Tianwen-4

• 1. Introduction

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  Tianwen-4 takes its original name from Gan De, a fourth-century BCE Chinese astronomer. Gan De reputedly observed Jupiter's moons with the unaided eye, centuries before any telescope existed. The mission bearing his name is China's planned expedition to the Jovian system, the family of worlds orbiting Jupiter. Two different mission designs are competing for selection, and the choice shapes everything about what the probe will study. One path leads to a moon that has barely changed since the early Solar System; the other targets volcanic Io. A secondary probe may accompany the main spacecraft and eventually separate, heading for a flyby of Uranus.

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• 2. Two Competing Profiles

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  The formal names for the two competing designs are the Jupiter Callisto Orbiter, known as JCO, and the Jupiter System Observer, or JSO. JCO would begin by flying past several of Jupiter's irregular satellites, smaller moons that orbit in unusual paths far from the planet. After those flybys, the spacecraft would enter a polar orbit around Callisto, the outermost of Jupiter's four largest moons. That profile may also include a lander or an impact probe delivered to Callisto's surface.

  JSO, by contrast, would skip a Callisto orbit entirely and focus instead on intensive study of Io. JSO's designers also proposed releasing one or several small satellites to study the Jovian magnetosphere from multiple points simultaneously. At the conclusion of its tour, JSO could travel to the Sun-Jupiter L1 point. That is a gravitational balance zone between the planet and the Sun where no spacecraft has yet operated.

  Reports from 2023 and 2024 suggest mission planners have leaned toward JCO combined with the Uranus flyby probe.

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• 3. The Quiet Moon

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  Jupiter's gravity tugs on nearby moons to generate the heat that powers Io's volcanic eruptions. Callisto sits far enough away that its interior has remained mostly quiet. Scientists believe the moon likely assembled from material left over after Jupiter formed. Only asteroid impacts have significantly modified its surface since then, preserving what the Jovian system looked like in its earliest days.

  That geological stillness is the core of Callisto's appeal to planetary scientists. A surface that has not been overwritten by volcanism or ocean activity is a surface that still carries ancient information. Callisto also has a thin atmosphere with small amounts of oxygen, adding another layer of interest.

  Callisto is less glamorous than the ocean moons Europa and Ganymede, or the volcanic surface of Io. But it offers a practical advantage those worlds cannot match. A spacecraft requires less fuel to reach Callisto than the other large Jovian moons. It also lies outside Jupiter's intense radiation belt, which simplifies spacecraft design. Getting to Callisto under one planned trajectory would span more than eight years from launch.

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• 4. Across the Solar System

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  One possible mission trajectory was presented at the 2020 General Assembly of the European Geosciences Union. Under that scenario, Tianwen-4 would lift off in September 2029 and perform a Venus flyby in April 2030. Venus's gravity would alter the spacecraft's path, sending it on toward Earth. The first Earth encounter would occur in February 2031, followed by a second in May 2033. Each close pass would add speed and redirect the spacecraft outward toward Jupiter.

  Before reaching Jupiter, the Uranus probe would separate from the main spacecraft and continue independently. It would not arrive at Uranus until March 2045. The main probe would enter Jupiter orbit in December 2035. After one or two braking passes, Tianwen-4 would settle into a large elliptical orbit with a period of 30 days. During this phase, the probe would conduct flybys of irregular Jovian moons. It would complete 10 orbits over approximately a year before two Callisto gravity assists lowered it into a tighter path.

  In February 2038, Tianwen-4 would achieve orbit around Callisto at an altitude of 300 km, with an orbital period of 17.7 hours. An impact probe would then be released to strike Callisto's surface. Two primary scientific questions anchor the mission: how the Jovian system formed, and how it functions today.

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• 5. The Science Payload

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  A thermal plasma spectrometer measuring particles from 100 electron volts to 100 kiloelectron volts is one of the instruments proposed for Tianwen-4. It belongs to the first of four payload packages, which covers plasma and dust analysis. That package also includes a high-energy charged particle detector, an ion and neutral mass spectrometer, and a magnetometer. A radio and plasma wave spectrometer and a cosmic dust detector round out the first package.

  The second package targets light across multiple wavelengths, with cameras and spectrometers for visible, near-infrared, far-infrared, and ultraviolet ranges. The third package focuses on geology, glaciology, and geochemistry. It includes a high-mass resolution spectrometer connected to a sampling system for ice surfaces and a pyrolyser for refractory material beneath.

  The fourth package covers radio science and navigation. Its centerpiece is a "PRIDE" astrometry experiment using very-long-baseline interferometry to track each flight element independently. That package also includes an altimeter with meter-level accuracy.

  A 2024 paper in China Space Science and Technology took the payload further. Titled "Research and Prospects of Jupiter System Scientific Exploration," it focused specifically on the Jupiter Callisto Orbiter configuration. That updated list includes a microwave radiometer, a polarimeter, a gamma/X-ray spectrometer, and a microwave radar. The flyby probe bound for Uranus carries its own scaled-down instrument set, including a camera, magnetometer, spectrometer, radio science package, and particle analyzer.

  The full payload will not be finalized until the choice between JCO and JSO is officially confirmed.

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• 6. From the Moon to Jupiter

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  China's National Space Administration, known as CNSA, built its interplanetary ambitions on a foundation of lunar missions. Chang'e 1 and Chang'e 2 were robotic spacecraft placed in orbit around the Moon. Chang'e 3 followed as a lander and rover mission, delivering hardware directly to the lunar surface. After those successes, CNSA set its sights on destinations far beyond the Moon.

  In 2018, Pei Zhaoyu, the deputy director of CNSA's Lunar Exploration and Space Program Center, made a sweeping announcement. He stated that China intended to complete four major interplanetary missions before the end of the 2020s. The first was Tianwen-1, a mission to Mars. The second, Tianwen-2, was designed to visit a main-belt comet and return a sample from an asteroid. The third, Tianwen-3, targeted a Mars sample return. Tianwen-4, the Jupiter system mission, was the fourth.

  The broader science agenda includes studying compositional variations in Jupiter's atmosphere. It also encompasses the space environment surrounding the Galilean moons, goals no Chinese spacecraft has yet been positioned to pursue.

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