Chang'e-5 probe, which was launched Tuesday to collect and return moon samples, is expected to achieve multiple breakthroughs in its journey. The lunar mission is comprised of an orbiter, a lander, an ascender and a returner. After it enters lunar orbit, the lander-ascender combination will separate from the orbiter-returner combination. The lander-ascender will touch down on the northwest region of the moon's Oceanus Procellarum, collect samples from the lunar surface, as well as drill into the ground. The sampling task is full of uncertainties, according to Peng Jing, deputy chief designer of the Chang'e-5 probe from the China Academy of Space Technology (CAST) under the China Aerospace Science and Technology Corporation. Chang'e-5 will be the world's first moon-sample mission for more than 40 years, and Chinese space engineers have made elaborate plans for possible challenges ahead, according to Peng. "We are not sure whether the landing site is made of hard rock or loose soil, so we have developed sampling instruments for different scenarios," Peng said. "We developed two sampling methods, including drilling underground and collecting samples from the lunar surface, to increase the chance of acquiring more diverse samples." The packaging and sealing of the moon samples also require an elaborate design to prevent leakage and contamination. Earth is surrounded by an atmosphere that contains both gases and floating solid particles, which might contaminate lunar samples if they are not perfectly sealed, limiting their scientific value. The packaging device should be able to perform well in harsh environments, said Wang Xianrong, a researcher from the CAST. After sampling, the ascender will take off and dock with the orbiter-returner in lunar orbit to transfer samples to the returner. The takeoff from the moon will also be a difficult task. Peng said the lander will act as the "launching pad" of the ascender, but the lander might not be horizontal and stable as it should be and is likely to land on a slope or complex terrain. "The spacecraft needs to conduct autonomous positioning and attitude determination during takeoff," Peng said. China has mastered rendezvous and docking technology in low-Earth orbit for manned space flights, but Chang'e-5 will carry out an unmanned rendezvous and docking in a lunar orbit 380,000 km away, a first in the history of space exploration, Peng said. During the rendezvous and docking, there is only a five-cm margin of error when controlling the two spacecraft, Peng said. The orbiter will carry the returner back to Earth. The returner, with the samples on board, will then reenter the atmosphere and land at Siziwang Banner in north China's Inner Mongolia Autonomous Region. The orbiter-returner is expected to arrive at Earth at a speed of more than 11.2 km per second, the second cosmic velocity or escape velocity. The designers have developed a skip reentry method mimicking a skipping stone to lower the craft's speed, according to Pei Zhaoyu, deputy director of the Lunar Exploration and Space Program Center of the China National Space Administration. "The returner will enter the atmosphere at a high speed, then spring out of the atmosphere with the lift of the air, and plunge back into the atmosphere at a lower speed roughly equal to that of a return satellite, for which we have experience in reentry," Pei said. "The method will make the reentry safer, and provide a technical foundation for future manned missions."