of Jupiter in January, 1979 and made its closest approach to the planet (217,000 miles) in March of that year. It was during this fly-by that Jupiter's planetary rings, a smaller version of the rings surrounding Saturn, were discovered. Using Jupiter as a gravitational slingshot, Voyager 1 took its last picture of Jupiter in April and began the long journey to Saturn.
Voyager 1 arrived at Saturn nineteen months later, in November, 1980. It soon discovered that the planet's massive rings were much more complex than anyone on Earth had imagined; instead of several broad rings, there are dozens of sub-groups of small rings in larger bands. To those of us old enough to remember such things, they looked like the grooves on a record.
As Voyager 1 made its way to Saturn, it was decided to alter its mission. Pioneer 11 had earlier detected a significant atmosphere on Titan, one of Saturn's moons. This was an important and surprising find, so Voyager 1's route was changed so it could make a close fly-by. However, this meant that the spacecraft would not be able to visit Uranus and Neptune. So, after a close encounter with Titan, which gravitationally pushed it out of the plane of the ecliptic, Voyager 1 headed for interstellar space.
Voyager 2 had a more extensive journey inside the solar system. Making its closest approach to Jupiter in July, 1979, the spacecraft made a surprising discovery: Io, one of Jupiter's many moons, is volcanically active. One of the images Voyager 2 captured was of a giant plume erupting from the moon's surface. This was the first time volcanic activity had been observed on any celestial body other than the Earth.
Voyager 2 made its close flyby of Saturn in August, 1981, after a 13-month trip from Jupiter. Almost all the iconic pictures of Saturn we see today were taken during this visit. The camera platform locked up from overuse during the Saturn flyby, threatening to cut the mission short. However, mission engineers were able to fix the problem and the spacecraft moved on to Uranus.
Uranus tilts towards the sun at a 90 degree angle, which makes it unique among the planets of our solar system. Voyager 2 discovered that, as a result of this radical tilt, the planets magnetic field trails behind the planet in a corkscrew pattern. It also studied the previously-known but still mysterious rings of Uranus and found them to be fundamentally different from those orbiting Jupiter and Saturn. Mainly, the Uranus ring system is thought to be a fairly recent addition to the planet's characteristics.
Voyager 2 made its last planetary visit in August, 1989, with a flyby of Neptune. It was decided to make a course correction so the spacecraft would also visit Triton, Neptune's largest moon. This resulted in Voyager 2's final trajectory out of the solar system being different than originally planned, but it made little difference as neither trajectory points to any specific interstellar destination.
In 2008, the International Astronomical Union reclassified Pluto as a “plutoid” and stripped it of full planet status. Thus, 1989 marked the year by which all eight planets of our solar system had been visited at least once by probes from earth.
In 1998, Voyager 1 became the farthest man-made object from Earth, exceeding the record set by the slower Pioneer 10. Since these two craft are headed in nearly opposite directions, they are also the furthest apart of anything ever created by humans. Both of the Voyager spacecraft carry golden records containing instructions on how to play the disc and samples of sounds and pictures of life on earth. The best chance for either spacecraft to reach an intelligent species will come in 40,000 years, when Voyager 1 passes within 1.6 light years of the star AC+79 3888. By then, the spacecraft will be dead and cold, having used up all her nuclear fuel by 2025 or so. But as of this writing, both Voyagers are still active and sending limited amounts of data back to Earth. There is disagreement about where interstellar space actually begins, but both spacecraft are now beyond reach of the sun's solar winds. They will never return, but will always remain our first ambassadors to the endless universe.