HELSINKI — Scientists are proposing China’s first ice giant mission, aiming to launch a radioisotope-powered spacecraft to orbit Neptune and study its enigmatic moon Triton.
The mission, detailed in a newly published concept study, envisions a 15-year interplanetary cruise followed by orbital insertion, followed by a gravity assist from Triton to adjust the spacecraft’s orbit.
After arriving at Neptune, the spacecraft would deploy an atmospheric entry probe into the planet’s atmosphere and enter a long-term resonant orbit. It would then observe the planet’s rings, geological dynamics and magnetosphere, and assess the potential habitability of Triton’s subsurface ocean.
Designed to operate more than 4.5 billion kilometers from Earth, a distance at which required power generation from solar arrays is not viable, the mission would rely on dual 300-watt radioisotope thermoelectric generators (RTGs).
The mission, which appears to remain a proposed concept rather than an approved project, outlines an 11-instrument payload suite, including a vector magnetometer, radio wave analyzer, charged and neutral particle sensors and a dust analyzer. It would carry a visible-light camera and a combination of infrared, UV, and terahertz spectrometers, and a microwave radiometer. The spacecraft would also feature advanced autonomous navigation, such as X-ray pulsar-based navigation.
In assessing which icy giant to target, the authors stated that, compared with Uranus, exploration of Neptune holds “greater scientific significance due to its potential implications for understanding extraterrestrial life and the origins of life.”
Engineers highlight major technical challenges ahead, including deep space communications, space-based energy solutions and onboard autonomy. It would also require shielding for a Neptune atmospheric entry vehicle with dual requirements of effective thermal shielding and mass efficiency. However, with heritage from Chang’e lunar and Tianwen planetary missions, there appears a strong foundation for more distant endeavors.
“With the development of deep space exploration technology, China has now acquired the capability to conduct orbiting exploration of ice giants,” the paper asserts. It also notes that the technological advancements required would substantively support the sustainable progression of China’s deep space exploration program.
If approved and realized, the mission would likely be the first orbiter mission for Neptune or Uranus, referred to as icy giants. The Uranus Orbiter and Probe mission was highly ranked in the Visions and Voyages and Solar and Space Physics Decadal priorities, however NASA’s planetary science budget has faced sustained pressure, and a shortage of plutonium-238 has likely delayed any prospective Uranus mission from its originally anticipated 2031 launch window into the latter half of the 2030s.
NASA also developed the Neptune Odyssey mission concept, to which the Chinese proposal bears a number of similarities, but the Uranus Orbiter and Probe was selected ahead of Odyssey.
The paper was published July 1 in the Chinese Journal of Aeronautics. It was authored by officials from the Beijing Institute of Spacecraft System Engineering, and Yang Mengfei, a senior figure at the China Academy of Space Technology (CAST), a major state-owned spacecraft maker responsible for the Chang’e lunar missions, Tianwen planetary missions and space station modules.
The proposal follows a 2021 pre-research paper by another set of Chinese scientists and engineers, calling for a Neptune orbiter powered by a nuclear reactor power generator. Neptune and Triton have recently also appeared in Chinese exploration roadmap presentations alongside approved missions.
Such a mission would appear to mark a logical next step for China’s growing planetary exploration ambitions. It has already landed a rover on Mars with its Tianwen-1 mission, and launched the Tianwen-2 near Earth asteroid sample return and comet rendezvous mission in late May.
The country also targets the launch of the pioneering Tianwen-3 Mars sample return mission in late 2028, followed by its first outer planet mission, Tianwen-4, around 2029, targeting the Jupiter system. The latter mission may also include a radioisotope-powered spacecraft to flyby Uranus, providing a stepping-stone to a full Neptune orbiter mission.
Neptune was discovered in 1846 and is the outermost known planet in our solar system. The planet, which hosts extreme, long-lasting winds, harbors a number of mysteries and has so far only been visited by NASA’s Voyager 2 back in August 1989.
In terms of science, Ravit Helled, a planetary scientist at the University of Zürich, told SpaceNews it would be very useful to have accurate measurements of Neptune’s gravitational field so we can further constrain its internal structure and bulk composition.
“Measurements of the magnetic fields are also important as they provide additional constraints for structure models, as well as atmospheric composition. The rotation rate, or length of day, of Neptune is also unknown, a fact that introduces large uncertainty in determining its internal structure,” Helled says.
“Triton is clearly an interesting object which represents an entire population that is yet to be explored.”
Triton orbits Neptune in a retrograde direction with respect to Neptune’s rotation and thus thought likely to be a captured Kuiper Belt object. The Voyager 2 flyby captured geyser-like eruptions from its icy crust, which is thought to encapsulate a subsurface ocean of liquid water.
“Triton could still have a subsurface ocean and there are features on Triton’s surface that indicate activity, such as cryovolcanoes. Having an ocean is not the same as being “habitable,” at the same time we can’t exclude that there is microbial life in such an ocean,” Helled says.
“There is a clear need to have dedicated missions to both Uranus and Neptune that will include orbiters and atmospheric probes. This requires planning and a sufficient budget.
“We recently gained a lot of information about Jupiter and Saturn thanks to the Juno and Cassini missions. Now it’s time to go to the ice giants, which are the least explored planets in our planetary system,” Helled says, speaking in general regarding planetary science.
The Chinese proposal underscores the country’s growing planetary science ambitions and its drive to expand deep space engineering capabilities. If realized, the mission could help fill some of the big gaps in the understanding of the solar system.