China's plans for outer Solar System exploration

China has been busy exploring the inner Solar System in recent years. Their first independent interplanetary mission, Tianwen-1, launched for Mars in 2020. Tianwen-2 is in development, and will launch in 2025 to target a near-Earth asteroid and a main belt comet. Tianwen-3, an ambitious Mars sample return project, is in the planning stages.

Now China’s plans for exploring the outer Solar System are taking shape, with Jupiter’s moon Callisto and one of the ice giants the main targets.

Tianwen-4 at Callisto

The next mission for China, which will be called Tianwen-4, will target Jupiter. As The Planetary Society previously reported, China was considering two main scenarios: the Jupiter Callisto Orbiter, which would focus on said Galilean moon and possibly include a lander, and the Jupiter System Observer, which would study the gas giant’s irregular satellites.

It appears that Callisto will be the prime focus. Zhu Xinbo, deputy chief designer of the Tianwen-1 Mars orbiter and a researcher of China Aerospace Science and Technology Corporation (CASC) — China’s state-owned main space contractor — revealed in a presentation at a joint United Nations/China space exploration workshop, that China’s Tianwen-4 mission to Jupiter looks to be focusing on Callisto. This is backed up by presentations from officials from the China National Space Administration (CNSA).

Zhu’s outline of the mission sees a Tianwen-4 launch in September 2029. The spacecraft will use a flyby of Venus in 2030 plus further gravity assists from Earth in 2031 and 2033, to arrive at Jupiter in December 2035.

At this point, the main spacecraft will enter orbit around the Solar System’s largest planet. But there will also be separation of another probe which will head for Uranus and make a flyby to study the ice giant. According to this launch profile, the spacecraft will zip by the ice giant in March 2045.

The plan to add a Uranus flyby to Tianwen-4 appeared at the International Astronautical Congress in Paris 2022. While brief, it will be an invaluable interaction and only the second-ever visit to Uranus, following Voyager 2’s encounter in 1986.

Back at Jupiter, the main spacecraft will enter orbit around Callisto — the outermost of the four Galilean moons. Before settling in around Callisto it could, though no longer optimized for that particular scenario, make a survey of the irregular satellites. These are thought to be planetesimals captured by Jupiter and are too small to be observed by telescopes. The scope and balance of science objectives is still being discussed by the Chinese planetary science community.

Though less glamorous than the other Galilean moons, Callisto is an intriguing target for many reasons. Its ancient surface preserves a history of the early Jupiter system and the wider Solar System, and it’s possible that a subsurface ocean lurks underneath. The moon also features the Valhalla impact basin, with a bright center surrounded by concentric fractures. And unlike Ganymede, Europa, and Io, Callisto sits outside Jupiter’s intense radiation field, making a longer term study of the moon easier.

Early plans for the Tianwen-4 Callisto scenario included a lander. Presentations by Chinese officials in recent years dropped any mention of this, but notably Zhu’s plan includes an impactor.

The targeting and deployment of an impactor and coordinating so that the main spacecraft can observe would be a challenging technical feat but also scientifically rewarding. Watching the impact and its aftermath would open a unique window for unprecedented insights into Callisto’s composition, structure, cratering mechanics, the presence of organics, and more from the material ejected by the collision. It could potentially even have an observable influence on Callisto’s tenuous atmosphere.

Zhu says work on the overall design and key technologies is underway, including generating enough power to operate a spacecraft nearly half a billion miles (800 million kilometers) away from the Sun, and the required protection to deal with the harsh radiation environment.

Michel Blanc, a planetary scientist at the Institute for Research in Astrophysics and Planetology in France and a co-investigator on NASA's Juno Jupiter mission, notes that studying Callisto is the best way to explore and test the different formation scenarios of Galilean moons.

With NASA’s Juno already in orbit, ESA’s Juice on the way, and Europa Clipper set to launch in October 2024, Jupiter is getting plenty of attention. But the gas giant still has big mysteries, says Blanc.

“If Tianwen-4 is tailored to answer our 'big question' of how the Jupiter system was formed, by targeting Callisto and more it will be a fully original mission that will bring a lot of new elements to our still very incomplete understanding of the Jupiter system, from formation to the emergence of habitable worlds among its icy moons,” says Blanc.

China is also committed to its challenging Mars sample return program, so readying both missions for the end of the decade — along with a separate but also intensive plan to put astronauts on the moon by 2030 and plan the International Lunar Research Station — will perhaps stretch resources.

An ice giant orbiter

Another interesting development in China’s outer Solar System exploration plans is that the notion of a mission — what’s more an orbiter — to an ice giant is becoming more likely.

The Planetary Society previously reported that Chinese scientists outlined a proposal for a Neptune orbiter mission. Zhu’s presentation, and another by a CNSA official this June to the Committee on the Peaceful Uses of Outer Space in Vienna, confirmed that the ice giants are both potential targets for a Tianwen-5 mission.

Zhu provides a potential launch date of 2035 for a Uranus mission, arriving in orbit in 2050. If a Neptune mission is chosen instead, it would launch in 2040 and arrive in 2058. Either would launch on a Long March 5 rocket.

Entering orbit would provide years of data — a huge improvement on the data we currently rely on from the Voyager flyby and studies by distant telescopes. Additionally, both mission profiles include aerostats which would be released into the planets’ atmospheres. These would return data on the structure, composition and dynamics and more concerning the ice giants’ atmospheres.

Additionally, the spacecraft could carry impactors which would be used to learn more about major moons such as Neptune’s Triton or Titania of Uranus.

Required breakthroughs include entering orbit around a planet while using electric propulsion, as well as developing a fission reactor for space, radioisotope thermoelectric generators, and a large aperture antenna to facilitate high-speed, deep space communications. China is already working on nuclear power for space and building infrastructure for deep space missions.

Zhu notes however that research is underway, and current and upcoming Tianwen missions lay a useful foundation and technological and engineering pathway to an ice giant orbiter mission. Such a mission would deliver major scientific discoveries related to the origin and evolution of the Solar System and the origin of life.

While this is not a declaration that a Chinese ice giant mission has the go-ahead — official approval sometimes only come once they fall within the scope of China’s five-year plans, and a number of advances are needed — it is notable that officials from both the policy side with CNSA and the engineering side with CASC are stating this as part of their plans. This may indicate China’s current thinking regarding its future exploration in the farther reaches of the Solar System.