A robotic explorer is dropped off on an alien moon in the outer reaches of the solar system. The surface is extraordinarily frigid this far out, but it is also rich in natural resources, which the explorer mines in order to fuel its rocket-powered leaps across the terrain. Jumping kilometers at a time, the robot can traverse the globe pole to pole, as the moon's sapphire blue host planet rises and sets on the horizon.
As hazily idealistic as this portrait may seem, it is actually well on its way to becoming a reality. The destination: Neptune and its moon Triton. The robotic explorer: the Triton Hopper, a proposed spacecraft recently funded by NASA's Institute for Advanced Concepts (NIAC).
The hopper is the latest of a long line of proposals for dedicated missions to Neptune, all of which have, so far, been thwarted by the immense challenges of visiting this distant blue giant.
"Neptune's awesome," Steven Oleson, the principal investigator for the Triton Hopper, told me. "But it's a long way out. It's [30 times farther] from the Sun than the Earth is. There are concept missions that take 18 years to get there."
Indeed, Neptune is about 2.8 billion miles from Earth, and occasionally wanders even farther from the Sun than Pluto. The reason that the New Horizons probe was able to visit Pluto's world in only nine years because it was a flyby mission, meaning that the probe zipped on past without slowing down to orbit or land on its target.
It's a lot harder to design a spacecraft that can accelerate enough to reach the outer solar system within a reasonable timeframe, but can also slam on the brakes for an orbital or landing maneuver. Compounding the problem is the fact that Neptune doesn't get a lot of sunlight, making solar-powered spacecraft a less practical option.
"What you really need is advanced propulsion, whether it is solar-electrical propulsion near the Earth, or nuclear-electrical propulsion," Oleson said. "It helps you get out there fast, but also slow down and move around."
These obstacles are daunting, no doubt, and the cost could be easily in the billions. Oleson said he will have a rough estimate of the price tag after a two-week intensive on the mission this October.
"It would be almost totally novel"
But the challenges are far outweighed by compelling reasons to visit Neptune, which is literally the coolest planet in our solar system. Its mesmerizing deep blue color hints at the mysterious inner dynamics of ice giants, which we know comparatively little about. Storms rage on Neptune's surface, producing winds of 1,500 miles per hour—the fastest in the solar system.
Triton is no slouch either. This unusual moon is bigger than Pluto, with a diameter of 1,700 miles to Pluto's is 1,473. But like Pluto, Triton is thought to be a Kuiper Belt Object (KBO)—meaning that it originated in the vast region of planetary detritus circling the outer reaches of the solar system.
The moon's retrograde orbit around Neptune implies that it was gravitationally captured by the blue giant at least one billion years ago, potentially displacing many of the other moons in the system.
Video: In a Nutshell/YouTube
That's right: While most planets develop moons in protoplanetary disks like total normals, Neptune took some initiative by catching one. That's a boss move, but that's just how Neptune rolls.
The Neptunian system is packed with these mind-boggling intricacies, yet it remains tantalizingly unexplored, having only ever been visited by Voyager 2 on August 25, 1989. While the Voyager visit was very fruitful, it lasted less than a day and imaged only one side of the system, leaving a lot left to discover.
"They should really draw those pictures of the solar system with a little tag on it that says, 'Here Be Dragons,'" NASA planetary scientist Geoffrey Landis, the science lead for Triton Hopper, told me. "We know so little about it."
Perhaps now, however, we are finally on the cusp of sending robotic emissaries to Neptune, and landing a hopper on its largest moon. Oleson and Landis have worked on a few concept missions involving hopper vehicles before, and see great potential in this method of exploring alien worlds. Leaping from location to location allows for a much wider range of mobility than the traditional lab-on-wheels rover model, and enables robotic explorers to collect atmospheric samples in addition to running ground experiments.
"It would be almost totally novel," Landis said of the hopper concept. "One time and one time only have we hopped a spacecraft on another moon—in this case, our Moon. The Surveyor spacecraft were the first robotic soft-landings on the Moon by the Americans [and] they did a little hop of about two meters. That was back in 1967, a long time ago."
The Triton Hopper, however, would do more than bunny hop a few mere meters. "We should be going tens of kilometers," Oleson told me, "and we can go roughly ten kilometers high. It's about a 2,000 kilometer trip to go from pole to pole, so it would be a lot of hops. But boy, you could see a lot of things."
That may all sound well and good in theory, but how would such Superman leaps be powered? Neptune is too far away from the Sun for reliable solar power. And even in Triton's low gravity—about eight percent of Earth's—multi-kilometer hops would require a lot of juice to pull off.
That's where the real magic of visiting Triton really kicks in, Oleson told me. "The neat thing about Triton is that it is so cold—33 degrees Kelvin (minus 400 degrees Fahrenheit)—that whatever atmosphere was there has basically collapsed and frozen on the surface," he said. "This turns out to be nitrogen, and cold gas nitrogen has been a staple for easy propulsion systems for many years."
"Triton is a unique place to go because you have volatile nitrogen laying all over the place," Oleson continued, "and all you have to do is figure out how to pick it up, cook it, and off we go."
