A global color mosaic of Triton, taken by Voyager 2 during its flyby of the Neptune system—the most complete view we have to date. Credit: NASA/JPL/USGS
A very different kind of moon
Neptune’s moon Triton is a bit off. First of all, it’s going the wrong way. Every other large moon around Neptune—and indeed throughout the entire Solar System—orbits in the same direction that its planet rotates. Not Triton. Triton is also much bigger than the other moons around Neptune—actually even bigger than Pluto. Triton’s unique qualities don’t end there: It also has a pink color to it, suggesting its surface is composed of nitrogen ice, something not found on any other moon we know of.
The problem, almost certainly, is that Triton was once a free-floating object, unburdened by the gravitational chains of its parent Neptune. Before getting mixed up with Neptune’s gravity, it was probably what scientists refer to now as a Kuiper belt object—a group of icy objects past Neptune’s orbit. The Kuiper belt includes thousands of known objects including comets, chunks of icy gas in varying sizes, and even dwarf planets like Pluto.
The existence of this massive region of our solar system wasn’t proven until as late as 1992 when astronomers David Jewitt and Jane Luu found a tiny chunk of ice they named 1992QB1. Since that groundbreaking discovery, which provided a new method to search for these objects, we have discovered thousands of small and large objects, including three other dwarf planets in addition to Pluto.
But the Kuiper belt isn’t only a vast region chock full of cool new objects waiting to be discovered. It is a surviving remnant of the earliest planetary building blocks of our solar system’s formative years—a cosmic fossil bed, so to speak. “These are the building blocks that were put together to build the solar system,” said NASA scientist Geoff Landis, who is co-designing a concept mission to Triton.
A visualization of the known Kuiper belt objects, their orbits, and their relative sizes. The objects appear in the order of their discovery. Credit: Alex Parker
The bigger picture
When our solar system formed, it was a mass of cosmic dust that collapsed under its own gravity. Most of the mass collected in the center to form our Sun; the remaining material formed a rapidly rotating disk. From that disc, rocky planets formed near the sun (where it was too hot for frozen volatiles), and gas or ice planets formed further away. Some materials—mainly ices made of methane, ammonia, and water—got flung out so far that they became too spread out to coalesce into planets. This stuff became the Kuiper Belt.
By preserving chemicals from the earliest days of solar system formation without subjecting them to planetary formation, objects in the Kuiper belt can help us better understand the conditions that led to the creation of our neck of the universe, and even life itself, said Landis. Anything that has settled into an orbit closer to the sun than Saturn, including many of the hydrocarbons that may have led to life, has been physically or chemically altered, he explained.
These reasons were among the many that made sending the New Horizons mission to Pluto, also a Kuiper Belt object, such an appealing prospect. Before our striking views of Pluto’s varied surface up close, the closest we had gotten to a Kuiper belt object happened when Voyager 2 flew by Neptune in 1989, and back then we didn’t even know that the Kuiper belt existed.
Actually landing there
But New Horizons could never have landed on or even orbited Pluto. The dwarf planet is too small and the New Horizons probe was traveling too fast, said Steve Oleson, a NASA engineer who is working with Landis to develop a mission to Triton. To get something to land on even a large Kuiper belt object, you need to hurl it through space at mind-bendingly fast speeds, and that means you would need to carry a lot of fuel to slow it down so that it can be inserted into orbit. These requirements make it extremely difficult, if not impossible, to develop a viable mission to any freely orbiting Kuiper belt object.
That’s why Landis and Oleson, both part of a NASA team that designs advanced and innovative concept missions, have proposed the “Triton Hopper” as a way to actually touch down on a Kuiper belt object. This concept involves using Neptune’s gravity and atmosphere to slow a spacecraft to a point where it can drop a “hopper” on Triton’s surface. “We’re using Neptune as a tool for stopping, slowing down, and landing on Triton,” Landis said.
A concept drawing for the Titan Hopper. Credit: NASA/NIAC/Steve Oleson
The idea is that the Triton Hopper will actually be able to collect fuel either from Triton’s atmosphere or the ices on its surface. “The big risk is we are not sure what the surface looks like,” Oleson explained, so, to keep their options open, they will design the craft so that it has the ability to acquire fuel from the atmosphere as well as the surface, in case the ice is too difficult to penetrate.
Once the Hopper collects the fuel, it would heat it up into a gas to create pressure—like those Alka-Seltzer rockets you made in third grade—and launch the craft by allowing the pressure to escape. Due to the low gravity, such a system could propel the craft one kilometer high and five kilometers across, allowing it to travel, in multiple jumps, from Triton’s polar regions to its equator.
The 1989 flyby of Triton recreated in 2014 using Voyager 2 data. Credit: NASA/JPL-Caltech/Lunar & Planetary Institute/Paul Schenk.
While airborne, the craft could take photographs and videos during flight. While on the ground, it could photograph and analyze the chemistry and geology of the surface. It could even jump through geysers on Triton’s surface to analyze the stuff ejected from them. This could give scientists an idea of what is going on under the surface, they said.
All of this is very preliminary and for the most part theoretical. But if we ever want to land a mission on a Kuiper belt object, Triton is probably the best hope we have right now. If you thought the surface of Pluto was cool, imagine actually landing on it. Triton makes that possibility real, and it would be incredibly cool if we got to see it in our lifetime.