This past weekend, China’s second deep-space mission, Tianwen-2, executed a significant engine burn to set course for a small, mysterious object in a quasi-Earth orbit. Although the Chinese space administration has not yet confirmed this achievement, amateur radio enthusiasts in Germany and the Netherlands, using telescopes, tracked the maneuver. They observed Tianwen-2 nearing the near-Earth asteroid Kamoʻoalewa. Over the next month, the spacecraft will approach the fast-spinning asteroid to study and map its surface, preparing for potential sampling missions.
Kamoʻoalewa, a space rock measuring between 40 to 100 meters, completes a rotation every 28 minutes. It is one of seven known quasi-moons of Earth, which orbit the sun in synchronization with our planet, forming slow retrograde loops around it. However, until Tianwen-2 can closely inspect it, scientists have limited information about the object, which is smaller than a soccer field.
“Every new image of an asteroid has been a surprise,” notes Patrick Michel, director of research at the French National Center for Scientific Research and principal investigator for the European Space Agency’s Hera mission, who has extensively studied Kamoʻoalewa. “We have everything to learn.”
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The asteroid’s rapid rotation may offer insights into its composition. If it were a “gravel pile,” it would likely lose debris as it spins. Instead, according to planetary scientist Christine Hartzell of the University of Maryland, it might be “a chunk of rock or a couple of chunks of rock held together.” A mission paper from the Tianwen-2 team also suggests this, indicating that while Kamoʻoalewa’s surface likely consists of millimeter- to centimeter-sized grains, its core might be a massive boulder or a fused rubble pile.
Another mystery is how this asteroid ended up in its unusual orbit alongside Earth. One hypothesis, supported by Michel and others, is that Kamoʻoalewa originated from the lunar farside, blasted into orbit by the impact that formed the 22-kilometer-wide Giordano Bruno Crater approximately 10 million years ago.
In contrast, some researchers believe the object is more likely from the main asteroid belt between Mars and Jupiter, having migrated to its current orbit around the sun. “If you just take the size of this body, it is very weird that it would be so large and have been ejected from the moon so recently,” says Mikael Granvik, a professor at the University of Helsinki and Luleå University of Technology in Sweden. His statistical models suggest that an asteroid in Kamoʻoalewa’s orbit is 10 times more likely to have come from the inner main asteroid belt than from the moon.
If successful, Tianwen-2 will resolve these debates by collecting samples from Kamoʻoalewa and returning them to Earth for analysis. Should the material match the isotopic composition of lunar rocks, the “moon fragment” theory would be validated. Granvik suggests this outcome would offer significant new insights into the physics of impacts. “If you want to track the solar system backward to see how it has evolved over time, then we need to understand how the collisional evolution has actually worked, and so this could provide key data for those models,” he says.
Alternatively, if Kamoʻoalewa originates from the main asteroid belt, its reddish color resembling moon rocks might be due to extreme space weathering, affecting how other reddened asteroids are classified.
Tianwen-2’s mission extends beyond scientific exploration, serving as a critical test of China’s growing capability to perform autonomous, high-precision maneuvers in deep space, essential for future missions to the moon and beyond. With guidance from cameras, radar, and lidar, the spacecraft’s sampling attempt will be China’s most ambitious to date, laying the groundwork for more daring plans to retrieve samples from Mars.
Tianwen-2 has three sampling methods—touch and go, hover, and anchor and attach—designed to handle Kamoʻoalewa’s swift rotation and unknown surface.
The touch and go method, similar to those used by Japan’s Hayabusa2 and NASA’s OSIRIS-REx missions, involves a disk-shaped, gas-driven head briefly contacting the surface. Brushes and gas bursts inside the head collect samples before the spacecraft quickly retreats.
The hover mode uses a robotic arm to gather surface samples. Hartzell likens this to floating in a water tank and trying to collect material from the bottom without leverage. “Of those three options, [hover] would be the most risky, because you have to worry about the reaction forces that your spacecraft needs to generate in order to actually scoop right,” he says.
In the anchor-and-attach approach, a claw-and-spike mechanism on the robotic arm would secure the spacecraft to the asteroid, assuming Kamoʻoalewa’s surface is firm and relatively free of obstacles. “You don’t expect any boulders on the surface, because you need cohesion to be able to survive at this spin rate,” Michel says. “But who knows? Maybe there are boulders glued with small dust. We don’t know.”
Michel believes the three-mode strategy reflects the mission’s priorities. “If science were driving the mission, you would try to maximize success and use the approach that has already been tested,” he says. Selecting a larger, slower-spinning target would simplify sampling. But Michel notes this strategy shifts when planning to extract resources from asteroids. “If you want to use these as gas stations to go further [into the solar system], you find more smaller objects than bigger ones, and they are all rotating fast. So it’s not a bad choice” he says.
According to an unconfirmed timeline on Chinese social media that has matched previous mission milestones, Tianwen-2’s close encounter with Kamoʻoalewa is set to begin on July 4, when the spacecraft is projected to come within 20 kilometers of the asteroid. While the U.S. celebrates its independence, China will embark on the latest phase of its ambitious exploration agenda, seeking to join the U.S. and Japan in successfully returning asteroid samples. The asteroids Itokawa, Ryugu, and Bennu—sampled by Japan’s Hayabusa missions and NASA’s OSIRIS-REx—each offered surprises and valuable materials. Kamoʻoalewa is smaller, faster-spinning, and more enigmatic in origin than these. “Even if they’re unsuccessful in capturing samples, if they can go and take high-resolution images of a body this size, that would be super interesting,” Hartzell says, as no one has yet seen such a small asteroid up close.
Should the mission succeed, Tianwen-2 will complete its sampling and depart from Kamoʻoalewa next April, heading for a brief encounter with Earth in November 2027. During its visit, the spacecraft will send its samples to Earth for retrieval in Inner Mongolia. The capsule will endure a high-speed atmospheric reentry at 12 km per second, a more intense descent than China’s prior lunar sample-return missions. After releasing its samples, Tianwen-2 will utilize Earth’s gravity to set its course for Comet 311P, expected to arrive in 2035.
