This artist’s rendering shows the Tianwen-2 spacecraft approaching the near-Earth asteroid 469219 Kamo‘oalewa.
Credit: CCTV
Six centuries ago, Admiral Zheng He commanded seven “treasure voyages” around Asia in the halcyon days of China’s Ming Dynasty. His fleet of colossal four-masted wooden junks — the largest ships of their day, each crewed by hundreds of mariners — plied the South China Sea and Indian Ocean, traveling as far as Arabia, the Persian Gulf, and East Africa on missions of diplomacy, commerce, and power projection across the known world.
China takes space seriously
In a similar vein, modern China’s projection of its space power has moved swiftly since the dawn of the millennium. China became the third nation after Russia and the U.S. to independently launch a man into space in 2003, before vigorously pursuing a continuous off-planet human presence since 2022. The country also has made little secret of its grandiose ambitions for a human habitat on the Moon in the next decade.
China also landed robotic rovers on the lunar surface in 2013 and on Mars in 2021. And it scored a pair of empirical firsts: the first landing of a spacecraft on the Moon’s farside in 2019, and the first return of farside soil samples to Earth last year.
This summer, a mission originally named in Zheng He’s honor but now known as Tianwen-2 will propel China deeper into space than ever before — its first foray into the asteroid belt. It follows on the heels of Tianwen-1, China’s first Mars mission, launched in 2020.
The mission’s name comes from an ancient Chinese poem of questions addressed to Tian, the mythical personification of the heavens. Tianwen-2 will address several questions about the birth of the solar system, the origins of water, and the emergence of life.
Over its decade-long voyage in deep space, Tianwen-2 will gather surface specimens from 469219 Kamo‘oalewa, a near-Earth object in a similar orbit to ours, then return them in a sample capsule for a parachute-assisted landing in China’s Gobi Desert in 2027. It will then embark on a seven-year-plus outbound trek to study 311P/PanSTARRS, a curious primordial body that bears the hallmarks both of an asteroid and a comet.
The spacecraft
Conceived in 2019, Tianwen-2 was originally slated to launch in 2022 but its schedule slid. After a design study by the Chinese Academy of Space Technology, eight science instruments were selected in October 2019. They include visible and infrared spectrometers, a thermal radiation spectrometer, cameras, a 4 ft (1.2 m) imaging radar, charged and neutral particle sensors, and a magnetometer.
The Russian Academy of Sciences is also contributing an instrument to Tianwen-2 and in 2020, Chinese middle school and elementary students were invited to propose popular science experiments for the mission.
Tianwen-2 will launch atop a Long March 3B/E rocket from the Xichang Satellite Launch Center in south-central China. First used in 2007, this rocket’s impressive pedigree includes the launches of China’s first Moon lander and rover (Chang’e-3 in 2013) and the first spacecraft to land on the lunar far side (Chang’e-4 in 2018).
After leaving Earth, Tianwen-2 will reach Kamo‘oalewa by July 2026. This tiny, elongated asteroid likely spans no more than 130 to 330 ft (40 to 100 meters). It circles the Sun at a mean distance of 0.9 to 1.1 astronomical units (AU: the average Earth-Sun distance) with a sidereal period of 365.7 days, similar orbital parameters to Earth itself.
About the asteroid
Discovered in April 2016 by the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) at Haleakala Observatory in Hawaii, it was initially called 2016 HO3, then renamed Kamo‘oalewa in 2019. The name is a native Hawaiian chant for an oscillating fragment — a fitting moniker given the asteroid’s oscillatory jostling motion as it orbits Earth from 38 lunar distances at its closest point to 100 lunar distances at its farthest.
Kamo‘oalewa’s physical closeness has prompted astronomers to label it a quasi-satellite of Earth. It is not gravitationally bound to us like the Moon, and is too distant to constitute a true natural satellite, yet this tiny rock has been our planet’s constant companion for more than a century and will probably remain so for at least another 300 years.
