China Rescues Stranded Lunar Satellites After Rocket Failure

China has managed to deliver a pair of satellites into lunar orbit despite the spacecraft initially being stranded in low Earth orbit following a rocket failure, using a mix of complex calculations, precise engine burns, and astrodynamic ingenuity.

China launched the DRO-A and B satellites on 13 March last year on a Long March 2C rocket, aiming to send the pair into a distant retrograde orbit (DRO) around the moon. However, the rocket’s Yuanzheng-1S upper stage—intended to fire the spacecraft into a transfer orbit to the moon—failed, leaving the pair marooned in low Earth orbit.

Little is known for certain about the satellites. They must be small, given the limited payload capabilities of the rocket used for the launch, and are thought to be for testing technology and the uses of the unusual retrograde orbit. (DRO orbits could be handy for lunar communications and observation.) Critically, the spacecraft’s small size means they have little propellant, making reaching lunar orbit from low Earth orbit unassisted a very tall order. However, Microsat, the institute under the Chinese Academy of Sciences (CAS) behind the mission, got to work on a rescue, despite the daunting distance.

“Having to replan that in a hurry must be a nightmare, so it’s a very impressive achievement.” —Jonathan McDowell, Harvard-Smithsonian

What followed was a 167-day-long effort that first got the spacecraft out to well beyond lunar distance and then successfully inserted the satellites into their intended orbit. The operation included five orbital maneuvers, five further trajectory corrections to fine-tune the satellites course, and three gravity assists from the Earth and moon.

The first steps were small engine burns at perigee—the spacecraft’s closest orbital approach to Earth—which gradually raised the apogee—the farthest point of the orbit from Earth. Once the apogee was high enough, a larger burn put the spacecraft on an atypical course for the moon.

From the Earth to the Moon

Normally, spacecraft going to the moon follow the simplest trajectory, a so-called Hohmann transfer that burns a lot of propellant to get moving and then uses another big burn to drop into orbit once the spacecraft arrives at its destination after three to four days. Instead, the Chinese took advantage of a chaotic dynamical region around the Earth-moon system to save propellant. The Japanese Hiten probe had been rescued using a similar approach in 1990, but it was sent into a conventional lunar orbit.The calculations to reach DRO—a high-altitude, long-term stable orbit moving in a retrograde direction relative to the moon—would have been arduous.

“A small error will make you miss your target by a long way.” —Jonathan McDowell, Harvard-Smithsonian

“The astrodynamics of getting to the Moon is already much more complicated than just Earth orbit missions,” says Jonathan McDowell, a Harvard-Smithsonian astronomer and space activity tracker and analyst. “Involving so-called ‘weak capture’ and distant retrograde orbits is far more complicated still, and having to replan that in a hurry must be a nightmare, so it’s a very impressive achievement.”

Weak capture refers to a celestial body gravitationally capturing a spacecraft without the need for a significant propulsive maneuver. This technique, crucial for a fuel-efficient lunar orbit insertion, demands precise timing and fine trajectory adjustments, as McDowell explains.

“The way to think of these ‘modern’ and fancy orbit strategies is that you trade time for fuel. It takes much longer but you use less fuel. Once you get out to the apogee of the transfer trajectory—they don’t say how far out that was but I am guessing over a million kilometers—you can change your final destination a lot with just a small puff of the rockets. But by the same token, a small error will make you miss your target by a long way.”

Slides from an apparent Microsat presentation emerged on social media, illustrating the circuitous path taken to deliver the spacecraft to lunar orbit. The institute, however, did not respond to a request for comment on the mission.

DRO-A and B separated from each other after successfully entering their intended distant retrograde orbit. The pair have, according to U.S. Space Force space domain awareness, orbits with an apogee of around 580,000 kilometers relative to the Earth and a perigee of around 290,000 km, while the moon orbits Earth at an average distance of 385,000 km, indicating a very high orbit above the moon.

There, the spacecraft are testing out the attributes of the unique orbit and testing technologies, including communications with another satellite, DRO-L, which was launched a month before DRO-A and B into low Earth orbit. Though not a major part of China’s lunar plans, the country is planning to establish lunar navigation and communications infrastructure to support lunar exploration, and the satellites could inform these plans.

DRO-A, at least, also carries a science payload in the form of an all-sky monitor to detect gamma-ray bursts, particularly those associated with gravitational wave events, such as colliding black holes, neutron star collisions, and supernovae. The instrumentation is based on China’s earlier GECAM low Earth orbit gamma-ray-detecting mission, but with an unobstructed field of view in deep space and less interference.

The rescue then is a boost for China’s lunar plans and space science objectives, and demonstrates Chinese prowess in astrodynamics. McDowell notes the closest approximation to this rescue is the Asiasat 3 mission, renamed HGS-1, where the satellite bound for geostationary (GEO) orbit was stuck in an elliptical transfer orbit in 1997. The satellite’s apogee was raised to make a pair of lunar flybys to eventually deliver it to GEO with fuel remaining to operate for four years.

“[This] definitely shows that China is now on a par with the U.S. in its ability to manage complex astrodynamics,” McDowell says.

China also pulled off a complex lunar far side sample return mission last year, requiring five separate spacecraft, and next year plans a landing at the lunar south pole to seek out volatiles including water. The successful salvaging of the DRO-A and B mission reinforces China’s growing expertise in deep space navigation and complex orbital rescues. With plans to establish a permanent moon base in the 2030s, such capabilities will be crucial for maintaining and supporting long-term Moon operations.