The Triton Hopper: Exploration of Neptune’s Largest Moon

Another short post this week¹. Today, we will be exploring what has become a common theme on this blog, or rather a common destination: Neptune. Neptune is indisputably the Spooky Planet. If the Solar System had a haunted house, this would be it—a cold, dark, storm-wracked world made extra fascinating by its sheer remoteness. Only Voyager 2 ever visited, on a brief flyby in 1989; since then we’ve been limited to gazing from afar through telescopes. But as the 21st century dawns, and NASA toys with a new generation of advanced missions, researchers at Glenn Research Center in Ohio have proposed a bold new space probe that would return to the environs of Neptune and explore its poorly understood moon, Triton.

Their design is called the Triton Hopper, a 450-kilogram lander powered by a radioisotope generator (solar panels are useless all the way out there). It would launch from Earth atop a Delta IV or a Falcon Heavy. Given the immense distances and delta-v’s involved, the propulsion bus would rely on solar electric propulsion for the outbound flight, taking advantage of Earth and Jupiter gravity assists along the way, then brake in Neptune’s atmosphere to place it finally on a Triton intercept course. Note that the Hopper itself is not actually capable of descending to the surface on its own—it would bring a larger descent stage. The flight phase would last between launch in 2029 and Neptune arrival in 2041.

Now—why Triton? What’s so special about a ball of ice? Well, as far as balls of ice go, Triton is a fascinating place. It is by far the largest of Neptune’s moons—hogging a whopping 99.5% of that system’s satellite mass—and is in fact more massive than Pluto, though not by much. It’s also noteworthy for its highly inclined and retrograde orbit, a dead giveaway that it did not form with Neptune but was captured from the Kuiper Belt in the distant past. Its surface is flat and extremely young, dating back as recently as six million years, with very few craters. This indicates geologic activity on the level of the other superstar moons, namely Io, Europa, Enceladus, and Titan. Mysterious fissures in the western hemisphere form so-called “cantaloupe terrain” which scientists still haven’t figured out. Furthermore, Voyager 2 observed a tenuous nitrogen atmosphere, along with cryovolcanic geysers spewing a combination of gas and dust—these are believed to be caused by a sort of greenhouse effect, with sunlight passing through a transparent layer of ice and heating the darker material below. This variety of phenomena is incredible for a world 0.36 percent Earth’s size; it’s practically begging for a space probe to unravel its secrets.

The Triton Hopper takes a bold approach to solving Triton’s mysteries, to say the least. It will make five-kilometer hops across the surface, using nitrogen propellant gathered in situ (either mined from the ground with a robotic arm, or extracted from the thin atmosphere with a cryopump; the arm would be much faster). Cameras and other instruments would make close-up observations during both ballistic and landed phases. For each hop, the RTG would heat the spherical nitrogen tank, forcing gas out of a ring of small nozzles and thus generating thrust. Additional, smaller thrusters are to provide attitude control. Using this system, the Triton Hopper will be able to leap across the moon’s surface, exploring different regions and possibly getting up close and personal with the cryovolcanic geysers.

The probe is presently a Phase II design study, with no more funding than is required to keep refining the idea. It is unlikely that we’ll see it fly as early as 2029; NASA missions have long life cycles, and there’s some stiff competition, such as a proposed Europa lander. The solar-electric transfer bus and Neptune aerocapture shell will require considerable development beyond current technology. Nevertheless, the Triton Hopper is an ingenious idea for how we might explore one of the Solar System’s most remote and fascinating objects, and as NASA grows steadily more ambitious, its day may yet come.

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1. Though my ability to bloviate my way far beyond word count goals is unparalleled, so I make no promises.

Further Reading:

• https://ntrs.nasa.gov/citations/20180001294
• https://www.hou.usra.edu/meetings/V2050/pdf/8145.pdf
• https://en.wikipedia.org/wiki/Triton_Hopper (Yes, it’s Wikipedia, but this isn’t high school history—I can use whatever sources I damn well please).