Earlier this year, on 3 January, China’s Chang’e-4 (CE-4) spacecraft made an historic first by touching down safely on the far side of the Moon.
All prior landings, manned or robotic, had been on the nearside, including China’s own Chang’e-3, which safely arrived in 2013. Landing on the farside is more difficult because, with the bulk of the Moon lying in the way, mission controllers on Earth have no direct way to communicate with the lander.
The CE-4 powered descent trajectory. Credit: Chinese Academy of Sciences
To solve this, the Chinese space agency began by launching a communication satellite called Queqiao into a stationary orbit about a million kilometers above the Moon. That allowed it to relay signals between Earth and Chang’e.
Using that, the Chinese team has been able to conduct investigations on the Moon, including piloting and collecting data from Chang’e-4’s rover, Yutu-2, which so far has trundled across 285 meters of the Lunar surface and, among other things, discovered rocks that appear to have been ejected from the Moon’s mantle by an ancient, deep impact.
But one thing has been elusive: Change’e-4 remains out of sight from Earth, and nobody was really sure exactly where it was. Sure, they knew it had come down in Von Kármán crater, a 180-kilometer depression in the Moon’s southern hemisphere, almost directly opposite the Earth. But with no Earth-based radio tracking or telemetry, the exact location wasn’t precisely known.
To find it, Jianjun Liu of the Chinese Academy of Sciences, Beijing, and colleagues used images from the spacecraft’s landing camera, transmitted back to Earth via the Quequiao satelite after the landing, to reconstruct the spacecraft’s track during its descent. They also used photos of Chang’e’s landing site sent back by the Yutu-2 rover as it began its explorations.
Based on these, and maps of the Lunar farside provided by NASA’s Lunar Reconnaissance Orbiter, the Chinese scientists were able not only to pinpoint the spacecraft’s landing site within a couple of meters, but to deduce its descent trajectory, as the autopilot sought out a safe landing site.
Drawing on 180 images snapped by the landing camera during the final three minutes of descent, Liu’s team reports, “we reconstructed the descent trajectory, showing even barely perceivable maneuvers of the spacecraft during the landing approach.”
The result, says one non-Chinese scientist, who asked not to be identified in order to commment as an “interested observer” rather than an expert on Moon landings, appears to be mostly an engineering study of “how they did the landing and how the lander figured out the terrain to stay close to target and not land on a big boulder.”
The scientist adds: “I can imagine engineers at NASA will find this quite interesting as they plan the next landing mission, somewhere.”
Liu’s team says the finding is also important for ground-truthing orbital maps of the Moon. “As a permanent artificial landmark on the lunar farside, the location of the CE-4 lander…can serve as a potential control point,” they write. “The result will provide a worthy geodetic data point for … subsequent lunar exploration.”
The findings are published in the journal Nature Communications.
