1989: Twelve years and one day after liftoff, Voyager 2 reaches Triton, the largest of Neptune’s eight moons and the coldest, most unusual satellite in our solar system.
Launched from Cape Canaveral on Aug. 20, 1977, Voyager 2, as the name suggests, was the second of two identical deep-space probes originally dispatched by NASA to gather data on Jupiter and Saturn. Their primary mission completed, Voyager 2 continued on to make observations of Uranus and Neptune, while Voyager 1 hightailed it toward the edge of the solar system. Voyager 2’s flyby of Triton was the spacecraft’s last contact with a major heavenly body before heading off in the direction of Voyager 1 and interstellar space.
Triton was certainly worth a look-see.
Neptune’s largest moon, discovered in 1846 by British astronomer William Lassell, was named for the Greek god Triton, the son of Poseidon (Neptune to the Romans). Its diameter of 1,677 miles makes it roughly half the size of our own moon, unremarkable enough. But Triton is the only large satellite in the solar system with a retrograde orbit, that is, orbiting in the opposite direction of its planet’s rotation. It is also the coldest known object in the solar system, with a mean surface temperature of minus 391 degrees Fahrenheit (minus 235 Celsius). Unlike Earth’s moon, Triton has an atmosphere, albeit a very thin one, composed mainly of nitrogen and methane.
Voyager 2 returned a series of crisp photos of Triton’s surface, including closeups of ice formations, impact craters and other general surface characteristics. It also photographed a plume of frozen material in the process of being ejected at the surface, which is believed to be either liquid nitrogen or methane. Voyager 2 remains the only spacecraft to have visited Triton. (Voyager 1, launched on a shorter trajectory, took a different route than its sister and bypassed Triton entirely.)
As Voyager 2 passed beyond Neptune and Triton, the mission’s planetary exploration phase officially ended.
If that had been the end of it, the twin Voyagers mission would have been an unqualified success. Although designed and built to complete an exploration of only Jupiter and Saturn, both Voyager 1 and 2 proved far more durable. So NASA extended the original mission include to the outer two planets. But more was, and is, yet to come. Thirty-two years after launch both craft are sailing through the heliosphere at 38,000 mph, still returning data to the Jet Propulsion Laboratory through the Deep Space Network.
NASA expects to continue receiving data from both probes until at least 2025, nearly a half-century after their launch.
Photo: Voyager 2, the only spacecraft ever to pass Triton, took this photo of the Neptunian moon in 1989.
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