"In the meantime, by controlling Chang'e 1's landing process and trajectory, scientists can better prepare Chang'e 2 for its future lunar landing," said Ouyang. He said it is very difficult to execute a soft landing on the Moon, as there is no air and a parachute cannot be used. A slope could also flip a lunar craft over and cause it to malfunction. The controlled collision is a landing rehearsal, Ouyang said.

Chang'e 1's controlled crash signals a new bar has been reached in China's remote control technology, said Wang Sichao, a research fellow at the Purple Mountain Observatory of CAS. To collide with the Moon, Chang'e 1 turned 180 degrees from its routine orbit. That is technically challenging, Wang explained, because radio signals are very weak 384,000 km from the Earth.

Scientists carefully chose the satellite's crash site so the process could be observed by scientists on the Earth, said Pang Zhihao, a research fellow in the China Academy of Space Technology. Chang'e 1 collided close to the Moon's equator on the side facing the Earth. The crash time was also carefully chosen and controlled, said Wang. The same site might be chosen to land the satellites in the next phase of China's lunar exploration project.

The recent collision of two satellites owned by the United States and Russia triggered worries about space junk, but the debris created when Chang'e 1 smashed into the Moon would not cause such concerns, Wang said. The debris from the crash would be trapped on the Moon by its gravity, and no space junk would be produced in the process.

Chang'e 1's adventure

During more than one year flying around the Moon, Chang'e 1 survived several troubles. It suffered from solar radiation disturbance in November 2007, said Sun Jiadong, an academician at CAS and chief designer of China's lunar orbiter project, who once worked on the country's first artificial satellite in 1970. When the sun gets above the Earth's equator, its electromagnetic waves are more likely to interfere with radio waves sent from observatories on the Earth, causing signal errors.

Chang'e 1 also experienced three lunar eclipses. "During a lunar eclipse, the satellite was shaded from the sun and its temperature would drop. Moreover, the solar cell panel could not work, and the satellite had to be powered by the electricity stored in the cells. A satellite usually does not carry many solar cells, so there is the risk of a blackout," said Sun.

Scientists designed Chang'e 1 with lunar eclipses taken into account. Data gathered during the February 9 lunar eclipse showed Chang'e 1's performance was better than expected, said Ye Peijian, chief designer of the Chang'e 1 satellite and the project's commander-in-chief. When the temperature dropped to minus 100 degrees Centigrade, the satellite consumed 40 percent rather than 60 percent of the stored energy, Ye said.

He said the first probe gathered valuable information for future lunar explorations and research.

Chang'e 1's first task was to capture 3-D images of the lunar surface. Although other countries have also taken 3-D pictures, they have not canvassed the entire surface, Ouyang said. Chang'e 1's mission was to obtain images covering the entire surface of the Moon, from the equator to 70 degrees north and south latitude. With these images, scientists can study the landscape, geological structure and the Moon's history.

One month after it went into space, Chang'e 1 transmitted China's first high-resolution picture of the Moon's surface, Ouyang said. Since then Scientists have stitched together more than 300 pictures to produce the world's clearest and most complete image of the Moon's surface.

Chang'e 1 also surveyed the distribution of chemical elements and minerals and measured the thickness of its soil. The satellite then obtained spatial data about the Moon's surface. Scientists are still analyzing the data. They have already been pleasantly surprised to find that they could study the Moon's gravitational field based on information collected by Chang'e 1, said Ping Jinsong, a research fellow at the Shanghai Astronomical Observatory of CAS, who was the technical director of Chang'e 1's VLBI (Very Long Baseline Interferometry) subsystem.

"Why do human beings explore the Moon? The Moon and the Earth are very closely related. The answer to the origin of life on the Earth may lie in the Moon," said Xiao Naiyuan, a professor in the Department of Astronomy at Nanjing University.

The Moon's impact on our lives is omnipresent, said Xiao. The ancient Chinese lunar calendar is based on the Moon's movement around the Earth while oceanic tides are created by the Moon's gravity. The Moon's origin has intrigued scientists for generations.

Four major hypotheses are competing to explain the origin of the Moon, according to Michael Oard, a U.S. meteorologist. Oard summarized the four hypotheses in his paper titled "Problems for 'Giant Impact' Origin of Moon." The fission theory suggests that the Moon is a chunk spun off from the Earth, the capture theory says that the Earth captured the Moon as it wandered through the solar system, the condensation theory holds that the Earth and the Moon formed at about the same time and from the same source, while the giant impact hypothesis believes that a smaller planetary body hit the Earth, blowing out rocky debris that eventually aggregated into the Moon.

Some scientists believe that studying the dust kicked up by a satellite collision might shed more light on its origin. Some of them think that debris produced when comets or small planets collided with the Earth might be strewn on the Moon and buried in its dust, while others speculate that lunar craters might contain fossils of the earliest microbes living on the Earth.