Pupils from a school in Zhuji city, Zhejiang province, find a special way to celebrate National Day and the space mission with their own model of the Chang'e-2, Sept 30, 2010. [China Daily]

The exact launch time for the country's second lunar probe was announced as authorities said the mission faces three major challenges.

Chang'e-2, a circumlunar satellite that will test key technology involved in a soft-landing on the moon around 2013, is scheduled to blast off from the Xichang Satellite Launch Center at 6:59:57 pm on Friday, a spokesman for the China National Space Administration said.

If the launch misses that slot it could be rescheduled for Saturday or Sunday, he said.

Wu Weiren, chief designer of China's lunar exploration program, told China National Radio on Thursday that weather conditions were a concern.

Drizzle or light rain would not interrupt the launch, but thunderstorms could, he said.

Showers are forecast to last until Monday in Xichang and the city was hit by a thunderstorm on Thursday night.

Scientists denied that the launch time was chosen to mark National Day, saying the launch date was purely coincidental and determined by other factors.

"According to scientific calculations, only three days (from Friday to Sunday) are suitable in the whole year for launching the lunar probe," said Qian Weiping, chief designer of the mission's tracking and control system, on Thursday.

"On each of the three days, the launch window is different, and the point where the probe enters the preset lunar transfer orbit could be hundreds of kilometers apart," he said.

Though the first lunar probe mission provided valuable information, the Chang'e-2 mission still faces unknown risks.

The first challenge is whether the rocket can directly send the probe into the Earth-moon transfer orbit. Simply put, Chang'e-2 will go directly into Earth-moon transfer orbit rather than orbit the Earth first. The Chang'e-1 used a different procedure, orbiting the Earth first on its mission, chief designer Wu said.

The second challenge is whether the probe can be captured by the moon's gravitational pull. "If the braking is not well handled, the probe could either crash into the moon, or fly away (from the preset orbit)," he said.

The third risk involves when the probe is maneuvered to an orbit just 15 km from the moon. At that point it will take high-resolution photos of the moon's Bay of Rainbows area where Chang'e-3 will land.

Wu said the maneuver to lower the orbit has to be done when the probe is on the dark side of the moon, so that when it flies to the bright side it can take the photos. As the tracking and control system is unable to reach the dark side, the success of the operation relies on the satellite's technology.

Sun Jiadong, chief commander of the program's first mission, also said that the maneuver to lower orbit will be a critical time and is a huge challenge to the country's orbital maneuver technology.

As the countdown begins, the attention of the whole country has turned to the small city in Southwest China.

Since Thursday, tourists planning to watch the launch have started to flock into the city and hotels say they are fully booked for Friday despite room rates increasing.

Local travel agencies said some 3,000 people can witness the event from a venue 4 km away from the launch pad but will have to pay 480 yuan (US$72) each.

(China Daily October 1, 2010)