1989: Voyager 2 makes its closest encounter with Neptune, passing just 3,000 miles above the cloud tops of the most distant planet in our solar system.
The Voyager 2 space probe has been our most productive unmanned space voyage. It visited all four of the outer planets and their systems of moons and rings, including the first visits to previously unexplored Uranus and Neptune.
What did the space probe discover about Neptune?
Originally it was thought that Neptune was too cold to support atmospheric disturbances, but Voyager 2 discovered large-scale storms, most notably the Great Dark Spot. It turned out to have a much shorter duration than Jupiter’s persistent Great Red Spot. Neptune not only has storms, it happens to have the fastest winds in the solar system.
The space probe was plotted to perform a close encounter with Triton, the larger of Neptune’s originally known moons. Along the way, Voyager 2 found six new moons (.pdf) orbiting the planet.
Voyager 2 found four rings and evidence for ring arcs, or incomplete rings, above Neptune. That means all four of the gas giants in our solar system have rings. Neptune’s, however, are very meager compared to the magnificent rings around Saturn.
In the late 19th century, astronomers thought that an unseen Planet X was influencing the orbits of Uranus and Neptune. The observed positions of the two planets and their calculated positions differed. Among those astronomers convinced of the existence of Planet X was Clyde Tombaugh. In 1930 while scanning areas of the sky for Planet X, he found Pluto.
When Voyager 2 flew by Neptune, it took very precise measurements of Neptune’s mass and found it to be about 0.5 percent less massive than previous estimates. When the orbits of Uranus and Neptune were recalculated using the more accurate mass figure, it became clear that the imprecise number for Neptune — and not the gravity of an unseen planet — had caused the observed orbital discrepancies.
After the International Astronomical Union demoted Pluto from planetary status in 2006, Voyager 2’s 1989 Neptune flyby became the point when every planet in our solar system had been visited by a space probe.
(All you Pluto-is-a-planet advocates can still argue for reinstatement, but you will have to bring a few more celestial objects into the planet category along with Pluto.)
The twin Voyager space probes were launched in 1977. Voyager 2 was actually launched first, on Aug. 20. Voyager 1 left two weeks later on Sept. 5. (Voyager 6 was never launched, much to the chagrin of Star Trek fans.) Voyager 1’s trajectory was a faster path, getting it to Jupiter in March 1979. Voyager 2 arrived about four months later in July 1979. Both then sped on to Saturn.
Neptune was Voyager 2’s final planetary destination after passing Jupiter (closest approach July 9, 1979), Saturn (closest approach Aug. 26, 1981) and Uranus (closest approach Jan. 24, 1986).
After its encounter with Neptune, the spacecraft was rechristened the Voyager Interstellar Mission by NASA to take measurements of the interplanetary magnetic field, plasma and charged-particle environment. But mostly it’s searching for the heliopause, the distance at which the solar wind becomes subsumed by the more general interstellar wind. Voyager 2 is headed out of the solar system, diving below the ecliptic plane at an angle of about 48 degrees and a rate of about 300 million miles a year.
We may be able to communicate with Voyager 2 for another 10 years, when its radioactive power sources are predicted to become too weak to supply electricity to run the craft’s critical systems. Then it will be out of our solar system and out of touch, racing to parts unknown and untold.
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