This tactic of harvesting alien worlds for useful materials is called in-situ resource utilization (ISRU), and it is crucial for extending our reach off Earth. Aside from powering spacecraft with sunlight, however, ISRU has never been achieved in practice. If the Triton Hopper were to pioneer this kind of technology in the Neptunian system, it would open up a wide range of new options for exploring similar bodies.
"This hopper doesn't stop with Triton," said Landis. "There's a bunch of really exciting places we can go beyond that, among them Pluto. We could really explore some of the features we saw on New Horizons. Triton is just the start."
That said, Triton happens to be a fantastic place to start, and not just because it has heaps of raw rocket fuel lying around. The moon's history as a KBO that fell into Neptune's gravitational embrace also makes it something of a twofer, scientifically speaking.
"This is a KBO, but it is different from Pluto in some ways because it's been a moon of Neptune for so long," Oleson said. "The neat thing is that you have a KBO, but you also get to see how Neptune has affected it over time."
Moreover, the 1989 Voyager 2 flyby revealed that Triton has extraordinary properties to match its changeling past. Landis told me he was particularly fascinated by Triton's reddish surface deposits of organic compounds, which are called tholins.
"We're talking about large quantities of the building blocks of life, in the outer solar system where you never expect them to be," he said.
I asked the team if there was an outside chance that life could exist on this far-flung moon, and Landis replied, "that's always the big question."
"Right now, we don't know anything like enough information to answer that," he told me. "The surface of Triton is phenomenally cold. In fact, when the Voyager probe flew past it, it was the coldest surface ever observed."
"There's probably no chance for life at the surface, just because we don't even have theories for how life could exist at very cold temperatures," he continued. "But once you start getting below the surface, there's been a lot of proposals that these icy moons may well have [subsurface] liquid water."
Fortunately, a hopper vehicle would have a way to sample this subsurface cocktail. "Another exciting thing about Triton is that it has a thin atmosphere that seems to be fed by geysers," explained Landis. "That would be just an amazing thing, if we could possibly fly through the plumes and see what's coming out, because that's giving us access to the inside of the moon."
Perhaps not only the moon, either, but access to the Neptunian world generally. Neptune is a fantastically weird and beautiful planet. Given how much we have learned about Jupiter with the Galileo orbiter, or Saturn with the Cassini orbiter, it's high time that it received its own orbital companion. (I'm calling it now—Neptune's mission will be called the Le Verrier orbiter, after the 19th century mathematician who predicted its location.)
Having the hopper work in concert with an orbiter is key, according to Oleson. "Our baseline concept is to have an elliptical orbiter that closely approaches Neptune in the low part of the orbit," he said, "and in the high part, flies by Triton where we'll drop off the hopper."
"The orbiter will provide us with a communication system in orbit, and also an imagery system," Oleson continued. "So if you are hopping around Triton, you can take your 'Google Van' images of what's surrounding you in that one area. But when considering where to go next, it would behoove you to have an orbiter to map Triton."
In other words, the Triton Hopper would be an efficient one-two punch of scientific research, allowing for ongoing analysis of an entrancing planetary system.
It will still likely be decades before this plucky hopper and its orbital partner wash up on Neptune's cosmic shores.
To that point, Oleson and Landis will be punching up the Triton Hopper concept with a group of specialists this fall, and then submitting it for "phase II" of NASA's NIAC program. If it is selected in spring 2016, the team will receive $500,000 to help conduct initial lab work and modeling for the concept. Oleson and Landis are already very familiar with this process, and are, in fact, currently working on a phase II concept submarine designed for the seas of Saturn's moon Titan.
"Titan and Triton, those are our two favorite moons," Landis said.
"Next will be Titania," Oleson joked, referring to a moon of Uranus.
Animation of Voyager flying by Triton. Video: NASA/JPL/YouTube
If the Triton Hopper follows the Titan submarine into phase II development, hopefully it will continue to gain traction from there. Along those lines, it's worth noting that NIAC isn't the only NASA subdivision that has an itch to develop missions to Neptune.
For instance, NASA's director of planetary science recently announced that the Jet Propulsion Laboratory is ramping up investment in missions to both Neptune and Uranus, and will seriously consider funding missions within a cost cap of $2 billion.
"The announcement [...] means that the next decadal will be better positioned to identify a realistic and capable mission, and its cost," planetary scientist Mark Hofstadter, who specializes in ice giants like Uranus and Neptune, told Spaceflight Now.
It will still likely be decades before this plucky hopper and its orbital partner wash up on Neptune's cosmic shores. But there's little doubt if and when a Neptunian mission finally comes to fruition, it would be well worth the wait.
"What we've discovered is that every place that we've examined in detail is far weirder than we ever thought," Landis told me. "Here's something that we flew by once. We were in the Neptune system for less than a day."
"There's a lot there that we don't know," he said.
It's going to take a lot of time, effort, and money. But Neptune, darling—we're coming for you.
Correction: An earlier version of this piece said that Neptune is 40 times farther from the Sun than the Earth is, but it's really 30. The piece has been updated.