Tianwen-2: China’s Near-Earth asteroid and comet double-header

China is gearing up for an ambitious combined mission to collect samples from a near-Earth asteroid, deliver the material to Earth, and then head off back into deep space to study a comet. The Tianwen-2 mission will build on some of the country’s biggest space achievements so far, but will also be very challenging for scientists and engineers.

Tianwen-1, the country’s first interplanetary mission, launched in July 2020. It was a double mission: sending an orbiter and a rover to Mars to attempt two major exploration milestones with a single shot. Tianwen-2 will similarly aim for two birds with one stone.

While China has already landed on the Moon and Mars and returned samples from the lunar surface, this new mission will involve a microgravity environment, greater distances, and communication delays that will put new demands on sampling technologies and automation.

If successful, Tianwen-2 will deliver invaluable new material for analysis and further our understanding of the origin of asteroids, comets, and the evolution of the Solar System.

What’s the plan?

Tianwen-2 is currently scheduled to launch in 2025 on a Long March 3B rocket. It will first target the near-Earth asteroid 469219 Kamoʻoalewa, a quasi-satellite of Earth. Kamoʻoalewa is roughly 40-100 meters (131-328 feet) in length and is thought to be a primitive planetary body, though a recent paper using data from ground-based observations indicates it could be part of the Moon blasted off by a major impact. Close-up analysis and grabbing material from the surface for study back on Earth should reveal much more.

The spacecraft will rendezvous with Kamoʻoalewa and carry out remote sensing, assessing the planetary body for potential landing sites. It will then attempt to collect samples from the small body using two different techniques — touch-and-go and anchor-and-attach — and return to Earth to deliver the samples for analysis. Altogether, this will take around 2.5 years.

China collected samples from the Moon’s Oceanus Procellarum in 2020 with its Chang’e-5 spacecraft. That mission used a drill and a scoop to collect material. However, attempting a landing and using similar techniques will not be possible for the visit to Kamoʻoalewa due to the minuscule gravitational field of the asteroid.

Instead, the mission will use the touch-and-go approach used by both NASA’s OSIRIS-REx and JAXA’s Hayabusa2 asteroid sampling missions and, for the first time, an anchor-and-attach method.

The second approach would require exquisite spacecraft guidance as well as navigation and control capabilities to get close to the asteroid. The anchoring system would need to be “extremely sensitive to the properties of the surface regolith” to allow for long-time operations and more controlled sampling, according to a 2021 paper on the mission. Adding further difficulty is the fact that the lander will need to descend and attach itself to the asteroid autonomously because of the time it takes commands to travel between Earth and Kamoʻoalewa.

If sampling goes well, getting the material back to Earth will require a reentry capsule designed to withstand greater speeds, and thus higher energies and temperatures on contact with the atmosphere, than experienced by the Chang’e-5 reentry capsule. The Tianwen-2 capsule will return at around 12.1 kilometers per second (7.5 miles per second) compared with 10.7 kilometers per second (6.6 miles per second) for the return from the Moon.

Objective 2: Main belt comet rendezvous

After sending the sample capsule to Earth, the Tianwen-2 spacecraft will use its close encounter with our home planet to fling itself into a transfer orbit that will take it to the main-belt comet 311P/PANSTARRS about seven years later. Arriving in the mid-2030s, the goal this time will be orbiting and conducting a remote sensing study.

311P/PANSTARRS is named for the telescope that discovered it and has a diameter of 480 meters (1,570 feet). A tail observed during its closest approach to Earth suggests it could be rich in volatiles. It orbits the Sun once every 1,180 days (3.23 years), coming as close as 1.94 astronomical units (AU) and reaching as far as 2.44 AU from the Sun.

Getting up close to 311P could provide not only new insights into comets themselves but also new and valuable data for discerning if comets are responsible for delivering water to the early Earth.

The spacecraft will carry eight payloads for the decade-long exploration goals: optical and multispectral cameras, a thermal emission spectrometer, a visible and infrared spectrometer, a sounding radar for analyzing the asteroid surface and subsurface, a magnetometer, a charged and neutral particle analyzer, and an instrument for determining the characteristics and the distribution of dust on both the asteroid and comet.

Russia will also contribute to these payloads following a China National Space Administration call for proposals to join the mission issued back in 2019. Further international participants could be revealed as the launch date nears.

Mission evolution

The mission has evolved since early proposals. It was initially named Zheng He for the 14th Century admiral and explorer. A mission profile published in 2019 targeted a 2022 launch, heading first to Kamoʻoalewa, returning to Earth in 2024, and then using a Mars gravity assist to head for a different comet, 133P/Elst-Pizarro, with arrival in 2030.

Although the mission’s launch date has been shifted to 2025, the mission has moved beyond the proposal stage and is also noted in China’s latest space white paper as one of the objectives for the next five years and, more recently, work on the spacecraft was noted as a major task for 2023 by China’s main space contractor.

It is now also designated Tianwen-2 and thus an official part of China’s deep space and interplanetary exploration program. Tianwen-1 visited Mars and Tianwen-3 will aim to return to the red planet, attempting an audacious mission to collect Martian samples and deliver them to Earth.

Tianwen-4 is scheduled to launch around 2030 and carry both a Jupiter orbiter and a Uranus flyby spacecraft. This will mean yet another highly ambitious attempt from China to explore two planetary bodies with one launch in the coming years.