Looking back on China's road to outer space, people can easily find it has not been very smooth. In the past years, Chinese people have made hard and determined efforts to realize the dream their ancestors had for thousands of years. After China's first satellite into the Earth's orbit in 1970 came four flights of unmanned Shenzhou missions  from 1999 to 2002. The country carried out its first one-piloted space flight in October, 2003, making China the third country in the world to have independent human spaceflight ability after the Soviet Union and the United States. Then came another breakthrough on October 12, 2005, when Shenzhou 6, China's second human spaceflight, was launched, with a crew of two astronauts. What's more, the landmark spacewalk done by Zhai Zhigang, one of the three boarding Shenzhou 7, launched on Sept. 25, 2008, leads the country further in its space exploration. Meanwhile, China's moon exploration project, started in 2004, has also been progressing satisfactorily. Fifty years after the Soviet Union launched Sputnik, the world's first man-made satellite, China's first circumlunar satellite Chang'e 1 took off on Oct.24, 2007, which became another milestone  in China's space achievements after the above-mentioned manned flights. Chang'e 1 was expected to fulfill four scientific goals, one of which was to probe mineral elements on the moon, especially those not existent on Earth. It's said that the lunar regolith  is abundant in helium-3, a clean fuel that may support the Earth's energy demands for more than a century. Nearly 3 years later, Oct. 1, 2010 witnessed the blasting off of Chang'e 2, China's 2nd unmanned lunar probe, marking another step forward in moon exploration. This time the aim is to test the key techniques of Chang'e 3 and Chang'e 4, as a preparation for a soft lunar landing in the future. With great expectations, people all over the world are looking forward to China's greater space achievements. Altogether how many astronauts have made successful spaceflight by Shenzhou spaceship by now?
4 5 6 7