"Neptune" by NASA/JPL / Public Domain

The Solar System’s Loneliest Planets, Revisited

Aug 22, 2019

By Shannon Hall

On August 25, 1989, in Pasadena, Calif., NASA’s Jet Propulsion Laboratory was bustling with activity. Scientists, reporters and even a bona fide rock star, Chuck Berry, had flocked to the facility’s mission control to commemorate the moment the Voyager 2 spacecraft flew shy of 5,000 kilometers above Neptune’s north pole the previous evening—marking its closest pass to the ice giant. “The level of excitement is the highest I’ve ever seen here,” Carl Sagan later said on a CNN television segment.

That excitement had been building for more than a year as the spacecraft slowly approached what is now considered the sun’s outermost known planet. Day by day, the exhilaration grew as Voyager 2 beamed back pictures—incrementally transforming a blurry cluster of pixels into a looming, beautiful blue orb. “It got to the point where, every day, when a new set of images came down, there would be new discoveries on the planet,” says Heidi Hammel, who was a member of Voyager 2’s imaging science team. Hammel’s logbooks from that time are filled with her sketches of those images—along with “Wow!” “Gosh!” and other exclamations scrawled in the margins. Each image revealed an unexpectedly dynamic world—one with methane-rich clouds, violent storms larger than Earth and planetary winds that, at more than 2,000 kilometers per hour, are the fastest in the solar system. Even Neptune’s large, frozen moon Triton churned with geysers and other surprising signs of geologic activity. “Every day was an adventure,” Hammel recalls. “It was just a remarkable time of discovery.”

But then Voyager 2 continued onward—leaving Neptune in solitude, as it had left behind our solar system’s other ice giant, Uranus, after flying by it in 1986. “Our detailed knowledge of the ice giant systems is pretty much frozen at that time,” says Anne Verbiscer, a planetary scientist at the University of Virginia. After 30 years, no space agency has returned to Neptune or Uranus, and the questions that Voyager 2 raised about each world remain mostly unanswered. “We think we’re so busy in space, but we’re busy at Mars,” says Candice Hansen, a scientist who was on the Voyager imaging team during the flybys. “Once you get beyond that, there just aren’t that many missions that have flown out that far. There’s so much still to learn.”

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