Despite its proximity, Kamo‘oalewa’s minuscule size renders our knowledge of it a virtual blank. It is too small for radar ranging from Earth to establish reliable echoes to estimate its size or shape. We do know that it rotates rapidly — every 28 minutes — and it might well be a liberated fragment from the Moon.
In April 2024, reconstructions of the impact dynamics that formed the Moon’s 13.7-mile (22 km) Giordano Bruno crater several million years ago might have ejected debris into space that eventually achieved a 1:1 orbital resonance with Earth — matching Kamo‘oalewa’s present location and orbital parameters. Indeed, ground-based spectroscopy hints at a lunar (rather than asteroidal) origin of its surface materials.
Kamo‘oalewa’s closeness has also driven an increased appetite for exploration, spawning several mission proposals in recent years using solar electric propulsion, or solar sails. In 2022, it was postulated that the asteroid’s subsurface could afford a radiation-safe habitat for future Mars astronauts, its rapid rotation imparting centrifugal forces to make the voyage more tolerable on the human body.
At the asteroid
When Tianwen-2 arrives at Kamo‘oalewa, it will employ dual methods to capture surface samples. The first, called “touch-and-go,” was used by Japan’s Hayabusa-2 spacecraft at asteroid Ryugu in 2018 and by NASA’s OSIRIS-REx at asteroid Bennu in 2020. But Tianwen-2 will also test a method called “anchor-and-attach,” using four robotic arms with drills at their tips to affix itself to the asteroid’s surface. This tricky manouvre demands exquisite spacecraft guidance, navigation, and control to get close enough for precision sampling. Further complicating matters are Kamo‘oalewa’s rapid rotation and low gravity, requiring significant automation aboard Tianwen-2.
The spacecraft aims to gather between 0.7 and 3.5 ounces (20 to 100 grams) of regolith from Kamo‘oalewa. It is thought the asteroid’s surface is a rubble-pile aggregate with boulders ranging in size up to a few tens of centimeters and finer particles up to 0.04 inch (1 mm). Tianwen-2 will also measure Kamo‘oalewa’s bulk parameters — orbital elements, rotation rate, mass, shape, gravitational field, and thermal properties.
If successful, it will mark the first use of the anchor-and-attach sampling architecture in deep space. Tianwen-2 will spend up to a year in Kamo‘oalewa’s vicinity, departing in April 2027 for return to Earth. It will release the sample capsule in November 2027 to re-enter Earth’s atmosphere at a blistering Mach 35 — 27,000 mph (43,450 km/h), 10 percent faster than Apollo astronauts during their fiery return from the Moon.
Onward to a comet
Meanwhile, Tianwen-2 will continue its trek to 311P/PanSTARRS, a main-belt comet also called an active asteroid: an object that inhabits an asteroid-like orbit but whose visual features closely mirror a comet. Discovered by PanSTARRS in August 2013, it measures 1,570 ft (480 m) in diameter — larger than a city block — and circles the Sun every 3.24 years at a distance ranging from 1.94 to 2.44 AU.
A month after its discovery, in September 2013, 311P/PanSTARRS was observed by the Hubble Space Telescope and unveiled its oddness — exhibiting substantial structural changes and six comet-like tails. These may represent ejected material from episodic mass loss induced by its rapid rotation, as dust avalanches along its equator then shear away and drift into space in icy tails.
Current plans envisage Tianwen-2 reaching 311P/PanSTARRS in early 2035 for a year of observations. It is hoped that studies of both Kamo‘oalewa and 311P/PanSTARRS will furnish new insights into the formation and evolution of primordial solar system bodies and provide important insights into the origins of water and organic life.
The future of the Tianwen program carries great promise. Tianwen-3, targeting a 2028 launch, will return the first Mars soil samples to Earth, while 2029’s Tianwen-4 will send an orbiter to Jupiter and its rocky moon, Callisto. And Zheng He, the 15th-century admiral who for a time lent his name to Tianwen-2, would surely approve of the riches that modern China’s spacefaring treasure ships will bring back home.
