China's Tianwen-2 space probe has captured the first image of rare asteroid 2016 HO3, a near-Earth companion that loops around the planet like a dance partner.
The photograph, released by the China National Space Administration (CNSA), depicts a grey space rock with jagged edges set against the backdrop of deep space following a 400-day journey covering 621 million miles (one billion kilometres).
Tianwen-2 launched on 29 May 2025 from the Xichang Satellite Launch Center in Sichuan province, China, as part of CNSA's first asteroid sample-return mission. The spacecraft made its first optical detection of the object on June 6 before closing in to within 1,242 miles (2,000 km) on June 19.
By July 2, the probe reached a distance of just 12.4 miles (20 km) from the asteroid. That close approach enabled onboard instruments to photograph the space rock, which scientists estimate measures between 130 and 328 feet (40 to 100 metres) in diameter.
"After a 400-day, one billion-kilometre (621 million-mile) journey, the Tianwen-2 probe recently made a successful encounter with asteroid 2016 HO3, reaching a distance of 20 kilometres (12.4 miles) from the asteroid to begin scientific exploration. During its approach to the asteroid, the probe acquired image data," CNSA said in a statement.
Close encounter with Earth companion
First discovered around a decade ago in 2016 by the Pan-STARRS 1 asteroid survey telescope on Haleakala, Hawaii, the asteroid is also known by its Hawaiian name Kamoʻoalewa, which alludes to an offspring that travels on its own.
Although it is too distant to be considered a true satellite of Earth like the moon, Kamoʻoalewa is the best and most stable example to date of a near-Earth companion, or quasi-satellite. It is one of only seven Earth quasi-satellites identified to date and is the closest to our planet, situated around 25.8 million miles (41.6 million kilometres) away.
The probe's mission goals were detailed in a scientific paper published in the journal Space Science Reviews earlier this year by physicist Rongqiao Zhang of the Lunar Exploration and Space Engineering Centre in Beijing and his colleagues.
"Among the known near-Earth asteroids, 2016 HO3 is an exceptionally rare Earth co-orbital object," Zhang and his team wrote. "As an Earth quasi-satellite, its orbital period is close to that of Earth, enabling low energy transfer...and providing favourable conditions for tracking, control, and communication."
"Its unusual orbit, enigmatic origin and largely uncharacterized physical properties make it a compelling candidate for addressing fundamental questions about the origins of Earth's quasi-satellites and the dynamical evolution of their orbits," they added.
Lunar origins and complex orbital path
Scientists suspect that 2016 HO3 may be a fragment of the moon knocked into solar orbit by an ancient collision. Renu Malhotra, a space expert at the University of Arizona, noted that analysis of light reflected from the rock indicates it is made from the same mineral material found in lunar rocks returned by NASA's Apollo missions.
Samples returned by Tianwen-2 will help researchers confirm whether the space rock originated as a piece of the moon.
When the asteroid was first discovered, Paul Chodas, manager of NASA's Centre for Near-Earth Object (NEO) Studies, explained how the body maintains its orbit alongside Earth.
"Since 2016 HO3 loops around our planet, but never ventures very far away as we both go around the sun, we refer to it as a quasi-satellite of Earth," Chodas said. "Our calculations indicate 2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come."
During its yearly trek around the sun, asteroid 2016 HO3 spends about half its time closer to the sun than Earth, passing ahead of our planet, and half its time farther away, falling behind. Its orbit is also slightly tilted, causing it to bob up and down once each year through Earth's orbital plane.
This configuration creates a multi-decade leapfrog effect and a slow back-and-forth twist. Earth's gravitational pull prevents the asteroid from wandering farther away than roughly 100 times the distance of the moon, while also keeping it from approaching closer than about 38 times the distance of the moon.
"The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon," Chodas said. "The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."
Sample return and extended comet mission
Tianwen-2 will spend nine months at asteroid 2016 HO3 conducting in-orbit observations and collecting physical samples from its surface. After completing its work at the quasi-moon, the spacecraft will travel back toward Earth to drop off a sample return capsule during a flyby.
Once the capsule is released to Earth, Tianwen-2 will continue its space voyage toward main-belt comet 311P, located beyond Mars.
Understanding quasi-moons and space objects
Quasi-moons such as 2016 HO3 and 2023 FW13 are a subcategory of near-Earth asteroids that orbit the sun while remaining close to Earth. They follow elliptical, non-circular orbits around the sun that mirror Earth's path, appearing to orbit Earth while remaining gravitationally bound to the sun.
Astronomers categorize solar system rocks into distinct groups based on their composition and behaviour:
Asteroid: A large chunk of rock left over from planetary collisions or the early solar system, with most located in the Main Belt between Mars and Jupiter.
Comet: A rock covered in ice, methane and other compounds whose elliptical orbit carries it far into the outer solar system.
Meteoroid: Debris originating from asteroids or comets, most of which are small enough to vaporise upon entering the atmosphere.
Meteor: The flash of light produced when a meteoroid debris fragment burns up in Earth's atmosphere, such as during a meteor shower when Earth passes through a comet's tail.
Meteorite: Any portion of a meteoroid that survives atmospheric entry and lands on the surface of Earth.
