China’s nuclear-powered mission to Neptune

Triton (Artist's Rendering)

Artist’s impression of what Triton, Neptune’s largest moon, might look like from high above its surface. The distant Sun appears at upper left and Neptune’s blue crescent moon is right of center. Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers was able to see that summer is in full swing in Triton’s southern hemisphere. Photo credit: ESO/L. Calçada

China is considering a nuclear-powered mission to Neptune

Some bold and cutting-edge space missions are proposed for the coming decade, as indicated by the Planetary Decadal Survey for 2023-2032. Examples of this are a[{” attribute=””>Uranus Orbiter and Probe (UOP) that would study Uranus’ interior, atmosphere, magnetosphere, rings, and satellites; and an Enceladus orbiter and surface lander to study the active plumes emanating fromthe southern polar region on Enceladus. Not to be outdone, China is also contemplating a nuclear-powered

Ice giants like Neptune are a potential treasure trove of scientific discoveries, as the authors describe in their paper. In addition to its intriguing interior structure (which includes diamond rain!), Neptune is believed to have played an significant role in the formation of the Solar System. In short, its composition includes large amounts of gas that were part of the protostellar nebula from which our system formed. At the same time, its position indicates where the planets formed (and since migrated to their current orbits).

There are also the enduring mysteries of Neptune’s largest moon, Triton, which astronomers suspect was a planetoid ejected from the outer solar system and captured by Neptune’s gravity. It is also believed that the arrival of this planetoid caused a tremor in Neptune’s natural satellites, causing them to disintegrate and merge into new moons. It is also believed that Triton will eventually break apart and form a halo around Neptune or collide with it. Basically, studying Neptune, its satellites, and its orbital dynamics could provide answers about how the solar system formed, evolved, and how life began.

Unfortunately, due to difficulties in sending missions into space (including launch windows, power supplies, and communications), only one mission has visited Neptune. This was the Voyager 2 probe that flew by the system in 1989 and acquired most of what we know today about this ice giant and its system. In addition, the nature of Voyager 2’s scientific instruments imposed certain limitations on the amount of data it could collect. in the past few years,[{” attribute=””>NASA has proposed sending a mission to explore Neptune and Triton (the Trident spacecraft).

However, this mission was not assigned priority by the Planetary Science and Astrobiology Decadal Survey 2023-2032 and was passed over for a Uranus Orbiter and Probe (UOP). But given the potential and immense improvements that have been made in spacecraft instruments since Neptune was visited last, Yu and his colleagues recommend it’s time for another mission to Neptune. (Note: all information and quotations translated from the original paper, written in Mandarin).

Hubble Neptune 2021

The Hubble Space Telescope’s 2021 look at Neptune, found that a new, “dark spot,” storm discovered in 2018 has reversed direction and is moving north. Credit: NASA, ESA, A. Simon (NASA-GSFC), and M. H. Wong (UC Berkeley); Image Processing: A. Pagan (STScI)

Design Considerations

Of course, the challenges mentioned above remain, which were used to inform the design of the spacecraft and its mission architecture. Looking at the power supply issue, Yu and his colleagues needed a source that could safely and reliably provide electricity for no less than fifteen years. They determined that a Radioisotope Thermoelectric Generator (RTG) with a 10-kilowatt energy (kWe) capacity would suffice. This nuclear battery, similar to what the Curiosity and Perseverance rovers use, converts heat energy from the decay of radioactive material into electricity. As they state in their paper:

“Considering the technical maturity of the space reactor power supply of different power levels, the power requirements of detectors and electric propulsion, the launch capability of the launch vehicle, and the funding, the output power of the space reactor power supply for the Neptune exploration mission is determined to be 10 kWe.”

They further recommend that the power supply system be based on a scheme of using one heat pipe, one set of thermoelectric conversion units, and one set of heat sinks as a single power generation unit. Multiple power generation units, where the heat energy is converted into electrical energy, can then be connected in parallel to supply power to the spacecraft. This system, they write, will be able to supply the mission with “8 years of 10 kWe full power operation and 7 years of 2 kWe low power operation, which can effectively ensure the reliability and safety of the system during the entire mission.”

10 kW Heat Pipe Fast Reactor Schematic

Schematic diagram of 10 kW heat pipe fast reactor and power supply of thermoelectric generation space reactor. Credit: SciEngine/Yu, Goubin et al. (2022)

The team also identified several key processes essential for this system’s safe and reliable operation. Among them, the generator must ensure continuous and controllable heat generation from nuclear fission, reliable heat transfer in the reactor, efficient thermoelectric conversion, and waste heat removal. To achieve this, the design for their reactor calls for Uranium-235 rods, monolithic uranium-molybdenum alloys, and rod-shaped ceramic elements that allow for efficient high transfer with a lightweight, compact core.

