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HELSINKI — A team behind the rescue of a pair of lunar satellites left stranded by a launch anomaly have revealed the challenges they faced in salvaging the mission.
China launched the DRO-A and DRO-B spacecraft from Xichang spaceport March 13, 2024, intending to send the pair, totalling 581 kilograms, into distant retrograde orbit (DRO) around the moon. The pair were designed to link up with the earlier-launched DRO-L satellite, operating in low Earth orbit, to test intersatellite links and demonstrate the utility of distant retrograde orbits—high lunar orbits in which spacecraft travel opposite to the moon’s orbital direction around Earth.
However, an anomaly experienced by the mission’s Yuanzheng-1S upper stage left the satellites in a highly elliptical Earth orbit, with a much lower apogee, or farthest point from Earth, than planned. Simultaneously, the joined satellites were spinning once every 1.8 seconds, threatening the structural integrity of the spacecraft and their ability to operate and communicate. The rescue operations are detailed by an April 16 report from China Youth Daily.
The team first eliminated the spin, using the DRO-B satellite attitude control engines to correct the rotation over the course of 20 minutes. Telemetry, however, then revealed issues with both satellites’ solar arrays which needed addressing.
Next, a team, including researchers from the Innovation Academy for Microsatellites (IAMCAS) and Technology and Engineering Center for Space Utilization (CSU), both under the Chinese Academy of Sciences (CAS), formulated a rescue plan over the following 40 hours. This needed to take into consideration the complexity of orbital dynamics and gravitational perturbations from the Earth, moon and Sun, and very limited fuel. The team also faced a deadline of mere days, needing to quickly execute a first maneuver or lose the opportunity of reaching DRO.
The first critical engine burn, on March 18, lasted 1,200 seconds. It raised their apogee from 134,000 to 240,000 kilometers. What followed, as detailed in earlier reporting, was the DRO-A and DRO-B satellites carrying out four further orbital maneuvers, gravity assists and additional trajectory corrections over the next four months to get the spacecraft on course for their intended lunar orbit.
The efforts ended on July 15, 2024, with the satellites safely in their predetermined orbits, having traveled around 8.5 million kilometers. The satellites appear to have traveled as far as over one million km from the Earth, allowing a low-energy capture and insertion into lunar orbit. The satellites separated successfully August 28. The pair then imaged each other, revealing that the solar panels of the DRO-A satellite were bent nearly 90 degrees, while DRO-B’s arrays were like “broken wings.”
The pair then established K-band microwave intersatellite measurement communication links with DRO-L, verifying a three-satellite network across the Earth-moon distance.
“For the first time internationally, we have achieved the ability to use satellites to track other satellites, instead of relying on ground stations,” Wang Wenbin, a CSU researcher, told China Central Television (CCTV).
“In essence, the ground station has been converted into a satellite and placed in low orbit. This breakthrough paves the way for new technological advancements in future Earth-moon space and deep space exploration.”
DRO-A also carries an all-sky detector to monitor gamma-ray bursts, similar to that used in China’s GECAM mission launched in 2020. Wang added that China intends to use DRO and its long-term stability to conduct fundamental scientific research in such fields as quantum mechanics, atomic physics and related fields.
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