The desert of fine, soft sand ripples with low dunes and not a plant is in sight. Simmering under a ceaseless noonday sun, the Kumtag desert descends through chill to extreme frigidity as night falls. This is the most moon-like place in China.
It was here in northwest China on the boundary of Xinjiang Uygur Autonomous Region and Gansu Province that Chinese scientists built the testing ground for moon rover Yutu (Jade Rabbit).
If successful, the Chang'e-3 lunar probe, launched on Monday on its two-week voyage to deliver Yutu to the surface of the moon, will be China's first soft-landing on an extraterrestrial body.
WHY THE DESERT?
Jia Yang, deputy chief designer, said that as this is China's first such mission, the designers conducted many ground experiments. "The field tests in the Kumtag are some of the most important," Jia said.
"We tried our best to simulate the lunar environment in the lab, and tested Yutu there. However, the operators of the rover became so familiar with the lab environment that they could pilot him entirely satisfactorily on experience alone, even if they had no clear images," said Jia.
"To avoid this kind of problem, we needed to create an environment in the field which was entirely unknown to the remote crew, who must use their judgment to control the rover purely on the basis of the images transmitted by Yutu himself," Jia said. Yutu's long-range abilities can really only be on trial in the desert, he added.
Yutu weighs approximately 140 kilograms and runs on six wheels. The solar-powered vehicle can climb inclines of 30 degrees and cover up to 200 meters in an hour. Equipped with clusters of cameras and spectrometers, an optical telescope, radar and other sensors, the Jade Rabbit will survey the moon's geological structure and surface substances, while keeping on the look out for signs of useful natural resources.
Sometime in mid-December, Chang'e-3 should touch down in the Bay of Rainbows, or Sinus Iridum, a flat volcanic plain thought to be relatively clear of large rocks. The Bay of Rainbows was selected because the level terrain should guarantee smooth communications and ample sunshine for Yutu's solar panels.
DESERT? WHICH DESERT?
Finding a test ground that could provide everything the mission required meant analyzing the geography and climate of all the deserts in China. "We took several factors into consideration. First, we wanted very fine sand. The average diameter of particles in the lunar soil is about 70 micrometers, as thin as a hair," Jia said.
"Second, the desert must be very dry. Although China has a surfeit of deserts, many of them have vegetation, which was precisely not what we wanted. After all, there are no plants on the moon. So we looked for places with a very low precipitation," Jia said.
Another requirement was the absence of any strong winds during the testing season in October, because there is no wind on the moon. There is more than one desert that fits the bill in China. Taklamakan, China's largest desert also has the right circumstances, but the scientists were also considering the shape of the dunes.
"We hoped to make the testing ground relatively flat with some stones to simulate the most typical lunar landform, but Taklamakan has huge pyramidal dunes which would be too much trouble to work around," said Jia.
After more than 20,000 kilometers of travel and collecting data on climate, vegetation, soil mechanics and security, the northeast part of the Kumtag, close to the Lop Nur was chosen. "We checked all the deserts in China, and this place is the most suitable," Jia said.
Covering 22,800 square kilometers, the Kumtag is China's sixth largest desert. Resembling a bird's feather on satellite images, the Kumtag, from the Uygur for "sand mountain", boasts many unique landscapes. Annual precipitation is less than 10 millimeters and no plant can grow on the dunes in the northeast.
SANDBOX
Within an area of about three square kilometers, some transformation was required to bring the testing ground in line with data and pictures collected by Chang'e-1 and Chang'e-2, as well as the Apollo missions. Duan Hailin, who was in charge of the work, said, "We built slopes according to the scientists' blueprint, and dug more than 10 'meteor craters', and scattered stones here and there."
"We made the testing ground as similar to the lunar surface as we could, except for the light conditions and the temperature," said Jia. To make Yutu weigh as same as he will on the moon, some electronic equipment was moved to another vehicle, and connected with wires.
Yutu's ability over short distances of around 10 meters and much longer distances of up to one kilometer were rigorously tested. Another important experiment was to look for a dormancy point during the lunar night. "Before sunset, Yutu will find a suitable place to park up with an inclination that will allow him to be ready to awaken again after the severe cold of the lunar night," Jia said.
The scientist was reluctant to speculate on how far the Jade Rabbit can go on. "He will not go very far, even though he has the ability. He will spend most of the time conducting surveys and taking pictures. After the pictures are sent back to Earth, it will take the ground crew a long time to find suitable paths," Jia said.
"Every step Yutu takes will be very carefully planned. He will not go very far during his life," Jia said. The scientists named the desert sandbox "Wangshu Village." In Chinese mythology, Wangshu is a goddess who drives the carriage of the moon. "And here we are, building and driving a moon rover in reality," Jia said.
