Behind China's 123-day space rescue: The math that defied the odds

For the past few months, we have seen many reports about China potentially saving a pair of satellites after launch failure.

It's official; the organization behind that almost-failed space mission announced on Tuesday that after over 100 days of rescue attempts, the satellites have entered the target orbit and are working as designed.

What's more, the satellites being rescued represent China's growing curiosity about the vast space around Earth and the moon.

Let's first look back to March 2024, when CGTN Digital reported on two Chinese satellites that failed to enter orbit after abnormalities after liftoff. In the brief story, we reported that "efforts related to the relevant disposal work are currently in progress."

But we didn't realize then that the efforts would last for months, ultimately leading to the satellites' rescue.

Caught by surprise

The launch glitch also came as a surprise to many at the Technology and Engineering Center for Space Utilization (CSU), which leads the space mission.

"That was the first launch mission I watched, and I didn't think about launching glitch at first," said Zhang Hao, whose team found the final rescue route.

The first thing that came to Zhang's mind was to find out the status of the satellites.

"If the satellites were destroyed, that would have been a waste of the years of effort that we put in and the money invested in the mission. It would also be a mental blow to the team," Zhang explained.

"Luckily, that's not the case," he told CGTN Digital in an interview.

When the satellites were found, they were spinning like an out-of-control discus and were much closer to Earth than planned.

"We divided into two teams. One team remotely controls the satellites' thrusters to slow down the spinning. The other team, my team, calculated the best route to move the satellites back on track."

Although Chinese are often stereotyped as skilled in math, calculating the new trajectory remained a stressful task. Zhang stayed up two nights straight, working on a solution. According to Zhang, with the help of adrenaline, the team managed to find a few plausible routes and presented them with mission control.

Harnessing gravitational forces to shoot into orbit

As the satellites were partly damaged during the glitched launch, they could not absorb enough sunlight to power a big turn, posing yet another retrieval challenge.

To solve the problem, the team used the gravity of the Earth, moon, and sun like a slingshot to shoot the satellites to their destination.

"If you don't want to consume much energy, you must replace it with something else. We chose to consume more time in order to save energy," said CSU researcher Mao Xinyuan, explaining why an activity that should have taken mere days took 123 days in total.

The first maneuver was the most dangerous one. It took exactly 20 minutes, or 1,200 seconds, to complete.

"I got more and more stressed as the clock ticked," said Zhang. "I just kept staring at the screen until it said 'normal.'"

"For the following maneuvers, I kind of got used to them and no longer got so stressed," Zhang recalled. "It's like raising a child. The baby phase is the hardest."

'Lighthouses' for space navigation

The two rescued satellites carry a special meaning for China's space program.

Named DRO-A and DRO-B, the pair works with the previously launched DRO-L spacecraft to form a constellation covering roughly 100 million kilometers of space between Earth and the moon to provide Beidou-like navigation services for spacecraft.

"They will act as lighthouses in space," Mao explained. "(With these satellites in position), we can locate a spacecraft in just three hours, unlike the two days or more with traditional, land-based positioning."

The constellation will also allow for unattended spacecraft piloting or autopilot.

"We just give it a target position, and the spacecraft will automatically find its way to the destination," said Wang Wenbin, a CSU researcher who participated in the mission.

The new pilot system will help China launch and maintain spacecraft in the vast space around Earth and the moon, especially the "distant retrograde orbit" (DRO), which is easy to enter, easy to maintain, and easy to get away from, according to Wang.

"It's a natural space harbor," he told CGTN Digital, adding that the team is in talks with China's manned space program to provide navigation services for future lunar missions.