Neptune and Triton (smaller crescent) three days after Voyager's 1989 flyby. ( NASA/JPL)
Neptune and Triton (smaller crescent) three days after Voyager's 1989 flyby. ( NASA/JPL)

Mission to Pluto Is Like a Next-Gen Voyager

Yesterday, a cosmic coincidence brought together two spacecraft. One, a veteran cosmic explorer, is hurtling ever outward toward the hinterlands of the solar system. The curtain is still waiting to rise on the other, a relative youngster that will soon be stepping into the spotlight.

On August 25, the Pluto-destined New Horizons spacecraft crossed Neptune’s orbit — 25 years to the day after its elder sibling, Voyager 2, swooped in for a close look at the big, blue ice giant and its curious, geyser-spewing moon.

That cosmic collusion of events helps mark the passing of a torch from one generation of space explorers to another, scientists said during a press conference commemorating the occasion.

A quarter-century ago, Voyager 2 beamed the first good images of Neptune back to Earthly eyes. Now, of course, Neptune isn’t anywhere near where it was then. But that didn’t stop New Horizons from snapping a quick photo as it zoomed over Neptune’s invisible footsteps. From 4 billion kilometers away, the giant planet and its weird moon Triton appear as nothing more than a few tiny pixels, a bit brighter than the inky black background.

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Neptune and its moon Triton, from 4 billion kilometers away. (NASA/JHUAPL/SwRI)

For many, exploring the Pluto system will be the modern equivalent of the Voyager mission, says New Horizons principal investigator Alan Stern.

“This is the first opportunity in a generation to really explore a new planetary system for the first time,” he said. “When I was growing up, we had the privilege of seeing the first orbiter at Mars, and the first landers. And then, the first missions to Jupiter, to Saturn and Uranus and Neptune. And they were enthralling. And they were mind-blowing in terms of the richness of nature. But there hasn’t been anything like this yet in a long time.”

I think it’s safe to say that the spotlight will be firmly fixed on New Horizons when it pulls up next to Pluto in July 2015 and sends those first detailed images of the dwarf planet back to Earth. It’ll be like sending a long-awaited interplanetary postcard to millions of people at once.

Pluto is the most farflung system we will have explored. That enormous distance means we know relatively little about the tiny planet, which is faint and hard to see, even for the most powerful telescopes. “Even with all of our modern technology, everything we know about the Pluto system today would probably fit on one piece of paper,” Stern said, gesturing to a regular old piece of paper.

The same could probably be said for several of the giant planets in the 1980s. Putting the issues of politics and funding aside, these are stories of discovery on the grandest scale, of visiting new worlds and revealing new vistas.

Erupting Ice

Take Neptune, for example. Until Voyager arrived in 1989, the planet was a small blue smear in the sky. But Voyager saw much more than that. Fragments of rings gracefully hugged the space near the planet’s equator. A storm the size of Earth left a large, dark blue blotch on the cerulean surface (the Great Dark Spot had disappeared by the time Hubble aimed its eye at Neptune five years later). Methane clouds high in Neptune’s atmosphere hovered in relief above the otherwise smooth, gassy world. “The planet also had the highest speed winds of any that we had seen in the solar system — over 1,000 milers per hour,” says Voyager project scientist Ed Stone. “We were surprised to find such an active atmosphere so far from the sun.”

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High altitude clouds streak Neptune’s atmosphere. (NASA/JPL)

And then there was Neptune’s strange little moon, Triton. Before Voyager arrived, teams had no idea what they would find. Unlike some of the other outer planet moons, Triton was not formed in the same neighborhood as Neptune. Instead, it grew up far, far away, in a region known as the Kuiper Belt. That distant band of rocky objects is home to the likes of Pluto and its dwarfy brethren.

“Triton was captured by Neptune and probably had geologic activity early in its history. But we had no idea, really, what it was going to look like,” Stone says. “There were many surprises ahead for us.”

The flyby revealed an active world with strange surface features (dubbed “cantaloupe terrain”), fractures, icy lava and geysers strewing dark material across the moon’s bright polar cap. “Even at the most remote edges, we have an active, alive surface on this cold little moon,” Stone says.

Now, a new animation using re-processed images from Voyager recreates that early flyby.

In the intervening 25 years, astronomers have discovered that several of the outer solar system’s icy moons are also erupting. Saturn has its famous little spitter, Enceladus, and Jupiter’s moon Europa is also releasing plumes of water vapor. The Jovian moon Io, of course, is the most volcanically active object in the solar system – another Voyager discovery – and New Horizons has snapped the photos to prove it.

Tiny Planet, Huge Mysteries

Now, some scientists say, it’s possible Pluto is erupting as well.

“The biggest surprise at Neptune was the plumes on Triton,” says JPL’s Bonnie Buratti, a team member on both the Voyager and New Horizons missions. “I would be surprised at Pluto if we didn’t see something like that.”

Pluto is a Kuiper Belt object, just like Triton. The best images we have of Pluto so far – though they’re just blurry pixels – reveal that the frosty features on the world’s surface are shifting. That suggests some kind of active process is going on out there, Buratti says, even at the freezing fringe of the solar system.

“We have been watching Pluto as a little pinpoint of light in the sky that we see from a telescope,” she says. “And it looks like the frost patterns are changing. I think the polar caps are sublimating away and we’re going to see plumes. I would be surprised if we did not see the same types of plumes on Pluto that we saw on Triton.”

The New Horizons mission will begin to reveal the characteristics of the objects populating the Kuiper Belt, which wasn’t discovered until the 1990s. In it are trillions of comets and who-knows-how-many dwarf planets. Lots. They’re the most common type of body in the solar system, and yet we know nothing about them, Stern says.

“We didn’t know they were there. We thought Pluto was a misfit,” he said. “There are many of them, in fact — more dwarf planets than all the giants and terrestrials combined. You might ask yourself, who’s the misfit now?”

In December, New Horizons will emerge from hibernation as it begins its final approach to the faraway system. The probe is already snapping photos, and last month released an animation of Pluto and its largest moon Charon, circling one another like perpetual planetary dance partners. When it arrives in the system in July, it will map the planet’s surface, study its many moons, and gather as much data as possible before continuing in the footsteps of Voyager and plunging headlong into the unknown.

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Next July, we’ll know what these blurry pixels are hiding.