China’s Tianwen-2 is off to collect an asteroid sample

Asteroids are the ancient remnants of our solar system’s birth, rocky fragments that never formed into planets. Most of these celestial wanderers inhabit the asteroid belt between Mars and Jupiter, where Jupiter’s immense gravitational influence prevents them from assembling into a single world. Ranging from house-sized boulders to Ceres, a dwarf planet nearly 1,000 kilometers across, asteroids preserve pristine records of the early solar system’s composition and conditions.

While asteroids are the rocky debris of planetary formation, comets tell a different story. They are pristine ice balls from the cold outer reaches of our solar system, born in regions so chilly that water, carbon dioxide, and other volatile compounds froze solid around dust particles.

These “dirty snowballs,” as they’re often called, originate from two primary sources: the nearby Kuiper Belt beyond Neptune and the more distant Oort Cloud, a spherical shell of icy objects that extends nearly halfway to the nearest star.

When a comet’s high orbit brings it close to the sun, solar radiation begins to vaporize its frozen surface, creating the spectacular glowing coma and streaming tail that can stretch millions of kilometers across space.

Studying these distant objects presents significant challenges due largely to their relatively small scale and sometimes rapid movement. There have been a number of spacecraft that have visited our planetary cousins over the past few decades.

The European Space Agency’s Giotto spacecraft made history in 1986 by flying through the coma of Halley’s Comet, capturing the first close-up images of a comet’s nucleus and revealing its dark core spewing jets of gas and dust. NASA’s NEAR Shoemaker mission broke new ground by becoming the first spacecraft to orbit an asteroid—433 Eros—before making a controlled crash landing on its surface in 2001.

China’s ambitious Tianwen-2 mission represents a groundbreaking leap, becoming the first to visit both an asteroid and a comet in a single journey. Launched on 28 May, it will first rendezvous with the near-Earth asteroid Kamoʻoalewa (2016 HO3), a fascinating object that follows Earth in its orbit around the sun like a quasi-satellite.

After spending a year studying it and collecting samples, it will return them to Earth. Tianwen-2 will then continue its voyage out to 311P/Pan-STARRS, an object in the asteroid belt that displays comet-like activity when heated by the sun.

This dual target mission is particularly significant because it will mark the first detailed study of a main belt comet. By investigating both a near-Earth asteroid and this unusual comet-asteroid hybrid, Tianwen-2 will provide unprecedented insights into the diversity of small bodies in our solar system and help us understand the complex processes that have shaped it over billions of years.

The mission demonstrates China’s rapidly advancing space capability and contributes to the growing international effort to understand our solar system’s early history.