After a 400-day journey of roughly one billion kilometres, China's Tianwen-2 spacecraft has reached one of Earth's most elusive travelling companions and sent back the first close-up picture of it.
The China National Space Administration (CNSA) announced on 6 July that the probe had settled into a station-keeping point about 20 kilometres from the near-Earth asteroid Kamo'oalewa, formally catalogued as 469219 (2016 HO3). The image was captured on 2 July, according to the Chinese news agency Xinhua, and shows a small, elongated and distinctly asymmetrical rock.
Kamo'oalewa is a quasi-satellite: it orbits the Sun rather than the Earth, but on a path that keeps it close to our planet, swinging between roughly 14 and 40 million kilometres away. Earth has at least seven such companions, and their orbits are far less stable than that of the true Moon. Discovered in 2016, Kamo'oalewa turns once on its axis every 28 minutes.
A picture that speaks to the asteroid's origin
The photograph is already doing scientific work. Ground-based measurements had put the object's diameter anywhere between 40 and 100 metres; the new image suggests a body of just over 20 metres, about the length of a bus. That is close to the roughly 18-metre estimate in a study by Sharkey and colleagues, made with the James Webb Space Telescope and posted on 1 July while still under review.
Where the rock came from has been debated for years. One hypothesis, set out in a 2024 paper in Nature Astronomy, held that Kamo'oalewa might be a fragment of the Moon, blasted into space by the impact that carved out the Giordano Bruno crater. But the Webb observations, and the high surface reflectivity that the new image appears to confirm, point the other way. Mikael Granvik, an astronomer at the University of Helsinki and Lulea University of Technology, told SpaceNews that the reflectivity is incompatible with the Moon's darker surface: "So it seems that Kamo'oalewa is of asteroidal origin."
A returned sample should help settle the question. Tianwen-2 carries 11 instruments, among them cameras, spectrometers, a laser range-finder, sounding radar, particle analysers and Italy's DIANA dust detector, and it can gather material in three ways: hovering, touch-and-go, or anchoring to the surface. The probe is expected to depart in April 2027 and drop its sample capsule into Earth's atmosphere in late November 2027, before travelling on to the comet 311P/PANSTARRS.