China prepares for country’s first Mars landing attempt with Tianwen-1

China will attempt to become only the second nation to land a spacecraft on Mars on Friday, joining the United States. Tianwen-1, China’s first mission to the Red Planet, launched in the middle of last year, sharing the particularly busy July 2020 Martian launch window with NASA’s Mars 2020 mission, including the Perseverance rover and Ingenuity helicopter, and the United Arab Emirates’ Al Amal orbiter.

Within the coming hours, Tianwen-1’s orbiter section is set to jettison it’s lander section, which will attempt to land on Mars’ Utopia Planitia, carrying with it a small rover called Zhurong. Landing is scheduled to occur at 23:11 UTC.

The spacecraft launched from Wenchang Spacecraft Launch Site on the southern Chinese island of Hainan aboard the fifth flight of the country’s Long March 5 heavy lift rocket on 23 July 2020. As well as marking the first time the Long March 5 had launched a payload beyond Earth orbit, the launch of Tianwen-1 also marked China’s first mission to Mars.

Tianwen-1 launching aboard the fifth flight of China’s Long March 5 rocket on July 23 2020 – via CNSA

Despite being the county’s first interplanetary mission, it is rather complex, with the approximately five ton probe consisting of three separate spacecraft, an orbiter, lander and rover.

These three spacecraft launched as one, with the lander/rover section of the spacecraft incapsulated in a small capsule, intended to allow it to pierce through the Martian atmosphere. The three spacecraft have been operating as one in Martian orbit since Tianwen-1 arrived on 10 February.

The China National Space Administration (CNSA) has been reluctant to share much information of the timeline of the entry, descent and landing procedures, but a rough outline of how Tainwen-1’s landing should play out is known.

Seven minutes of terror

Those who have worked on spacecraft that have attempted, successfully or not, to land on the Red Planet, have nicknamed the time in which the spacecraft enters the atmosphere, descends to the surface, and touches down, as the “seven minutes of terror”, mainly because of the complexity of the approximately seven minute journey from the top of the Martian atmosphere to the surface.

Artist’s impression of the three spacecraft China will send to Mars – via Nature Astronomy/CNSA

The first order of business for Tianwen-1 will occur approximately five hours prior to landing, when the orbiter, still connected to the capsule containing the lander and rover, will ignite it’s engines to place it in on a trajectory that would see it intersect with the Martian atmosphere. Shortly afterward, the orbiter will separate from the lander/rover capsule, and re-ignite it’s engines to place it back into a safe Martian orbit. This maneuver will place the lander and rover on a course to enter the Martian atmosphere within five hours.

This descent strategy, temporarily de-orbiting the Tianwen-1 orbiter, avoids the need for the entry capsule to have its own orbital maneuvering system, and is not dissimilar to the strategy used by NASA’s Galileo spacecraft to drop an atmospheric probe into Jupiter in 1995.

The main entry, descent and landing sequence will begin at around 23:04 UTC, around seven and a half minutes prior to the planned landing time, when the lander and rover are set to hit the Martian atmosphere, travelling at around 4,800 meters per second, protected by a heatshield to keep the two vehicles safe from entry heating.

Once the spacecraft comes within four kilometers of the Martian surface, the capsule, still encapsulating both the lander and rover, will deploy a parachute to begin slowing the spacecraft as it barrels towards the surface. Shortly afterward, at around 23:06 UTC, the heatshield that had protected the lander during atmospheric entry will be jettisoned and fall to the Martian surface.

The jettisoning of the heatshield will allow for the lander and rover to separate from the landing capsule and parachute, which is set to occur when the spacecraft is around 1,500 meters above the Martian surface. At around 100 meters above the surface, the lander will ignite its engines and slow down the spacecraft into a hover above Utopia Planitia, allowing it to begin its final descent to the surface.

Diagram outlining Tianwen-1’s entry, descent and landing sequence – via CNSA

A suite of cameras and LIDAR (light detecting and ranging) equipment will be used to navigate the spacecraft to touch down.  Assuming this all goes to plan, the lander and rover will touch down on the Martian surface at 23:11 UTC, brining an end to China’s “seven minutes of terror.”

Surface operations

Once the lander has touched down in Utopia Planitia, the spacecraft will begin a planned 90 Sols (Martian days) of surface operations, conducting geology, minerology and geophysical investigations, among others.

The rover, named Zhurong, will be kept on top of the lander, in similar fashion to China’s Chang’e 3 and Chang’e 4 lunar landers, which both carried a small rover atop them during the landing sequence of their missions. To allow for Zhurong to safely make it’s way onto the Martian surface, a ramp will remotely unfold, allowing the rover to drive down to the surface.

To facilitate its 90 Sol scientific mission, Zhurong is equipped with over six scientific instruments, including a subsurface radar that will allow the rover to peer over 100 meters below the Martian surface, a spectrograph to gain data about the chemical composition of Mar’s surface, and a device provided by the French Institute for Research in Astrophysics and Planetology (IRAP). The device is a calibration target, a duplicate of one IRAP provided NASA’s Curiosity rover. The agency will compare the dataset from the Zhurong calibration target with the dataset from the Curiosity calibration target.

The landing site, Utopia Planitia, is also very significant in terms of exploration of the Red Planet. On 3 September 1976, NASA’s Viking 2 spacecraft touched down in the region, marking the second ever landing on Mars. Viking 2’s scientific mission lasted nearly four years until its batteries failed and contact was lost in April 1980.

Mock-up of the Zhurong rover in March 2021 – via AFP

A return to Utopia Planitia is significant because Viking 2’s scientific mission uncovered interesting results during its analysis of the soil in the region during it’s stay on the planet. Viking 2 carried several biological experiments to aid in the search for life on Mars, and during one experiment, known as a Labeled Release (LR) experiment, Viking 2 injected a soil sample with a solution manufactured to influence any metabolism with micro-organisms that could be present within the soil.

When Viking 2 performed the LR experiment, it returned results indicative that their were micro-organisms within the soil sample the spacecraft had collected from the surface. In the years since Viking 2, several theories have been thought of to provide a non-biological explanation for what Viking 2 could have found within the soil in Utopia Planitia, although many hope that the first return to the Planitia since Viking 2 hopes to definitively answer some of these 40 year old questions.

Like all Mars landing attempts, a lot will need to go right if the Tianwen-1 lander is to safely touchdown in Utopia Planitia on Friday. If the CNSA is able to successfully pull it off, it will mark a breakthrough, not only for the growing space agency, but also for the wider space community.

A successful landing of Tianwen-1’s lander will mark the first time an agency will have been able to safely land a spacecraft on the Martian surface on their first mission to the planet, an incredibly difficult and noteworthy achievement.

(Lead render of orbiter/capsule separation via Mack Crawford for NSF)

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