China is Considering a Nuclear-Powered Mission to Neptune
Some bold and cutting-edge space missions are proposed for the coming decade, as made clear by the Planetary Decadal Survey for 2023 – 2032. Examples include a
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 ongoing mysteries of Neptune’s largest moon Triton, which astronomers suspect is a planetoid flung from the outer Solar System and captured by Neptune’s gravity. The arrival of this planetoid is also thought to have caused a shakeup with Neptune’s natural satellites, causing them to break up and coalesce to form new moons. It’s also theorized that Triton will eventually break up and form a halo around Neptune or collide with it. Basically, the study of Neptune, its satellites, and its orbital dynamics could provide answers to how the Solar System formed, evolved, and how life began.
Unfortunately, due to the difficulties of sending missions to deep space (which includes launch windows, power supply, and communications), only one mission has visited Neptune. This was the Voyager 2 probe, which flew past the system in 1989 and obtained most of what we now know about this ice giant and its system. What’s more, the nature of Voyager 2‘s scientific instruments imposed certain limitations on the amount of data it could acquire. In recent years,
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.”
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.
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