By Yu Sinan, Li Yi, Zhang Wei
In the early hours of May 29, China successfully launched its first asteroid sample-return mission, Tianwen-2, from the Xichang Satellite Launch Center in southwest China’s Sichuan province, an endeavor designed to shed light on the formation and evolution of asteroids and the early solar system.
The Tianwen-2 mission aims to achieve multiple goals over a decade-long expedition: collecting samples from the near-Earth asteroid 2016HO3 and exploring the main-belt comet 311P, which is more distant than Mars.
The mission includes 13 flight stages and will reach a distance of approximately 150 million to 500 million kilometers from Earth, marking a major step forward in China’s deep space exploration endeavors.
Known as a quasi-satellite of Earth, the asteroid 2016HO3 orbits the Sun and appears to circle around Earth as well, remaining a constant companion to our planet. Its relatively stable orbit requires lower energy requirements for spacecraft to reach it, while enabling a viable scenario for subsequent exploration of the main-belt comet 311P.
Scientific research suggests that the asteroid 2016HO3 may preserve primitive materials from the early solar system. Investigating this asteroid could yield valuable insights into the composition, formation, and evolutionary history of the solar system’s early materials.
The second target, 311P, a celestial anomaly discovered in the main asteroid belt between Mars and Jupiter, occasionally spews out materials and resembles a comet with tails. While it exhibits compositional traits with traditional comets, it maintains the orbital characteristics of an asteroid. Exploring this unique object will deepen human understanding of the composition, structure, and evolutionary mechanisms of small celestial bodies.
Tianwen-1 marked China’s first Mars exploration mission, successfully achieving orbiting, landing, and roving operations in a single mission. Tianwen-2, however, has a different set of objectives and faces new technical challenges.
First, the Tianwen-2 mission will conduct sampling under weak gravity conditions. Observational data suggest that the asteroid 2016HO3, with an average diameter of about 41 meters, exists in near-zero gravity and rotates rapidly. Given these complex conditions, it is extremely challenging for the probe to achieve stable attachment with the asteroid and sampling within a limited time.
Second, the mission involves vast distances. Asteroid 2016HO3 is located about 18 million to 46 million kilometers from Earth, while the main-belt comet 311P lies approximately 150 million to 500 million kilometers away. The immense distance from Earth imposes stringent requirements on the probe’s communication system, trajectory design, power management, and the capability to maintain highly reliable long-term operational performance.
Furthermore, there is uncertainty regarding the exploration target. Current observations of the asteroid 2016HO3 provide limited information on its rotational behavior and surface characteristics, which significantly complicate the design of both the spacecraft and its sampling methods.
Tianwen-2 is equipped with 11 scientific instruments, including cameras, a visible and infrared imaging spectrometer, a thermal emission spectrometer, and a detection radar, all designed to facilitate scientific data collection during the mission.
In previous missions sending payloads into the Earth’s orbit, a rocket is required to achieve a separation velocity equal to the first cosmic velocity, or approximately 7.9 kilometers per second.
However, this mission requires a separation speed exceeding 11.2 kilometers per second, known as the second cosmic velocity, to escape the Earth’s gravitational influence, necessitating high speed and energy.
After evaluating the rocket’s payload capacity, performance capability, and reliability, Long March-3B was chosen as the dedicated vehicle for the Tianwen-2 mission.
As the primary launch vehicle for China’s high-orbit launches, the Long March-3B has completed 108 missions, including key lunar missions such as Chang’e-3 and Chang’e-4.
Experts noted that asteroids are characterized by their small size, low mass, and weak gravity, making them challenging to capture and requiring exceptional precision in the rocket’s trajectory. During this launch, the rocket reached a speed of 11.2 kilometers per second, with a permitted speed deviation of no more than one meter per second, which could otherwise lead to a positional error of up to one million kilometers.
“Achieving such trajectory precision is akin to shooting a basketball from Shanghai to hit a hoop in Beijing, while ensuring the ball’s angle and speed upon entering the hoop are correct,” said Wei Yuanming, an expert with China Aerospace Science and Technology Corporation.
To tackle this challenge, the rocket design team spent two years completing multiple design iterations so as to achieve seamless “handoff” between the rocket and the probe, Wei added.