The spacecraft would also carry several instruments to study the planet, its system, and objects along the way. This includes a Neptune Atmospheric Probe (NAP) for studying the planet’s interior and a Triton Penetration Probe (TPP) that would examine the moon’s crust. A complement of smaller satellites (CubeSats or nanosatellites) would also be deployed along the way to explore a Main Belt asteroid and a Centaur asteroid.

Mission Profile

To start, the team explored several possible methods for exploring Neptune (remote sensing, flybys, orbital observation, soft landing, etc.). Remote sensing and flybys were ruled out immediately because these would not allow the mission to effectively measure Neptune’s deep composition and internal structure. “The requirements are high, and the task scale, technical difficulty, and funding requirements are extremely large,” they state. “Based on the scientific objectives, technical level, and funding scale, the detection method is determined to be polar orbiting detection.”

Another consideration was that given the distances involved (an average of 30 AUs from the Sun) and the carrying capacity of a mission to deep space, the probe’s flight speed should be increased as much as possible during the early stage. They further concluded that the best way to do this (and decelerate to achieve an orbit around Neptune) was to conduct a launch around 2030, which would allow for a gravity assist with Neptune Explorer Possible Flight Path

The flight path for a possible Neptune Explorer, based on the locations of the planets before 2040. Credit: SciEngine/Yu, Guobin et al. (2022)

Scientific Objectives

According to Yu and his colleagues, there are four major scientific objectives that a Neptune Explorer should investigate. These include Neptune’s internal structure and composition, its magnetosphere and ionosphere, its moons and rings, and its populations of Trojans and Centaurs (small asteroid families that share its orbit). In terms of its structure/composition, astronomers hope to shed light on Neptune’s strange thermal properties, which are believed to be the result of its “weather patterns.” As they write:

“The internal heat sources of Neptune (gravity collapse, tidal force, isotope decay heat, etc.) are considered to be one of the important sources to maintain the surface temperature of Neptune. There is a deviation between the calculated infrared detection result 57?K and the actual result 47?K, so the infrared radiation measurement in a wider frequency band is helpful to understand the operation mechanism of the heat release rate inside Neptune.”

Examining Neptune’s interior would also explain why the planet is much smaller than

As for Neptune’s moons and rings, the potential for scientific discovery includes the retrograde orbit, revolution, and dynamic migration of Triton (Neptune’s largest moon). The fact that Triton orbits counter to Neptune’s rotation is one of the main arguments that Triton could be a dwarf planet formed in the Kuiper Belt – the other is its composition similar to that of[{” attribute=””>Pluto. Per this theory, Triton was ejected from the Kuiper Belt and captured by Neptune’s gravity, which caused the breakup of Neptune’s existing satellites and the formation of new, smaller ones.

In essence, studying Triton’s orbital dynamics could shed light on the early solar system’s history, where ejected objects and planetoids were still settling into their current orbits. This could be supplemented by a comparative analysis of 2014 MU69 (also known as Arrokoth), the KBO that the New Horizons probe studied during its close flyby in July 2015, and other KBOs to learn more about the origin of Triton.

There’s also Triton’s cryovolcanic activity, resulting from tidal flexing in its interior caused by Neptune’s gravitational pull. However, this activity increases when Triton is closest to the Sun (perihelion), resulting in greater eruptions from the interior. This will leave higher concentrations of nitrogen and other gases in the moon’s tenuous atmosphere, which could be studied to learn more about its interior composition and structure. As for the rings, the team noted several objectives there:

“Establish a complete list of planetary rings and their inner Shepherd satellites, study the characteristics, formation mechanism, material exchange, and gas transport of planetary rings of different orbital types, analyze the origin of different celestial bodies, and detect possible organic matter… The multiple planetary rings of Neptune are not uniformly distributed in longitude. Instead, it presents an arc-block-like discrete structure. Why these arc-block structures can exist, and whether they exist stably without spreading out, are all interesting dynamical problems.”

Arrokoth

This composite image of the KBO 2014 MU69 (aka. Arrokoth) was compiled from data obtained by NASA’s New Horizons spacecraft during its flyby. Credit: NASA/JHUAPL/SwRI/Roman Tkachenko

China’s space agency has made some rather impressive moves in recent years that illustrate how the nation has become a major power in space. These include the development of heavy launch rockets like the Long March 9, the deployment of space stations (the Tiangong program), and their success with the Chang’e and Tianwen programs that have sent robotic explorers to the Moon and

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